Weather: An Introduction

Updated About encyclopedia.com content Print Article Share Article
views updated

Weather: An Introduction

It starts with the Sun
Heat and temperature
The atmosphere: Where weather occurs
Air pressure and weather
Wind: Air in motion
Air masses
Fronts: Where the action is
What is a storm?
Water in the air
Condensation
Cloud formation
Clouds and precipitation
Land and weather
Oceans and weather
For More Information

Weather plays an important role in our lives. It influences how people dress, what they can do outdoors, and even their moods. When people talk about weather, they usually mean things like wind, rain, snow, thunderstorms, and sunshine. But what causes the weather? Where does it all come from?

All forms of weather are produced by complex, constantly changing conditions in Earth's atmosphere. However, the driving force behind the weather is the Sun.

It starts with the Sun

The Sun continually generates energy, which escapes from its surface and flows through space. Solar energy travels 93 million miles (149 million kilometers) to reach Earth. It warms all of Earth's atmosphere, some parts more than others. The area of Earth that receives the Sun's rays most directly, the equatorial region, is heated the most. The poles, conversely, never receive sunlight directly. Sunlight strikes the poles only at a steep angle. Hence, they are warmed the least.

Another factor that determines how much solar energy strikes any particular part of Earth at any time is the season, a period of year characterized by certain weather conditions. Most places in the world have four seasons: winter, summer, spring, and fall. In winter, the Sun shines for the fewest hours per day and never gets very high in the sky. In summer, day is longer than night, and the Sun shines high in the sky. In spring and fall, the Sun rises to an intermediate height, and there are roughly the same number of hours of daylight as darkness.

The change in seasons is caused by a combination of Earth's tilt and its yearly journey around the Sun. Earth's axis of rotation is tilted 23.4° away from the perpendicular. At different points along Earth's orbit around the Sun, the Northern Hemisphere, the half of the earth which lies north of the equator (which includes the United States) is tilted either toward or away from the Sun. For instance, on or about June 21, the first day of summer, the Northern Hemisphere receives more sunlight than on any other day. On or about December 21, the first day of winter, the Southern Hemisphere, the half of the earth that lies south of the equator, receives its greatest amount of sunlight.

For two days each year the hemispheres receive approximately equal amounts of sunlight. These days are on or about March 21, the vernal equinox, and on or about September 23, the autumnal equinox. These two days mark the beginnings of spring and fall, respectively.

The uneven heating of the atmosphere sets the atmosphere in motion. Air moves through the atmosphere in such a way as to even out the distribution of heat around the planet, with warm air moving from the equator to cold areas at the poles and cold air back toward the equator. The movement of air between the equator and the poles is influenced by other factors as well, such as differences in composition of air over land and sea, and Earth's rotation. The result is a complex web of air currents whirling around the globe—the ingredients of weather.

When solar energy strikes Earth

Energy that comes from the Sun is often referred to as "sunlight." Yet, it is really a combination of many types of electromagnetic radiation. Electromagnetic radiation is energy in the form of waves of electricity and magnetism. Solar energy that reaches Earth's surface is made up almost entirely of visible light (which can be seen), and infrared radiation.

Try this: How the seasons change

Place a lamp (minus the lampshade) on the desk in front of you. Now take a tennis ball or a pingpong ball and draw a horizontal line around its middle. This line represents Earth's equator. Mark an "N" on the top half of the ball (for the Northern Hemisphere) and an "S" on the bottom half (for the Southern Hemisphere).

First hold the ball with the "N" pointing straight up and the "S" pointing down. Now tilt the ball so the "N" is tilted slightly away from the perpendicular. This arrangement represents the 23.4 degree tilt of Earth's axis away from the perpendicular. Hold the ball this way in front of the light and move it in a circle around the light in this order: to the right of the light, behind the light, to the left, and finally to the front again.

You'll notice that when the ball is to the right of the light (similar to Earth on the first day of winter), light from the lamp strikes the "S" half of the ball more directly than it does the "N" half. When the ball is directly behind or in front of the light (representing the start of spring and fall, respectively), light strikes both "N" and "S" halves at the same angle. When the ball is to the left of the light, light strikes the "N" half more directly. This position marks the start of summer in the Northern Hemisphere.

Infrared radiation is a form of electromagnetic radiation that takes the form of heat and has a wavelength longer than that of visible light. Small amounts of x rays, ultraviolet rays, and radio waves from the Sun also penetrate Earth's atmosphere.

Only two forms of solar energy reach and heat up the lower levels of atmosphere and Earth's surface. These are visible light and infrared radiation. They are the only two forms of solar energy that affect Earth's weather. When radiation is absorbed by gas molecules in the atmosphere, by clouds, or by the ground, it is converted into heat.

Most x rays and ultraviolet rays are absorbed high in Earth's atmosphere and never reach the surface. This is fortunate for humans, since a large dose of either type of radiation would be deadly. Radio waves also penetrate the atmosphere, but in such tiny amounts that they have no warming effect on Earth.

Only about two-thirds of the total solar energy reaching Earth's outer atmosphere is absorbed by Earth. One half of that radiation is absorbed by the atmosphere and the other half by Earth's surface. Ultraviolet radiation is selectively absorbed by the ozone layer, an atmospheric layer that exists between 25 and 40 miles (40 and 64 kilometers) above Earth's surface. Infrared radiation is absorbed by clouds and gases in the lowest atmospheric levels, and then reradiated in all directions.

Most of the solar radiation that reaches Earth's surface is in the form of visible light. About two-thirds of that light is absorbed by living and nonliving materials and transformed into heat. This heat causes snow and ice to melt and water to evaporate.

About one-third of solar radiation striking Earth is reflected back into space. A number of factors are responsible for this effect. One of the most important is clouds. When solar energy strikes a thick cloud, as much as 95 percent of the energy is reflected. Thinner clouds turn away up to 50 percent of the radiation that strikes them.

On the ground, the greatest reflectors of sunlight are snow and ice. Snow and ice reflect up to 95 percent of the solar energy that strikes them. Thus, air is colder when there's snow on the ground. Water, on the other hand, is a good absorber of energy. Water reflects only 10 percent of the solar energy that strikes it. Sand reflects more radiation than water (about 15 to 40 percent), but much less than snow.

The Earth also radiates heat

Solar energy that reaches Earth's surface in the form of visible light is reradiated in the form of infrared radiation (heat). Heat leaves Earth's surface and is absorbed by clouds and water vapor in the air. Clouds absorb large amounts of infrared radiation, which is why cloudy nights tend to be warmer than clear nights, all other things being equal. Clouds radiate infrared energy in all directions, throughout the atmosphere, back toward Earth, and out into space.

All parts of Earth's surface are constantly absorbing and emitting heat. For the temperature to remain constant at any one location, the surface must absorb and emit energy at the same rate. When absorption outpaces emission, the surface warms. One can experience this effect when walking on an asphalt parking lot in bare feet. When emission outpaces absorption, the surface cools. This effect occurs at night when there is no incoming sunlight to offset the heat radiating from the ground.

Earth maintains a long-term balancing act with regard to heat. Over a period of years, the quantity of heat absorbed is almost identical to the quantity released back into space. Many scientists, however, believe that a warming trend has begun in recent decades. This effect may be caused by increased amounts of certain gases in the air, primarily carbon dioxide. Carbon dioxide and other gases formed during industrial processes trap heat.

The mechanics of heat transfer

The primary way that heat is transferred in the atmosphere is by convection. Convection is the movement of masses of air caused by differences in temperature. It can be explained by two key concepts. First, heat causes air molecules to move more quickly. Second, warm air rises.

When air is heated, the molecules within it move rapidly and spread out. As a result, warm air loses density and becomes thinner and lighter. The surrounding cool air, which is denser, slides beneath the warm air, pushing it upwards. As the warm air rises, it cools. When it is no longer warmer than the air around it, it stops rising.

Convection is a critical element in the formation of weather patterns. It is the process that carries warm air up from the ground, to be replaced by cold air. The cold air is then warmed and cycles upward again.

Heat can also be transferred by a second method called conduction. This method depends upon collisions between individual molecules, in which heat is transferred from a fast-moving, warm molecule to a slow-moving, cold molecule. As the cold molecule is heated, it also moves more quickly, and a chain reaction of molecular heat transfer follows. Conduction is a very slow process because, even in the densest layer of the atmosphere, collisions between molecules are relatively rare.

Heat and temperature

Most people consider heat and temperature to be the same. After all, one can feel the heat increase at the oven door when one raises the temperature in the oven. However, while heat and temperature are closely related, they are not exactly the same.

What's the difference between heat and temperature?

The key to this difference is kinetic energy, the energy of motion. All substances are made of tiny particles (molecules or atoms) that are in constant motion. Motion ceases only at absolute zero, −459°F (−273°C). Heat is defined as the total kinetic energy of the particles of a substance, whereas temperature is the average kinetic energy of a substance.

The crux of this distinction is that heat takes into account the total volume of a substance. That is, given two volumes of the same liquid at the same temperature, the larger one contains more heat because it contains more matter. The larger volume contains a greater number of moving molecules and, hence, more total kinetic energy.

To illustrate this concept, imagine a cup of coffee at 140°F (60°C) and a bathtub of water at 85°F (29°C). If you let them both cool, the coffee would reach room temperature much more quickly than the water. This is because the water in the bathtub possesses a larger quantity of kinetic energy, which it must lose in order to cool down. The coffee cup, which contains relatively little kinetic energy, cools quickly. Although the coffee had a higher starting temperature (average kinetic energy) than the bathwater, the bathwater possessed more heat (total kinetic energy).

Specific heat

The specific heat of a substance is the amount of heat required to raise the temperature of 0.0353 ounce (1 gram) of the substance by 1.8°F (1°C). The amount of heat (measured in units called calories) necessary to raise the temperature differs from substance to substance.

The specific heat of a substance is measured relative to that of water: It takes 1 calorie to raise 1 gram of liquid water 1°C. Water, therefore, has a specific heat of 1.0. This is one of the largest specific heats of any naturally occurring substance.

By way of comparison, the specific heat of ice at 32°F (0°C) is 0.478; wood is 0.420; sand is 0.188; dry air is 0.171; and silver is 0.056. Thus, it can be seen that it takes much less heat to raise the temperature of sand than it does to raise the temperature of water. This explains why on a sunny day at the beach, the sand heats much quicker and feels much hotter than the water.

Latent heat

A large amount of heat is needed to raise the temperature of water even slightly. This effect is magnified when water undergoes a phase change between any two of the three phases: liquid, solid, or gas. During a phase change, water or ice absorbs, or emits, very large amounts of heat energy without any corresponding change in temperature. The energy associated with a phase change of water is called latent heat.

This heat is "latent" because it does not perform a warming function but instead is "stored" or "hidden" as it produces a phase change. A tremendous amount of energy is absorbed in the process of melting ice or of evaporating or boiling water. Conversely, when water freezes or when water vapor condenses, that heat energy is released back into the environment. If a cup of water and a cup of ice—both at 32°F (0°C)—were placed side by side, the ice would take much longer to reach room temperature than the water. The reason for this is that ice must first absorb enough heat to transform it to water.

Latent heat is also responsible for keeping a cold drink with ice colder than a cold drink without ice. It works like this: as heat is added to the beverage, it breaks down the crystal structure of the molecules of ice. The added heat changes ice from a solid to a liquid without changing the temperature of the surrounding liquid. If heat were added to a drink without ice, that heat would warm the liquid instead.

Who's who: Joseph Black

As a graduate student, Scottish chemist Joseph Black (1728–99) discovered the existence of carbon dioxide, which he called "fixed air." Black also was the first person to explain the concept of specific heat: that each substance requires a particular amount of heat to raise its temperature 1°C.

Black is most famous for solving the mystery of latent heat. He was first drawn to the question by German physicist Gabriel Daniel Fahrenheit's finding that water can remain in the liquid state even below its freezing point. This state is called supercooled water. On striking a surface, supercooled raindrops freeze. The freezing of water is a process that occurs when heat is lost. While heat loss can usually be measured with a thermometer, in this case the water froze with no accompanying drop in temperature. This discovery led Black to the conclusion that water possesses hidden or "latent" energy that comes and goes as water changes phases.

               FURTHER READING ON LATENT HEAT AND JOSEPH BLACK: WILLIAMS, RICHARD. "THE MYSTERY OF DISAPPEARING HEAT." WEATHERWISE. AUG./SEPT. 1996: 28-29.

Latent heat has important implications on a global scale. More than half of the solar energy that strikes Earth is stored in the form of latent heat. Since ice is able to store large amounts of solar energy in the form of latent heat, it is slow to melt. Imagine if the Arctic or Antarctic ice caps were to melt just because of the heat they absorbed on one mild day. This would create unfathomable floods. Similarly, latent heat of vaporization is why the oceans do not evaporate.

Windchill

On blustery winter days, weather forecasts usually include the windchill factor, the cooling effect on the body determined by temperature and wind, as well as the temperature. Wind magnifies the effects of low temperature because moving air removes heat from the body quicker than still air does. The body is ordinarily surrounded by a very thin layer of still air, called the boundary layer. Heat is constantly lost through the boundary layer by conduction, but this process is very slow. However, increased wind reduces the boundary layer in thickness and heat loss accelerates.

WORDS TO KNOW

air mass:
a large quantity of air throughout which temperature and moisture content is fairly constant.
air pressure:
the pressure exerted by the weight of air over a given area of Earth's surface. Also called atmospheric pressure or barometric pressure.
convection:
the upward motion of an air mass or air parcel that has been heated.
Coriolis effect:
the apparent curvature of large-scale winds, ocean currents, and anything else that moves freely across Earth, due to the rotation of Earth about its axis.
cyclone:
a weather system in which winds spiral counterclockwise, into a low-pressure area. Also called storm.
front:
the dividing line between two air masses.
kinetic energy:
the energy of motion.
latent heat:
the energy that is either absorbed by or released by a substance as it undergoes a phase change.
latitude:
an imaginary line encircling Earth, parallel to the equator, that tell one's position North or South on the globe.
precipitation:
water particles that originate in the atmosphere (usually referring to water particles that form in clouds) and fall to the ground.
supercooled water:
water that remains in the liquid state below the freezing point.
troposphere:
the lowest atmospheric layer, where clouds exist and virtually all weather occurs.

Due to the danger of frostbite, the freezing of the skin, in cold, windy conditions, forecasters added an index called the windchill equivalent temperature (WET), also called the "windchill index." This value represents the temperature at which the body would lose an equivalent amount of heat if there were no wind. For instance, if it was 32°F (0°C) with winds blowing at 15 miles per hour (mph), or 24 kilometers per hour (kph), the WET would be 15°F (−9°C). If it was 0°F (−18°C) and the wind was blowing at 10 mph (16 kph), the WET would be −20°F (−29°C).

The atmosphere: Where weather occurs

The air called atmosphere extends more than 600 miles (977 kilometers) above Earth's surface. Yet relative to the diameter of Earth, it is no thicker than a coat of paint. The atmosphere, where all weather occurs, is what sustains life on Earth.

The atmosphere contains the air we breathe and the water vapor that drives weather patterns. It shields us from most of the lethal components of the Sun's rays while allowing through the harmless components. It regulates the temperature of the planet, keeping us from getting burned up by the Sun's heat during the day or frozen to death during the dark night. In addition, the atmosphere protects us from most potentially devastating debris from space.

The atmosphere consists of 78 percent nitrogen, 21 percent oxygen, and 1 percent argon, with minute quantities of water vapor, carbon dioxide, and other gases. It is held to Earth by the force of gravity, which acts most strongly close to the surface. For this reason, the pressure and density of gases in the atmosphere decreases with altitude (height). In fact, half of the mass of our atmosphere is contained within 4 miles (6 kilometers) of the planet's surface. While 99 percent of the atmosphere is calm, the air in the lowest 6 miles (10 kilometers) is constantly on the move.

Layers of the atmosphere

Beginning with a series of hot-air balloon experiments in the late 1800s, scientists have determined that the atmosphere is made up of five distinct layers. The bottom layer, where clouds exist and virtually all weather occurs, is called the troposphere. As one rises through the troposphere, the temperature drops rapidly. About 9 miles (14 kilometers) above ground is the stratosphere. Jet planes cruise in the stratosphere to take advantage of strong winds found there and to reduce friction experienced with air in the troposphere. The temperature rises gradually from a low of about −75°F (−60°C) at the lowest level of the stratosphere to a high of about 32°F (0°C) at its upper boundary. The rate of temperature increase in the stratosphere rises sharply in the region between about 20 and 30 miles (32 and 48 kilometers). The reason for this change is the presence of a band of ozone in that portion of the stratosphere. Ozone is a form of oxygen that has three atoms per molecule instead of the usual two. It absorbs ultraviolet rays, which have a warming effect.

The ozone layer, which protects earth from the Sun's harmful rays, may be the atmosphere's most famous layer. The reason is that the ozone layer has been damaged by chemical pollutants. The loss of ozone from the stratosphere is a concern because ozone protects life on Earth from serious harm. For example, some forms of skin cancer are caused by exposure to certain kinds of ultraviolet radiation that are absorbed by ozone. Fortunately, governments around the world have now banned most of these dangerous substances, giving the protective shield an opportunity to regenerate.

The region of the atmosphere above the stratosphere is the mesosphere. This belt extends upwards from about 30 to 55 miles (48 to 88 kilometers) above Earth's surface. Within the mesosphere, the temperature falls from about 32°F (0°C) at its lower boundary to nearly −150°F (−100°C) at its upper boundary.

In the next higher zone, called the thermosphere, temperatures rise to about 1,800°F (1,200°C). The thermosphere extends from a height of about 55 miles (88 kilometers) to about 300 miles (483 kilometers) above Earth's surface. The extreme heat in this layer burns up debris, such as meteors and non-operational satellites, falling toward Earth. Many of the molecules in both the upper mesosphere and lower thermosphere become ionized (electrically charged) by x rays and ultraviolet rays in solar radiation. For this reason, that region is also called the ionosphere.

The highest atmospheric layer is the exosphere. Molecules of gas in the exosphere break down into atoms. In addition, because gravitational attraction is so low, many molecules escape into space.

Who's who: Antoine Lavoisier

French chemist Antoine-Laurent Lavoisier (1743–94) is widely considered the father of modern chemistry. Originally trained to be a lawyer, Lavoisier soon discovered his passion in science. In the 1780s, Lavoisier identified the life-giving element present in air as "oxygen." Lavoisier is equally famous for describing what occurs when things burn, for formulating the system of naming chemical compounds, and for improving the accuracy of scientific methods.

Through his experiments, Lavoisier learned that many substances give off carbon dioxide when they burn. He also learned that oxygen has to be present in order for burning to occur. Also, Lavoisier isolated a second element present in large quantities in the air. To that element, originally discovered by the Scottish chemist Daniel Rutherford, he gave the name "azote." Today that element is known as nitrogen.

Air pressure and weather

Air pressure (also known as "barometric pressure" or "atmospheric pressure") is an all-important concept in the world of weather. Air pressure is the pressure exerted by the weight of air over a given amount of Earth's surface. Changes in air pressure produce winds, cause the development of clouds, and clear the way for sunny skies. The air pressure at any given time provides weather forecasters with important clues about what the weather holds for the next several hours or days.

Much of what is known about temperature and wind conditions in the troposphere was collected by hot-air balloonists beginning in the late 1700s. Two of the most famous of these upper-air explorers were Englishmen James Glaisher and Robert Coxwell, who made twenty-eight flights over England between 1862 and 1866.

The highest and riskiest ascent made by Glaisher and Coxwell was in September 1862. At an altitude of 29,500 feet (9,000 meters, or more than 5.5 miles), Glaisher lost consciousness due to the lack of oxygen. The balloon continued to rise, and at 37,500 feet (11,400 meters, or more than 7 miles) Coxwell was on the verge of passing out, too. At the last moment Coxwell managed to guide the balloon into a descent.

Unpiloted hot-air balloons were invented shortly thereafter. These balloons carried instruments to greater heights than humans could ever withstand. Using these balloons, French meteorologist Teisserenc de Bort learned that at about 9 miles (14 kilometers) above ground the air temperature no longer decreases but begins to increase. De Bort had discovered the second atmospheric layer, the stratosphere.

What is air pressure?

Simply put, air pressure is the pressure exerted by the weight of air over an area of Earth's surface. It is a function of the number of molecules of air in a given volume, the speed with which they are moving, and the frequency with which they collide. Although they are too small to see, air molecules are always in motion at tremendous speeds. In fact, at ground level, there are 400 sextillion (400 plus twenty-one zeroes) air molecules per cubic inch. They are moving at an average speed of 1,090 mph (1,753 kph).

Who's who: John Dalton

English chemist and Quaker John Dalton (1766–1844) chose to explore the simplest unit of matter: the atom. In the early 1800s, Dalton postulated that all forms of matter, in all three phases (solid, liquid, and gas) are composed of tiny particles called atoms and that these atoms can combine to form "compound atoms," which are now known as "molecules."

It was Dalton's interest in the weather that led to the development of his atomic theory. For fifty-seven years he kept daily records of temperature, barometric pressure, dew point, rainfall, and other conditions. He contemplated the nature of air and concluded that it, like every solid, liquid, or gas, is made up of tiny particles.

Using his weather observations in combination with his atomic theory of air, Dalton learned how condensation occurs. First, he demonstrated that water vapor is a gas and can mix with other gases in the air. Then he proved that the amount of water that air can hold (the saturation point) depends upon the temperature of the air. From there he extrapolated that at every temperature, there is a corresponding saturation point. By dividing the amount of water in the air by the amount of water at which air at that temperature would be saturated, he came up with an explanation of relative humidity.

The rapid movement of air molecules means that they frequently collide with one another and any objects they encounter. These collisions are responsible for air pressure. If air is heated, molecules move more quickly, collide more often, and cause an increase in pressure. If air is cooled, molecules move less rapidly, and air pressure decreases.

Air pressure can be altered by adding air to, or removing it from, a closed container, such as a bicycle tire. Each time one drives down the plunger on a bicycle pump, more air molecules are squeezed into the same volume of space. With each movement of the plunger, air pushes out against the inside of the tire, and the tire feels hard. If one were to continue pumping long enough, the air pressure would increase to the point where the tire would explode.

Air pressure changes with altitude

Measurements of air pressure may be given in a variety of units. The unit most commonly used by meteorologists is the millibar (mb). The unit of air pressure in the English system is pounds per square inch (psi). At sea level, air pressure is equal to about 1,000 mb (14.7 psi). Atmospheric pressure decreases with higher altitudes. At about 1,000 feet (300 meters) above sea level, air pressure is about 900 mb (14.1 psi). At 4 miles (6 kilometers) above the ground, the point at which half of the atmosphere's mass is above and half is below, the air pressure is about 500 mb (7.3 psi).

A key reference to: Laws of air pressure

Boyle's Law was first published in 1660 by British chemist Robert Boyle (1627–91). The law states that at a constant temperature, the volume occupied by a gas is inversely proportional to the pressure applied to the gas. For example, when the pressure applied to a given volume of air is doubled, the air shrinks to half its volume. In other words, air under pressure becomes compressed. Boyle is also noted for his invention of two of the earliest types of barometer: the water barometer and the siphon barometer.

The companion to Boyle's Law is Charles's Law (also called Gay-Lussac's Law). This 1802 finding by French physicist Jacques Alexandre César Charles (1746–1823) and French chemist Joseph-Louis Gay-Lussac (1778–1850) states that at a constant pressure, the volume of a gas is proportional to the temperature of the gas. This means that as heat is applied to a sample of air, the air expands, and as heat is taken away, the air contracts. Gay-Lussac, incidentally, set a record for height in a hot-air balloon flight in 1804, of over 4 miles (6 kilometers) above ground. His record remained unbroken for the next fifty years.

Air molecules are constantly bombarding us from all directions, exerting a constant pressure of about 1,000 mb (14.7 psi) at sea level. People don't feel anything hitting them because the air pressure inside the body balances that outside it. The only way people notice air pressure is if it changes rapidly, such as when they ascend or descend in an airplane, or drive on a steep mountain road. In those cases, the air pressure around them changes more quickly than does the air pressure in their ears and sinuses.

Anyone who has flown on an airplane has experienced the "popping" of his or her ears during take-off and landing. This "popping" is the body's attempt to equalize the pressure imbalance by releasing air from the eustachian tube (the passage connecting the eardrum and the throat).

High-pressure and low-pressure systems

Altitude is not the only factor associated with differences in air pressure. Air pressure also differs from location to location on the ground and even from one hour to the next at a single location. It is these changes at ground level that are connected with weather patterns. Even a very small difference in air pressure between two points can signal profound changes in the weather.

Television weather forecasters regularly refer to systems of high pressure and low pressure. There is no set definition of a "high" or "low" pressure system: they are only defined relative to one another. For example, if one area has an air pressure of 1,000 mb (14.5 psi) and a second area has an air pressure of 975 mb (14.3 psi), the former is considered a "high-pressure" area and the latter, a "low-pressure" area.

High- and low-pressure systems are the result of multiple air masses (which are large quantities of air consistent throughout in temperature and moisture) of different temperature and moisture content entering and leaving an area. In the middle latitudes, between 30° and 60°, which includes the United States, this parade of air masses is nearly constant. As one air mass is replaced by another, the air pressure rises or falls, and the weather changes.

High-pressure systems are usually associated with clear skies and low-pressure systems with clouds and precipitation, or water droplets that originate in the atmosphere and fall to the ground. These are only generalities and, due to the interaction of other factors in the atmosphere, do not always hold true. To understand how pressure systems affect the weather, it is necessary to combine the concepts of convection and air pressure.

Who's who: Blaise Pascal

French mathematician and philosopher Blaise Pascal was the first person to explain the connection between air pressure and altitude. He hypothesized that the weight of the atmosphere above Earth's surface is responsible for air pressure at the surface and, by extension, that air pressure decreases as elevation increases.

To test his hypothesis, Pascal conducted an experiment using the newly invented barometer. He took barometer readings at the base and the peak of a mountain. He found that he was correct. The air pressure measured 935 mb at ground level and only 828 mb at the mountaintop.

As air is heated, it rises. It leaves in its wake an area of low pressure. One consequence of rising air (and hence a low-pressure area) is that it causes water vapor in the air to condense and form clouds.

Clouds form over low-pressure areas because cool air can hold less water than warm air. The warm air carries water vapor upward until it reaches the dew point. This is the temperature at which air can no longer hold water in the vapor state, and the water begins to condense into clouds.

In a region of high pressure, colder, drier air from above sinks toward the surface. The cold air becomes warmer as it falls. This causes water and ice in the clouds to dissolve and the clouds themselves to thin or evaporate entirely, leaving only clear, sunny skies.

Wind: Air in motion

Wind is the natural movement of air. Winds are produced and acted upon by numerous forces. Among these are air pressure differences, Earth's rotation, and friction. Essentially, air attempts to flow from areas of high pressure to areas of low pressure but is prevented from traveling along such a path directly because of Earth's rotation. Wind speed, although largely a function of pressure differences, is also influenced by Earth's surface features.

Pressure changes produce winds

The flow of air across a pressure differential is a crucial factor to understanding wind. A pressure differential or pressure gradient is the difference in atmospheric pressure at any two given locations. The movement of air from a high-pressure to a low-pressure area is the atmosphere's attempt to equalize differences in pressure. When the pressure between the two areas is equalized, the wind stops blowing.

Two main factors determine how fast the wind moves: the difference in air pressure and the distance between two areas. Either a greater pressure differential or a smaller distance between the areas makes for a stronger wind. Conversely, either a smaller pressure differential or a greater distance between the two areas makes for a weaker wind.

These two factors taken together are called the pressure gradient force (PGF). To illustrate, in the case of a pressure gradient of 10 mb between two locations set 1,000 kilometers apart, the pressure gradient force is 10mb/1,000km, or 0.01mb/km. On the other hand, a pressure difference of 40 mb at the same distance apart would produce a pressure gradient of 40mb/1,000km = 0.04mb/km. A pressure gradient four times as large would produce a wind speed four times as great in the same time.

Friction slows the winds

Topography (the physical features of land) doesn't produce winds, but it does affect wind speed. As wind blows across a pressure differential, it encounters hills, trees, tall buildings, sand dunes, and other objects that create friction and slow it down. Relatively flat surfaces—such as water, prairies, and deserts—exert little friction on the wind. Over flat terrain, winds reach greater speeds than they do over hilly terrain. Farmers know that planting a row of trees on otherwise flat land goes a long way toward preventing soil erosion caused by strong winds.

Earth's rotation curves the winds

Imagine a wind blowing from north to south because of a pressure differential between two areas. If the space between the two areas were perfectly flat, one might expect the wind to blow in a perfectly north-south direction. But such is not the case. Instead, the wind is diverted slightly because of an effect known as the Coriolis effect. The effect is named for the French scientist Gustave-Gaspard de Coriolis. De Coriolis used mathematical formulas to explain that the path of any object set in motion above a rotating surface will curve in relation to any object on that surface.

To an observer beyond one of Earth's poles, say on a space shuttle, the wind would not appear to curve—it would blow in a straight line while Earth spun beneath it. But relative to observers on Earth—that is to say, all of us—the wind does appear to curve. One way to understand the Coriolis effect is to think of a person riding on a carousel who throws a ball straight up into the air. When the ball comes down, it lands behind the person who threw it. To the person on the carousel, it seems that the ball's path has curved backwards. However, to a person standing next to the carousel the ball appears to have traveled in a straight vertical path while the carousel rotated beneath it. To relate this example to Earth, then humans are all on the carousel, and the wind appears to curve as it travels.

The Coriolis effect influences the direction of winds as follows: In the Northern Hemisphere it curves them to the right. In the Southern Hemisphere it curves them to the left. The Coriolis effect is felt most strongly at the poles. It does not exist at all at the equator, where opposing forces (the turn to the right and the turn to the left) are canceled out.

Putting it all together

The forces of pressure gradient and the Coriolis effect create complex global wind patterns. In the Northern Hemisphere, winds spiral clockwise around high-pressure systems (where air is falling) and counterclockwise around low-pressure systems (where air is rising). In the Southern Hemisphere, the opposite is true. What causes the wind to move like this?

The spiral pattern represents the path of equilibrium between opposing forces. Imagine a hot-air balloon that's being carried along by the wind. [Note that this example applies only to the Northern Hemisphere. The Coriolis effect works in reverse in the Southern Hemisphere.] At the start of its journey, the balloon is pushed away from a high-pressure system. It moves into a low-pressure system, which is characterized by rising, warm air. Yet, rather than following a straight line into the center of the system, the balloon is pushed to the right by the Coriolis effect.

It is now caught in a tug-of-war between forces pushing it toward the low-pressure system and those pushing it to the right. The balloon finds and settles into a pattern where these two forces are in balance. As it moves from point A to point B to point C and so on, it's simultaneously driven in toward the low-pressure system and to the right. Connecting the points of equilibrium between these forces is a circular path, running counterclockwise around the low pressure area.

Now consider the opposite case, where the balloon is swept into the descending, cold air of a high-pressure system. This time, the balloon is simultaneously being pushed away by the high-pressure system and being tugged to the right by the Coriolis effect. The balloon travels to the point where these two forces are in balance. The path of equilibrium around a high-pressure system runs clockwise.

One more factor is necessary to complete the description of how the wind travels: friction. Friction causes wind near the ground to behave differently than wind at higher altitudes. The reason is that winds near the ground are slowed down, lessening the Coriolis effect. In fact, for wind blowing toward a low-pressure system just above the ground, the Coriolis effect is so weak that the wind blows right into the low-pressure area. Wind blowing toward that same system in the upper air, due to the Coriolis effect (unimpeded by friction), would circle around the system, as described above.

Global wind patterns

The Sun heats Earth unevenly and the atmosphere strives to even out heat distribution. (Winds are responsible for about two-thirds of the world's heat distribution and ocean currents, the major routes through which ocean waters move, for about one-third of the burden.) In general, winds move between the equator and the poles, bringing warm air to cold areas and cold air to warm areas. Global wind patterns are made more complex by a number of factors, such as Earth's rotation and the location of land and sea. The result is a complex pattern of swirling winds encircling the globe. These wind patterns are what create the variety of weather conditions at specific regions north and south of the equator.

The global motion of the winds begins with the flow of warm air from the equator to the poles. The air doesn't travel all the way to the poles in one interrupted journey, however. It travels through a series of loops in which warm air rises and cold air falls at different latitudes. It is also important to note that, due to the Coriolis effect, these winds do not travel due north or south, but between points southwest and northeast in the Northern Hemisphere and between points northwest and southeast in the Southern Hemisphere. Remember that the Coriolis effect acts differently in the Northern and Southern hemispheres. In the following discussion, examples are given only for the Northern Hemisphere. It can be assumed that the opposite is true in each example for the Southern Hemisphere.

Hadley cells

The first of these loops, which extend from the equator to 30° north and south latitudes, are called Hadley cells. They are named for George Hadley, an English scientist who first explained this air flow pattern in 1753. The air that flows through the Hadley cells begins at the equatorial region (from about 10° south latitude to 10° north latitude), also known as the tropics. This area is the warmest region on Earth, because sunlight hits the surface most directly. The air is warmed and rises by the process of convection. The upward movement of air creates a low-pressure zone, which produces the clouds and rains for which the tropics are famous.

Warm air continues rising to the top of the troposphere, cooling as it goes. Then it begins to spread out toward the poles. At approximately 30° north and south latitudes the air sinks to the surface, warming as it descends. The latitudes at which air rises and falls, marking the boundaries of the cells, are only approximations and shift throughout the year.

High-pressure systems are created in these regions, meaning that skies are generally clear and little precipitation occurs. It stands to reason that 30° north and south are the latitudes at which most of the world's deserts are located.

Air descending at these latitudes displaces air at the surface. Most of the displaced air moves back toward the low-pressure belt at the equatorial zone, forming the trade winds. Northern- and Southern-hemisphere trade winds meet at the heat equator, the warmest part of the equatorial zone. The location of the heat equator is generally north of the geographic equator, due to the greater mass of land in the Northern Hemisphere, and it shifts north to south with the changing seasons.

Where the trade winds meet, they form a broad band of light, variable east-west winds. This area, which is generally cloudy and rainy, is called the doldrums. Another name for this region is the intertropical convergence zone.

Ferrel cells

Ferrel cells encompass the next wind cycle, in which equatorial air moves one step closer to the poles. These cells are named for American meteorologist William Ferrel, who first described them in 1856. The Ferrel cells cover the region of the globe from about 30° to 60° north and south latitude, in other words, the temperate regions.

A key reference to: How the horse latitudes and doldrums got their names

"Horse latitudes" and "doldrums" are colorful terms used to describe two regions of Earth at which the winds are nearly still. The horse latitudes are a high-pressure belt that exists at around 30° north and south latitudes of the equator. It is in this region that air from the upper troposphere descends to Earth's surface, bringing clear skies.

While sunshine does not create a problem for sailors, the lack of wind does. Over time many ships stalled in the horse latitudes. When food ran low, the first to forego feedings were the horses on board. They often were slaughtered to feed the crew or simply thrown overboard. The preponderance of horse corpses floating in the waters throughout this region led to the name "horse latitudes."

The term doldrums, an old English word for "dull," is another name for the intertropical convergence zone. This is the zone near the equator where the trade winds coming from north and south meet and nearly cancel each other out. The warm tropical air, rather than traveling horizontally (and creating wind) rises straight up. So-named by sailors stranded in this part of the world, the doldrums are known for their warm, rainy, and still conditions.

The Ferrel cells begin where the Hadley cells leave off, with the air that falls at 30° latitude. Some of this air, rather than returning to the equator, continues in the direction of the poles. The winds traveling to the poles generally come from the southwest and are curved to the northeast (in the Northern Hemisphere) by the Coriolis effect. For this reason, they are called westerlies. At around 60° north and south latitude the westerlies encounter cold polar air. The points where this occurs are called the polar fronts.

The contrast in temperature between these air masses causes the warmer air to rise. This results in a low-pressure system, bringing clouds and precipitation to regions such as southern Alaska and central Canada. The air that rises forms a circulation pattern called the upper-air westerlies. These winds, which flow from west to east, are responsible for driving most of the weather systems of the middle latitudes. These winds travel in waves that carry warm air toward the poles and cold air toward the equator.

Polar cells

The final leg of the trek bringing warm air from the equator to the poles takes place within the polar cells. These cells extend from the poles, to 60° north and south latitude.

Some of the warmer air (relative to the cold polar air) rising at the sixtieth parallels heads to the poles. It cools drastically along the way. Once this air reaches a pole, it descends, forming a high pressure area. The displaced air at the surface then heads south. These cold winds, which head from the northeast to the southwest across the polar regions, are known as the polar easterlies.

At around the sixtieth parallel the polar easterlies, which have warmed slightly, meet the westerlies (warmer air coming from the thirtieth parallel). The warm air rises and heads back to the pole, completing the polar cell.

Some heat is lost through every cell between the equator to the poles. This means that the atmosphere's attempt to distribute heat across the planet is only partially successful—the poles remain forever colder than the equator.

Global pressure patterns

Air rises and falls at certain latitudes as it makes its way from the equator to the poles and back. On Earth's surface, rising air creates low-pressure areas and falling air creates high-pressure areas. These major pressure areas exist along the boundaries between wind cells. The highs are located around 30° north and south latitude and at the poles, where cold air descends, and the lows are around the equator and 60° north and south, where warm air rises.

It's important to distinguish between the major pressure areas encircling the globe and the minor ones responsible for our day to day weather. The major high- and low-pressure areas, caused by global wind circulation, cover thousands of square miles each and can persist for months or longer. Small, localized high- and low-pressure areas form and die out in a matter of hours or days.

The world's major high- and low-pressure areas under go significant shifts north and south with the seasons. They move to the north when it's summer in the Northern Hemisphere and to the south when it's winter in the Northern Hemisphere. However, four large pressure areas—two high and two low—maintain their basic position throughout the year. These systems are called semipermanent highs and lows. They are called "semipermanent" because they undergo changes in strength, as well as slight shifts in position, throughout the year.

The semipermanent systems are all located in the Northern Hemisphere. The Southern Hemisphere has far less land mass than the Northern Hemisphere overall, and has virtually no land between 50° latitude and Antarctica. It is the contrast in temperature of land and sea that results in changes in air pressure. Thus, the Southern Hemisphere has a fairly continuous low-pressure belt running across the globe at around 60° latitude. In contrast, the Northern Hemisphere, due to the positions of land masses, has areas of great temperature contrast at this latitude. In the subtropical region, at around 30° south latitude, the Southern Hemisphere has a series of well-defined but shifting high-pressure areas.

The semipermanent highs and lows are called the Pacific High, the Azores-Bermuda High, the Aleutian Low, and the Icelandic Low. In general, the lows produce storms, and the highs influence the direction in which the storms travel.

The Azores-Bermuda High strongly affects North American weather, especially the states bordering the Gulf of Mexico and the Atlantic coast. It occupies a huge area in the east Atlantic Ocean, between the eastern coast of North America and the western coast of Europe. It changes in size throughout the year. When the pressure system is large, it strongly repels all storms that come its way. Even the strongest hurricanes, generated in the band of warm waters running from the south of Florida eastward to northern Africa, are bent around this system. As a hurricane heads north, it encounters the large high-pressure system and is forced westward through the Caribbean and toward the eastern seaboard of the United States.

Upper-air winds

The winds that blow in the middle and upper levels of the troposphere, also known as winds aloft take on a different pattern than the three main types of surface winds that blow between the equator to the poles—trade winds, westerlies, and polar easterlies—and the major pressure systems that generate and steer surface winds.

In general, the winds aloft run in the opposite direction from surface winds. For example, the surface trade winds blow toward the equator from the northeast to the southwest (in the Northern Hemisphere) and the upper-air trade winds blow back to the subtropics (30° latitude) from southwest to northeast.

Upper-air westerlies

In the middle latitudes, however, the winds circulate in a different pattern: they move in wavelike patterns from west to east. Where these waves crest to the north they form ridges, and when they dip to the south they form troughs. It is essential to understand the flow of upper-air westerlies since they have a very significant impact on the weather of the United States and Canada.

These waves, through a series of ridges and troughs, cycle warm air masses northward and cold air masses southward. From the base of a trough, the southerly winds blow warm air masses northward. From the top of a ridge, northerly winds blow cold air masses southward. The net result is that upper-air westerlies transfer heat toward the poles and cold air toward the equator. In addition to cycling warm and cold air, upper-air westerlies transport high- and low-pressure systems from west to east. These pressure systems exist within the ridges and troughs.

Large-scale waves in the atmosphere are easy to observe as large-scale meanders of the jet stream, which is discussed in the next section. When these loops become very pronounced, they may detach as masses of cold or warm air and become cyclones or anticyclones. These air masses are primarily responsible for day-to-day weather patterns at mid-latitudes.

Jet streams

The world's fastest upper-air winds are embedded within westerlies as well as within subtropical winds aloft. They are called jet streams. Jet streams are narrow bands of wind that blow through the top of the troposphere in a west to east direction at an average speed of about 60 mph (97 kph). The fastest moving jet streams greatly exceed that value, however, and have been clocked at more than 280 mph (450 kph).

Jet streams occur in regions with the largest differences in air temperature and pressure at high altitudes. In the middle latitudes of each hemisphere, this region occurs over the polar front, where the mild westerlies meet the cold polar easterlies. In the subtropical latitudes of each hemisphere, this region occurs around 30° north and south, where the warm trade winds meet the westerlies.

In either hemisphere, jet streams tend to move faster during winter than during summer. The reason for this pattern is that a larger temperature differential exists in winter. For example, in winter it may be 32°F (0°C) in Michigan and 80°F (27°C) in Florida, a difference of almost 50°F (10°C). On a typical day in summer in contrast, it can be about 80°F (27°C) in Michigan and 100°F (38°C) in Florida, a difference of only 20°F (11°C).

In addition, the latitudes at which jet streams travel shift throughout the year. In winter they are closer to the equator and in summer, closer to the poles. The reason for this pattern is that during the Northern Hemisphere's winter, cold polar air is swept further south. In the process, the cold air/warm air boundary also moves farther south. Conversely, in the summer, mild air is swept farther north and the cold air/warm air boundary moves northward. Note that these northward and southward shifts in cold air/warm air boundaries occur simultaneously in the Southern Hemisphere.

Jet streams are not the only phenomenon to inhabit the boundary between warm air and cold air. Storms also occur in this region. For this reason, weather forecasters consider the path of the jet stream a useful tool in predicting where storms will occur. Jet streams are also reliable indicators of temperature changes. When a jet stream dips southward, it brings cold air with it. When a jet stream shifts to the north, it brings warmer air in its wake.

Convergence and divergence

Within a jet stream, winds regularly shift in direction and speed. They alternate between north and south as they pass through the stream's ridges and troughs. They speed up or slow down as they pass in and out of the jet maximum, the fastest region within the jet stream. Any change in wind speed or direction causes air to either pile up or spread out. In the former case, when air moves inward toward a central point, it is called convergence. In the latter case, when air moves outward from a central point, it is called divergence.

Convergence and divergence are perhaps easier to understand in this example involving traffic patterns. Convergence is when a stream of cars enters an already crowded freeway, causing a slowdown. Divergence is what occurs when a two-lane highway expands to a four-lane highway. Cars spread out between all four lanes and traffic speeds up.

When winds converge at high altitudes, they diverge at the surface. When winds diverge at high altitudes, they converge at the surface. This is because convergence and divergence affect air pressure over a vertical gradient. Specifically, convergence raises air pressure and divergence lowers air pressure. Thus, when winds diverge aloft, lowering the pressure, surface winds converge to the point beneath the divergence. The surface winds then rush upward to the low-pressure area. The surface winds stop rising when the pressure between points above and below has been equalized.

To speak of the factors that produce large-scale weather patterns in the middle latitudes, the story starts with convergence and divergence aloft. With the upper-air winds (westerlies and the jet stream), areas of convergence and divergence coincide with ridges and troughs. Winds tend to strengthen as they curve clockwise in ridges and weaken as they curve counterclockwise in troughs. Thus, as winds approach a trough they decrease in speed, converging to the west of a trough. As winds pass through the trough and head into a ridge, they curve clockwise and pick up speed. This situation results in divergence just before the winds enter to the west of a ridge.

Let's look at the case of convergence aloft. In the upper air, winds blow toward a central point and pressure builds. The air can't keep on piling up indefinitely, so it looks for an escape. If the air is already at the top of the troposphere it can rise no farther. Thus, it is forced to travel downward to an area where pressure is lower.

This downward travel of the air creates an area of high pressure on the surface, from which winds flow out in a clockwise direction. In other words, an anticyclone is created. The air from above will continue to descend, strengthening the anticyclone, until the pressure at the surface equals the pressure aloft. The air from above warms as it descends, causing the water vapor within it to evaporate. Thus, an anticyclone is associated with clear, settled weather conditions.

Now take the opposite case, where winds aloft are diverging. The air below travels upward toward this area of low pressure, leaving a surface area of low pressure in its wake. A convergence forms on the surface, where air rushes counterclockwise into the center of this surface low and rises. As the rising air cools, it forms clouds and precipitation.

This system is called a cyclone, also known as a storm. As long as the divergence aloft is stronger than the surface convergence, air will be pulled upward and the cyclone strengthened. Once these two forces come into balance, the storm dies out.

Air masses

An air mass is one of the few stable elements in the ever-changing world of weather. An air mass is a large quantity of air where temperature and moisture content is fairly consistent throughout. Air masses commonly cover thousands of square miles, the size of several states. Air masses are produced by the heating or cooling effect of the land or water beneath them.

Air masses form primarily over polar and tropical regions. Since the air does not stay still for long over temperate latitudes (including the United States), air masses generally do not form over those regions.

When a single air mass remains over a region for an extended period, it produces conditions called air mass weather, a period of unchanging weather conditions. This pattern occurs in various parts of the United States depending on the time of year. For example, the Southeast can count on hot weather and daily afternoon thunderstorms in the summer, and the Pacific Northwest is treated to cold, rainy weather for long periods in the winter.

Each air mass is given a two letter classification. The first letter refers to the air mass's point of origin, and the second tells whether it has traveled over land or sea. The second factor is crucial to determining the mass's moisture content. The first letter of the designation may be "c" for continental, meaning that it has traveled over land and is dry or "m" for maritime, meaning that it has traveled over oceans and seas, and is moist. The second letter of an air mass's identifier tag may be "P" (polar), "A" (arctic) or "T" (tropical). Some meteorologists do not use a separate designation for arctic air, since arctic air that travels southward becomes warmer and is virtually indistinguishable from polar air.

The combination of the two letters of a designation describes the temperature and moisture content of an air mass. For instance, the label mT refers to a warm, moist air mass that formed in the tropics and traveled over water.

Continental arctic air mass (cA)

This frigid air originating near the North Pole greatly affects the weather of Canada and, to a far lesser degree, the weather of the northern United States. It forms above Greenland, Siberia, northern Alaska, northern Canada, and islands in the Arctic Ocean. The temperature of this dry air can dip as low as −45°F (−43°C) in the winter. For the most part it produces cold, dry conditions. Occasionally it picks up moisture while crossing a body of water and brings snowy weather.

Maritime arctic air mass (mA)

This air mass is largely responsible for the cold weather experienced by western Europe. It brings low temperatures in the summer and very low temperatures in the winter. It forms over the ice-covered Arctic region and travels over large bodies of water (such as the northern Atlantic Ocean and Greenland Sea), which warm it somewhat and cause it to absorb more moisture before reaching Europe. This air mass brings rain in the summer and snow in the winter.

Continental polar air mass (cP)

This type of air mass forms over land in Alaska, northwestern Canada, northern Europe, and Siberia. A cP air mass begins as very cold, dry, stable air and picks up heat and a small amount of moisture (mostly from the Great Lakes) as it travels south over warmer ground. A cP air mass that starts out with a temperature between −40 and −34°F (−40 and −37°C) over Alaska, may warm up to between 20 and 23°F (−7 and −5°C) by the time it reaches Florida.

A cP air mass exhibits greatly different qualities in summer than in winter. In the winter it consists of very cold, dry air, almost as cold as arctic air. It brings low temperatures and clear skies to the north and central United States, and even dips into the southern states on occasion. Its southernmost penetration occurs when much of the United States is covered by snow. Snow reflects incoming sunlight and keeps the ground and air above it colder. In the summer, a cP air mass starts out cool and warms considerably as it travels south.

Maritime polar air mass (mP)

Siberia and the northern Pacific and Atlantic oceans are the points of origin of mP air masses. These air masses start out cold, but not as cold as their continental counterparts. Usually their temperatures hover just above freezing. As they travel south over warmer waters, they become warmer and wetter.

The regions affected most by these air masses are western Europe, southern Australia, New Zealand, and the east and west coasts of North America. Maritime polar air masses bring snow and rain in winter and fog and drizzle in summer. The mP air mass that travels over the Atlantic Ocean brings about the days of dreary weather the East Coast experiences in spring and early summer.

Continental tropical air mass (cT)

These air masses build up over desert regions and are the world's hottest. A mass of continental tropical air tends to hover where it forms but sometimes moves away. If it hovers over another region for any length of time, it can bring about a drought. As an example, a cT mass may form over the deserts of the southwest United States and then travel to the plains states, where it remains for weeks. A cT air mass picks up moisture as it crosses over lakes and rivers, making it cooler and more humid.

Maritime tropical air mass (mT)

An mT air mass consists of warm, moist air and forms over tropical and subtropical waters. The mT air masses that invade the eastern United States begin over the Gulf of Mexico, the Caribbean Sea, and the Atlantic Ocean. Those that affect the western United States form over the Pacific Ocean, from Mexico to Hawaii.

In the summer, mT air masses bring very warm, humid air and rain. Along the Gulf Coast they bring daily thunderstorms. In the winter, mT air is usually prevented by a wall of cold polar air from reaching all but the extreme south of the United States. On the rare occasion when mT air is drawn northward, it brings unseasonably mild weather and clouds. It also brings the rains that wash away the snow, disappointing skiers.

Fronts: Where the action is

Air masses are transported around the globe by winds. As one air mass blows into a region, it encounters an air mass that is already there. What occurs next is often described as a battle, since in order to advance, one air mass must push the other out of the way.

The line where two air masses meet is called a front. A front is so named because of its similarity to the line where two armies meet in battle. Fronts can be moving or stationary. When they are moving, it means that one air mass is gaining ground, while another is losing ground. In contrast, a stationary front marks a region of stability between the air masses.

As a front passes through an area, it produces weather conditions ranging from gentle winds and light rains to violent storms. When a mass of cold air displaces a mass of warm air, it is said that a cold front has come through. Conversely, when a cold air mass is displaced by a warm air mass, a warm front has come through.

The interaction of warm and cold air masses is most common in the middle latitudes, in other words, the temperate regions. Included among these regions are the continental United States and southern Canada in the Northern Hemisphere and southern Australia in the Southern Hemisphere. The mixing of air with great differences in temperature and pressure produces storms. It is the temperate regions, therefore, that experience some of the world's most violent weather.

Warm fronts

A warm front is the leading edge of a mass of warm air that overtakes a mass of colder air. The warm air, being less dense, slips over the cold air, which hovers close to the ground. The warm air cools as it rises and the water vapor within it condenses. This results in the formation of wispy clouds at high altitudes.

A warm front has a very gradual slope. In fact, when the leading edge of the air mass is 3,000 feet (914 meters) above a particular point, the base can still be more than 100 miles (161 kilometers) away. As the middle level and base of the warm front come in contact with the colder air mass, the warmer air cools and water vapor condenses. This forms layers of clouds at middle and lower altitudes.

As a warm front approaches an area, the air pressure decreases and precipitation begins, sometimes lasting for several days. Fairly strong winds may accompany this precipitation for a day or so when the base of the front sweeps past an area. Once the front passes through, skies generally clear up and temperatures rise.

Cold fronts

Cold fronts are more closely associated with violent weather than are warm fronts. When a cold front moves into an area, the cold air (being denser than the existing warm air) wedges underneath the warm air and forces it sharply upward. This occurs because a cold front is very steep. At a distance of only about 30 miles (48 kilometers) behind the leading edge of the air mass, the cold air may reach 3,000 feet (914 meters) above ground. The precise steepness of cold front depends on the speed at which it's moving.

The warm air that is forced upward by the cold air mass produces tall clouds, and sometimes cumulonimbus (thunderstorm) clouds. These clouds, in turn, bring rain and possibly thunderstorms. The storms are accompanied by strong winds, produced by a drop in pressure created by the rising warm air. Where a cold front advances rapidly, fierce thunderstorms develop in a band called a squall line.

Cold fronts tend to pass through an area very quickly, so their effects are harsh but short-lived. The drop in temperature cold fronts bring is variable. Temperatures may drop just a few degrees to more than 35°F (10°C). In the summer, a cold front may simply bring drier air into a region, lowering the humidity considerably while barely affecting the temperature. The cold front leaves in its wake a band of clouds that produce rains, though of lesser intensity than the initial storms.

Stationary fronts

A stationary front represents a standoff between two air masses. It occurs when one air mass pushes against a second air mass, and neither side budges.

If both air masses are relatively dry, then clear to partly cloudy conditions prevail at the front. However, if the warmer air is moist, then some of this air rises above the cold air and forms clouds and, possibly, precipitation.

When one air mass begins to move over or under the other air mass, the front ceases to be stationary. It becomes either a cold front or a warm front, depending on which air mass is advancing and which is retreating.

Occluded fronts

The final possible outcome of a meeting between air masses is called an occluded front (or "occlusion"). An occluded front is formed by the interaction of three air masses: one cold, one cool, and one warm. The result is a multitiered air system, with cold air wedged on the bottom, cool air resting partially on top of the cold air, and warm air pushed up above both colder air masses.

There are two types of occluded front: cold and warm. Cold occlusions, which are much more common than warm ones, occur when a fast-moving cold front overtakes a slower-moving warm front. The cold front thrusts the warm air upward and continues to advance until it encounters cool air. The leading edge of the cold front then noses under the cool air, forcing it upward also.

When a cold occluded front is approaching (but before it reaches an area), the weather is similar to that when a warm front passes through. Clouds form in the upper, middle, and lower layers of the troposphere and there is precipitation. As the front passes overhead, however, it is accompanied by stormy weather and a sharp drop in temperature, similar to that associated with a cold front.

A warm occlusion forms under similar conditions as a cold occlusion, except that the cool air mass is the one advancing on the warm air. It pushes the warm air up and runs into cold air. The advancing cool front is then pushed upward, above the cold front. As a warm occluded front passes through an area, it produces weather conditions similar to those produced by an advancing warm front.

What is a storm?

The word "storm" has come to represent many different types of weather phenomena. It is most often associated with unsettled weather conditions, such as heavy rain, thunderstorms, and snowstorms. Storms thus defined can be severe, causing floods, damaging homes, and even causing injury or death; or they can be mild, bringing rain or snow but causing little or no damage.

Storms are large-scale weather systems centered around an area of low atmospheric pressure, drawing in contrasting warm and cold fronts. They produce wind, clouds, precipitation, and the types of unsettled weather listed above, and cover hundreds to thousands of square kilometers. In a global sense, storms are a major mechanism of air circulation, pushing cold air southward and warm air northward.

Another word for a large-scale storm is a cyclone. A cyclone is a weather system in which winds spiral counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere, around a low-pressure area. The technical name for the kind of storm system that sweeps through the middle latitudes is an extratropical cyclone (or "midlatitude cyclone"). This term literally means a cyclone that is formed outside of the tropics. It differs from a tropical cyclone, one formed in the tropics, in that tropical cyclones are storms that don't involve fronts. While tornadoes are sometimes called cyclones, it should be noted that a tornado is one particular kind of cyclone.

Conditions ripe for storms

The formation and sustenance of an extratropical cyclone requires vast amounts of energy. This energy is generated by the contrast between cold air and warm air. Temperature contrasts are particularly strong along fronts, and that is where cyclones are generally found. Occasionally, extratropical cyclones are formed in the absence of fronts. In such cases, contrasts in air temperature within a single air mass are produced as the air mass travels across warm and cold surfaces and is heated unevenly.

The birth of an extratropical cyclone also depends on conditions in the upper atmosphere. Specifically, an area of horizontal divergence is required. The divergence of winds aloft reduces the pressure at the top of a vertical column of air. Air from below ascends to this low-pressure area aloft, creating a surface area of low pressure. This, in turn, results in the convergence of both cold air and warm air at the center of the surface low. The contrasting air temperatures enhance the pressure gradient, causing the winds to blow faster. The process of cyclogenesis has begun.

Cyclogenesis: The birth of a cyclone

As air converges upon the center of the surface low-pressure area, warm air rises over cold air. The warm air cools as it rises, and the vapor within it begins to condense and form clouds. With the transformation of water vapor into liquid water comes a release of latent heat, the energy that is either absorbed or released by a substance as it undergoes a phase change, which also provides energy to the storm system.

The heavy, cold air then slides beneath the rising warm air and noses farther into the warm front. As a result, it pushes more warm air upward. The winds spiral in a counterclockwise fashion (in the Northern Hemisphere) and the fronts rotate. The greater the contrast between temperatures of the fronts, the greater the pressure differential. The greater the pressure differential between the center of the storm and the surrounding air, the faster the winds blow.

Another force that affects the cyclone's wind speed is the conservation of angular momentum. This scientific law states that as the radius of a spinning object decreases, its speed increases, and as its radius increases, its speed decreases. For example, think of a figure skater spinning on the ice. When she places her arms straight over her head, she spins faster. When she stretches her arms out to her sides, she spins more slowly.

Similarly, the speed at which a cyclone turns is related to how tightly the winds are wrapped about its center. As winds blow into the center of the low-pressure system, they spiral more and more tightly. However, if the storm center is forced to expand, the winds spin more slowly and the storm loses intensity.

Tracing the path of a storm

The paths that storms follow shift throughout the year, as the boundaries between cold air and warm air shift. As warm air covers more of the Northern Hemisphere in summer, this boundary shifts to the north, through Canada and the northern United States. As cold air makes its way southward in winter, the boundary shifts accordingly, running through the central and southern states. Storms tend to follow these boundaries, since they feed on contrasting warm and cold air.

Weather conditions along a storm's path

A storm generally involves three air masses: one cold, one warm, and one cool. This is the pattern one finds in the formation of an occluded front, a cold air mass that overtakes a warm air mass. As air masses move from west to east, the cold front is to the west and nudges along the warm air, which, in turn, butts up against the cool air to the east. The air masses cover huge north-south areas, often running from the northern edge to the southern edge of the United States. Thus, the same storm system can affect the weather across the entire country.

Weather conditions look quite different on either side of the storm. Locations ahead of (to the east of) the storm experience a high layer of thin clouds that grow thicker as the warm front approaches. For locations to the west, where the cold front has already passed through and the storm is over, clear skies and chilly air remain.

Now let's look at conditions for locations where the storm is overhead. As the steep cold front advances, it forces the warm air up sharply. This powerful convection produces tall clouds that often give rise to thunderstorms and possibly even tornadoes, all along the cold front. At the same time, 625 miles (1,006 kilometers) to the east, the warm front passes through. The gentle slope of the warm front noses upward, over the cool air. This produces clouds and light rain. Between the cold and the warm front is a pocket of warm air. That area experiences warm temperatures and hazy or clear skies.

As the whole system moves eastward over the next few days, the cold front outpaces the warm front. As a result, the pocket of warm air between the cold and cool air grows smaller and smaller. When the cold air finally meets the cool air, the warm air is forced completely off the surface. An occluded front is thus formed, and the storm begins to die out. Rain and clouds at the occluded front can persist for days.

Anticyclones

An anticyclone, or high-pressure area, typically follows on the trail of a cyclone. The anticyclone strengthens the cyclone by providing a contrast in pressure with the cyclone's low. It is also brings about the calm, clear weather seen once a storm passes.

Anticyclones are the opposite of cyclones in every respect. They are centers of high pressure from which winds flow outward in a clockwise (in the Northern Hemisphere) pattern. Anticyclones form when there is a convergence of air above. That air descends, forming a high-pressure area on the surface, from which winds diverge.

Because clouds and water droplets in the air evaporate, or change from liquid to gas, as the descending air warms, anticyclones usually bring about clear skies. While cyclones are associated with competing fronts, anticyclones favor the formation and sustenance of a single, uniform air mass.

Water in the air

Water plays an important role in the creation of all weather conditions. Some concentration of water always exists in the air, even on sunny days. When that concentration is so great that the air can hold no more water vapor, the water begins to condense (enter the liquid phase). Condensation, the process of becoming liquid, may take the form of clouds, fog, dew, or frost. When water (or ice) in the clouds aggregates into units that are large enough, it falls to the ground as rain or snow.

How water becomes a gas

Water molecules are always in motion. The speed with which they move is a function of temperature. That is, a function of their average kinetic energy. When they possess very little heat, water molecules are nearly still. The molecules are drawn together by their opposing electrical charges into the hexagonal (six-sided) crystalline configuration of ice. Except for slight vibrations, frozen molecules cease to move.

As heat is added to the ice, the molecules begin to move more rapidly. With the addition of enough heat, the molecules move fast enough to break the bonds of the ice structure. The result is that ice melts and becomes liquid water.

The molecules in liquid water are still connected to one another, although not in the rigid configuration of ice. They remain linked because there is an electrical attraction between oxygen atoms and hydrogen atoms in different molecules.

With the addition of more heat, the molecules in liquid water move even faster. The molecules at the surface eventually move fast enough to overcome the electrical attractions connecting them to other molecules. Those molecules break free, leaving the liquid water and entering the air as a gas.

The process by which water changes from a liquid to a gas is called evaporation. Evaporation occurs naturally from any body of water. For example, water evaporates from a lake when the lake is heated by the Sun. When water molecules enter the gaseous phase, they retain the heat they absorbed in the liquid, which is the heat they require to break free. Thus, evaporation takes heat away from water and adds it to the air.

Absolute humidity

Absolute humidity is a measure of the amount of water vapor in the air. It is expressed as the mass of water per unit volume of air. For instance, the absolute humidity on a given day may be 0.5 cubic inches of water per cubic yard of air. There is a limit as to how much water vapor can exist within a given volume of air at any given temperature. That limit is called the saturation point. For example, if you fill a cup of water halfway and seal the top with plastic wrap, the air in the top half of the cup will soon reach the saturation point. After that, water molecules will evaporate from the water's surface and condense back into it, at the same rate.

The saturation point is a function of air temperature. The warmer the air, the more water it can hold, and the higher the saturation point. For example, at about 50°F (10°C), a cubic yard of air can hold about 0.5 cubic inches of water vapor. At about 75°F (about 23°C), the same parcel of air can hold an entire cubic inch (17 grams) of water. At 100°F (37°C), a cubic yard of air can hold 2 cubic inches (50 grams) of water.

A key reference to: Why you feel cooler after a shower

Have you ever wondered why your skin feels cool after showering or taking a swim? Before you dry off you may find your skin covered with goose bumps, even when it's hot outside. The goose bumps are caused by the evaporation of water from your skin.

The process of evaporation requires energy called latent heat. This type of heat does not raise the water's temperature but makes possible the conversion of water from one phase to another. When water evaporates from your body, it takes away latent heat, which makes your skin feel cooler.

Another factor affecting saturation is air flow. On a still day, air remains in place and becomes saturated relatively quickly. However, on a windy day, air reaches the saturation point more slowly. When there is wind, the humid air is blown away, and the water vapor goes with it. As drier air moves in, more water molecules can evaporate into it. For this reason, puddles of water, as well as clothes on a line, dry more quickly when it's windy (and warm) than when it's still (or cold).

Relative humidity

The absolute humidity of an air parcel (a small volume of air that has a consistent temperature) is merely a measure of how much water vapor is in the air. But this tells us little without the proper context. A much more meaningful description of the moisture content of the air is the relative humidity. This tells us how saturated the air is. In other words, it expresses humidity as a percentage of the total moisture the air can hold. To find the relative humidity of a parcel of air, divide the amount of water vapor present in the air by the maximum amount of water the air at that temperature can hold. Then multiply by 100 to find the percentage.

As an example, consider two different parcels of air. The first air parcel has a volume of 1 cubic yard and a temperature of 50°F (10°C). It contains 0.4 cubic inches of water vapor. At that temperature, a cubic yard of air is capable of holding 0.44 cubic inches of water. Thus, the relative humidity is 0.4 divided by 0.44 times 100, which equals 91 percent. The second cubic yard of air has a temperature of about 75°F (23°C) and contains 0.7 cubic inches of water vapor. At that temperature, the air can hold 1.07 cubic inches of water. The relative humidity of the second parcel is 65 percent. Thus, while the second air parcel has a higher absolute humidity, the first parcel has a higher relative humidity.

A key reference to: Why people use humidifiers and dehumidifiers

Have you ever wondered why the air in your home feels so dry in the winter? Or why your basement gets damp in the summer? Both of these phenomena are due to changes in relative humidity, changes that are brought about by the heating or cooling of air in our homes with no corresponding change in absolute humidity.

First let's examine the case of dryness in winter. Remember that at low temperatures, air can hold very little water. When the cold outside air enters your house and is heated, its absolute humidity stays the same but its relative humidity greatly decreases. Humidifiers, which put water vapor back into the air, raise the relative humidity inside the house back to a comfortable level.

Now take the opposite case, which occurs in the summer. Recall that warm air can hold more water than cold air. When warm air from outside enters your basement, which is generally cooler than the rest of the house, the absolute humidity stays the same but the relative humidity increases. This results in dampness, a condition that favors the growth of mildew. To counter this effect some people use dehumidifiers, which take water out of the air.

You can extrapolate from the above example to understand why the relative humidity is higher at night than during the day. Consider a day in which the absolute humidity is 0.5 cubic inches of water per cubic yard of air. Say the temperature peaks in the afternoon at about 85°F (29°C). At that temperature, a cubic yard of air can hold 1.42 cubic inches of water. Thus the relative humidity is 35 percent. In the evening, the temperature drops to about 60°F (15°C), at which point a cubic yard of air can hold only 0.6 cubic inches of water. If the absolute humidity remains the same, the relative humidity rises to 83 percent.

Dew point

Another measure of humidity is called the dew point. The dew point is the temperature at which a given parcel of air becomes saturated (reaches 100 percent relative humidity) and water vapor begins to condense (return to the liquid phase). The dew point is so-named because it is the temperature at which dew forms on the ground.

Consider the following example in which a cubic yard of air contains 0.6 cubic inches of water vapor. During the day, when the temperature reaches 75°F (23°C), the air is capable of holding 1.07 cubic inches of water vapor. At that point the air has a relative humidity of 56 percent. As the temperature falls (and the absolute humidity remains the same), the relative humidity increases. When the temperature reaches about 60°F (15°C), the relative humidity is 100 percent, the air is saturated, and dew begins to form. Thus, 60°F is the air's dew point. Where the absolute humidity is higher, the dew point is higher; and where absolute humidity is lower, the dew point is lower.

Condensation

At the dew point, water vapor condenses to the liquid state. The form this liquid water takes depends on two factors: the distance above Earth's surface at which condensation occurs and the temperature of that medium. When water condenses on Earth's surface itself, it forms either dew or frost. When water condenses in the air just above the ground it forms fog. At higher levels, it condenses to form clouds.

Dew

The wetness felt on the grass, particularly in the spring or fall, is dew, the condensation of water vapor on a cold surface. It occurs whenever the ground is cold enough to reduce the temperature of the air directly above it to the dew point. This assumes that the dew point is above freezing. If the dew point is below freezing, frost will form.

Dew forms only on surfaces that lose heat quickly and become colder than the dew point of the air, such as the surface of grass and plants. Dew doesn't form on the pavement or a baseball diamond, because hard surfaces retain more heat than the air. Thus, the air above hard surfaces seldom reaches its dew point.

Dew is more likely to form on clear nights than on cloudy nights. The reason for this tendency is that Earth's surface radiates heat upward at night, when there is no incoming solar heat to warm the surface back up. Clouds trap some of that heat and reradiate toward the ground. In the absence of clouds, that heat is lost into space. Thus, on clear nights surface temperatures drop more dramatically than they do on cloudy nights.

Dew formation plays an important role in the regulation of air temperature. When water changes from a gas to a liquid, it releases latent heat, the same energy it absorbed during the evaporation process. When dew forms, it warms the air around it, thus slowing the rate at which the temperature drops throughout the night. It does this so efficiently that nighttime temperatures generally don't drop below the dew point. An exception to this rule occurs when a cold air mass enters a region during the night, causing a sharp decline in temperature.

Weather forecasts often give both the temperature and the dew point. The temperature may tell the current condition, but the dew point tells what to expect at night. Remember, at the dew point, relative humidity is close to 100 percent. On a day when temperatures are high, the dew point will also be high, say around 70°F (21°C). It is expected that the night air will be in the 70s with nearly 100 percent humidity.

Frost

Frost formation is very similar to dew formation, except that it occurs at temperatures below freezing. In contrast to dew, frost will form on any surface, even dirt and concrete. During winter, these surfaces become sufficiently cold for moisture to gather. Dirt and concrete don't absorb enough heat during a winter day for them to remain warmer than the frost point (at which an air parcel can hold no more air when the temperature is below freezing) of the night air.

Weather report: Supercooled water

Often when it rains in the winter, cold rain falls and forms icicles on houses and trees, as well as sheets of nearly impassable ice on the ground. Why is the water liquid in the air but becomes ice when it strikes a surface? The answer lies in the mechanics of supercooled water.

Supercooled water is water that exists in a liquid state below 32°F (0°C). It has not frozen because it takes more than cold temperatures to freeze water. It also takes a freezing nucleus, a particle of ice or other solid on which water vapor can condense. In the absence of a freezing nucleus, a water droplet will not turn to ice until it cools to about −40°F (−40°C). Most freezing rain contains some ice crystals. Once those ice crystals hit the ground, they provide the freezing nuclei around which supercooled raindrops can form ice.

Central to an explanation of frost formation is a concept of supercooled water. Supercooled water is water that exists in a liquid form below 32°F (0°C), the freezing point of water. When the dew point is below 32°F (0°C), water vapor first condenses on a surface as "supercooled dew" and then freezes. This initial layer of frost grows as water vapor from the air freezes directly onto it.

The process of water changing directly from gas to solid ice without first passing through the liquid phase is called deposition. The reverse of deposition, when ice passes directly from the solid state to water vapor without first melting, is called sublimation. During the process of deposition, latent heat is released to the environment. During sublimation, it is absorbed.

Frost that is formed by the process of deposition is called "true frost" or hoar frost. Hoar frost has the intricate structure that can be seen on a windshield on a cold winter day. Hoar frost also forms on the inside of the windows—or between the panes of double-pane glass—in the home. Water vapor freezes onto a window when the air just inside the window is cooled to the dew point, provided the dew point is below 32°F (0°C).

Another type of frost is produced by the freezing of dew that has already formed on a surface. This occurs when the dew point is above freezing, and the temperature later falls below 32°F (0°C). This type of frost does not form crystal structures like hoar frost, but droplets of ice.

Fog

Fog is condensation that occurs in lower levels of air. It is essentially a cloud that has formed close to Earth's surface. Fog in temperate regions is composed of water droplets; in polar and arctic regions it may also be composed of ice crystals. Condensation in the air is generally defined as "fog" when it restricts visibility to 1 kilometer. If visibility is greater than 1 kilometer, the condition is defined as "mist." In this discussion, all condensation in the lower levels of air will be referred to as "fog."

The process of condensation in the air begins with condensation nuclei. Similar to dew and frost, which won't form in the absence of a surface, the water droplets that constitute fog and clouds need something to cling to. Condensation nuclei are tiny solid particles suspended in the air. Even in relatively clean air, there are about two thousand of these particles in every cubic inch. Examples of condensation nuclei include pollen, sea salt, sand, volcanic dust, factory smoke, and other industrial pollutants.

As has been shown in experiments using purified air, individual water vapor molecules do not readily stick together. Even when they do collide and form tiny droplets, those droplets will likely disintegrate. It has been theorized that in the absence of condensation nuclei, water would not condense into raindrops. Rather, the air would grow increasingly saturated with water vapor until it was unable to hold another molecule. Then water would fall to the ground in massive, destructive sheets.

There are several types of fog, which differ according to the conditions under which that fog was formed. Fog is produced in one of two ways: either when air is cooled to its dew point by contact with a cold surface; or when air is brought to its saturation point by evaporation from a wet surface. What follows is a brief outline of three major categories of fog.

The first type of fog, with which most people not in the coastal areas of the United States are familiar, is radiation fog (sometimes called ground fog). This type of fog forms on clear summer nights when winds are nearly still. After sunset, heat radiates away from the ground, cooling the ground and the air above it. Once this air is cooled to the dew point, water vapor condenses and forms a fog. When the Sun rises the next morning and warms the air, the fog quickly dissipates.

The second type of fog is called advection fog. This is the thickest and most persistent type of fog and may form at any time of day or night. Advection fog is formed by advection, the horizontal movement of air. Specifically, it forms when a warm, moist layer of air crosses over a cold surface. The air loses heat to the cold surface. Once the air cools to the dew point, fog is formed.

A third class of fog is called evaporation fog. Like advection fog, it involves the interaction of cold air and warm air. But unlike advection fog—where warm, humid air travels over the cold air—the cold air in this case travels over a warmer body of water. Evaporation fog usually forms over inland lakes and rivers in the fall, when the air is cool but the water still retains heat. Water evaporates from the lake or river, saturates the cold air, and condenses. This fog often appears as "steam" that rises from a body of water.

Cloud formation

The subject of cloud formation has already been touched upon several times in this volume. As air rises, it cools. Once the air reaches the dew point, water vapor within it begins to condense into clouds. When a cold front advances into a warm front, the warm air is thrust upward in a powerful convection, which produces tall clouds. When water condenses on the ground it forms dew or frost, and when it condenses in the air it forms fog or clouds.

Why rising air cools and falling air warms

Both the cooling of air as it rises and the warming of air as it falls are adiabatic [add-ee-uh-BAT-ick] processes. In an adiabatic process no heat is exchanged between a moving air parcel and the ambient (surrounding) air, even as the temperature of the air parcel changes. "Air parcel," refers to a volume of air that has a consistent temperature throughout and experiences minimal mixing with the surrounding air. The mechanism by which ascending air cools is called expansional cooling. Conversely, the mechanism by which descending air warms is called compressional warming.

Expansional cooling is the most significant process in the formation of clouds. It works like this: as a parcel of air rises, the pressure of the air around and above it decreases. This decrease occurs because the density of air decreases with altitude. With fewer molecules, air exerts less pressure. In order to equalize its pressure with that of the ambient air, molecules within the parcel push outward, enlarging the parcel. However, the number of molecules within the parcel does not change. The result is that the same number of molecules is spread over a greater area. In other words the density of the air parcel decreases.

The expansion of air requires energy. That energy comes in the form of molecular kinetic energy (energy of motion), which is the same as heat. Before expanding, the molecules store that kinetic energy, meaning they are warmer. Once the molecules spend kinetic energy moving away from one another, they slow down and collide less frequently. They have a decreased kinetic energy, which is to say they have become cooler.

Conversely, as an air parcel falls, it is compressed by the increasing pressure of the surrounding air. The parcel is squeezed into a smaller volume, thereby increasing the density of the air within it. This leads to a greater number of collisions between molecules, hence greater kinetic energy. The increase in kinetic energy within the air parcel translates into an increase in temperature.

Temperature changes in unsaturated air

The change in temperature of a rising or falling air parcel is a measurable quantity. For air that is not yet at the saturation point (having less than 100 percent relative humidity), the rate of change is called the dry adiabatic lapse rate. This rate of change is constant. Air cools by about 5°F (−15°C) for every 1,000 feet (304 meters) it ascends and warms by 5°F for every 1,000 feet it descends.

Temperature changes in saturated air

Once air becomes saturated, the rate at which temperature changes with altitude occurs more slowly and is no longer a constant. The scale that applies to saturated air is called the moist adiabatic lapse rate.

The reason that saturated air cools more slowly than unsaturated air as it rises is that water vapor condenses within saturated air (and forms a cloud), releasing latent heat. Whereas latent energy is absorbed in the process of evaporation, it is liberated in the process of condensation.

Thus as water vapor condenses out of saturated air, it releases latent heat and raises the temperature of the air parcel. This increase in temperature, however, is not enough to offset the decrease in temperature due to expansional cooling. It merely slows the rate at which the cooling occurs.

The amount by which the release of latent heat slows the cooling of an ascending air parcel depends upon that parcel's temperature. In the warmest saturated air, cooling proceeds at a rate of about 2°F (−17°C) for every 1,000 feet (304 meters) ascended. In the coldest saturated air, cooling proceeds at a rate of about 5°F (−15°C) for every 1,000 feet ascended. The average moist adiabatic lapse rate, about 3°F (−16°C) per 1,000 feet, is often used as a constant, for convenience in weather forecasting.

The reason that the moist adiabatic lapse rate depends upon temperature is that when the air parcel first becomes saturated (and is at its warmest), condensation within it releases the most latent heat. In this case, it offsets the declining temperature by the greatest amount. As the saturated air continues to rise, it cools. At the same time, the air parcel can hold a smaller amount of water vapor. The rate of condensation decreases, and the release of latent heat declines. As a parcel of saturated air decreases in temperature, it provides less of a buffer to the expansional cooling.

By the same token, as saturated air descends, it warms at the moist adiabatic lapse rate. As air sinks and its temperature rises, water droplets (and clouds) evaporate into it. The process of evaporation absorbs latent heat and impedes the rate at which the temperature of the air rises. In other words, evaporation partially offsets compressional warming. Once the falling air is no longer saturated and the water droplets within it have all evaporated, it begins warming at the dry adiabatic lapse rate.

Using the dry adiabatic lapse rate, it is possible to determine the temperature of an unsaturated air parcel at various heights within the atmosphere, provided we know its temperature at the surface. If we know the air parcel's dew point, it is possible to determine at what height clouds will form. Knowledge of air temperature at all levels of the troposphere is a critical element in creating weather forecasts.

Air stability and vertical motion

The vertical movement—or lack thereof—of an air parcel is dictated by differences in temperature and density between the air parcel and the ambient air. Those differences and the resultant degree of vertical movement of air are referred to as air stability (also called "atmospheric stability"). Air stability is the key to both the size and shape of the clouds and the intensity of the precipitation that results when a rising parcel of air reaches the dew point.

The rules of air stability state that as long as an air parcel has a higher temperature and lower density than the surrounding air, it will rise. When this parcel is no longer warmer than the air around it—when its pressure and density have become equal to those of its surroundings—the air parcel stops rising. On the other hand, as long as an air parcel has a lower temperature and higher density than the air around it, it will continue to fall.

A stable air layer, through which a parcel of air cannot rise or descend, marks the end point of an air parcel's vertical journey. A layer of air is stable at the height where an air parcel reaches the temperature of the ambient air and ceases to move. An unstable air layer is one through which an air parcel moves upward or downward. In other words, a layer of air is unstable at heights where an ascending parcel is warmer than the ambient air or a descending parcel is colder than the ambient air. If we know the surface temperature of a rising air parcel and the temperature of the troposphere at various heights, we can determine the height of a stable air layer.

In order for clouds to form, unstable conditions must exist at least long enough for a rising parcel of air to reach its dew point. For cloudy skies to turn clear—caused by a descending parcel of air—unstable conditions must also exist. In stable atmospheric conditions, where the relative humidity is high enough, fog may form and, prevented from rising, will persist.

What causes air stability and instability

Unstable conditions exist within the troposphere when relatively cold air layers are situated above relatively warm surface air. This is what most commonly occurs, as air generally cools off with increasing altitude. Sometimes, however, a layer of warm air exists above colder air. This produces stable conditions. The warm air layer acts as a ceiling, or an upper limit, beyond which warm air parcels will rise no farther. There are various factors that lead to the development of both stable and unstable conditions.

Air becomes stable when either an upper layer of air warms or the surface air cools. The former may be caused by a warm air mass blowing in above, while the layer of air below experiences no change in temperature. The other route to stability—the cooling of the surface air—may occur when the ground loses heat at night or a cold air mass arrives.

An absolutely stable atmosphere is produced by an inversion. An inversion is a condition in which air temperature increases with height. An inversion can occur when a thick layer of unsaturated air sinks, covering a large area. Since the upper levels of the air layer cover a greater vertical distance than the lower levels, the air within them has farther to fall—and more time to undergo compressional warming. The top of the layer therefore becomes warmer than the air below it. The relatively warm air layer acts a lid on any rising air parcel, preventing it from rising further. The presence of low-lying fog, haze, and smog (a hazy layer of pollution at Earth's surface) are all indicators that an inversion has occurred near the surface.

Air instability is caused by the opposite conditions that produce stability, either the warming of the surface air or the cooling of an upper layer of air. The warming of the surface air may be caused by the absorption of solar heat by the ground during the day or the influx of a warm air mass. Or a mass of cold air might be brought in by the winds aloft, while the layer of air below experiences no change in temperature.

The stability of air over land changes significantly throughout the day, from the most unstable at the warmest time of day to the most stable at the coldest time of night. At night the ground loses heat and the air just above it cools. At sunrise this stability can be witnessed (in clear, calm weather) as fog rests on the ground. As the day progresses, the surface layer of air is heated by the Sun. When it becomes warmer than the air above, unstable conditions prevail. As the lower air continues to warm and the difference between lower and upper layers increases, instability increases. Thus, instability is greatest at the hottest time of day. As day passes into night, the surface air cools and the cycle begins again.

Air stability and cloud shapes

When the air is unstable, parcels of air rise throughout the day. If they rise high enough to cool to the dew point, clouds will form. In some cases, the unstable air layer is shallow, meaning that at a relatively low altitude the ambient air becomes warmer than the rising air parcel. In such cases, the clouds that form are puffy and small. If, however, the layer of unstable air is deep, tall clouds such as those that bring thunderstorms may form.

When the air is stable, one won't find individual pockets of rising air, hence no puffy clouds. Clouds form in stable conditions only when an entire layer of air rises. This occurs when air flows into the center of a low-pressure area and rises or when a warm front advances and slides over a cold air mass. In those cases, the lifting of warm air produces a nearly continuous, flat sheet of clouds.

On a typical day, one may see both types of clouds at the same time. The reason for this pattern is that layers of stable and unstable air may be stacked on top of one another over a single location on the ground. As a result, small puffy clouds and sheets of clouds form at different altitudes.

The lifting of air

Air does not just rise spontaneously: it needs a push. That push comes in three different forms: convection, frontal uplift, and orographic lifting.

Convection is the lifting of air that has been heated. When heat is applied, air molecules move more quickly. The molecules spread out, the air loses density, and the air becomes thinner. As long as it is warmer and lighter than the surrounding air, it continues to rise.

Convection occurs when the ground is heated by the Sun. That heat is then radiated upward from the ground, warming the air above it. This causes air parcels, often referred to as "bubbles," to rise and form individual puffy clouds upon reaching the dew point.

Frontal uplift and orographic lifting each cause an entire mass of warm air to rise. Frontal uplift occurs when a warm air mass and a cold air mass come together at a front. The cold air mass occupies the space closest to the surface while the warm air mass rises over the cold air. As the warm air mass reaches the dew point, it forms a sheet of clouds. Orographic lifting occurs when a warm air mass encounters a mountain and rides upward along the surface. This process results in a variety of unusual cloud types, including those shaped like banners and those shaped like flying saucers.

Clouds and precipitation

Although they cover, on average, 60 percent of the sky, clouds hold just one one-thousandth of 1 percent (0.001 percent) of the world's water. Nonetheless, they are critical elements in the cycling of water from the ground into the air and back. Without clouds to regulate the intensity of solar heat, all water would evaporate, and Earth would experience an interminable drought, or extended period of time with abnormal dryness. Clouds also trap heat that is reradiated up from the ground, preventing the surface from growing too cold.

Anatomy of a cloud

A cloud is a collection of many billions of water droplets, condensed from air that has cooled to its dew point. Those water droplets may take one of two forms: liquid water or ice crystals. Whether they form as a liquid or as ice depends on the temperature of the air. When condensation occurs within air that is warmer than 32°F (0°C), it takes the form of liquid droplets. When the temperature of the air is 32°F or below, condensation usually takes the form of ice.

Clouds of liquid water droplets include another vital set of ingredients: condensation nuclei. A condensation nucleus is any solid particle in the air. As explained in the section on fog, sea salt, dust, pollen, sand, and industrial pollutants all act as nuclei around which molecules of water condense.

There are various ways in which ice crystals form within clouds, depending on atmospheric conditions. Ice crystals may either be produced by the freezing of liquid water droplets or the deposition (the process by which water changes directly from a gas to a solid) of water vapor.

First, let's consider the freezing of water droplets. At temperatures below −40°F (−40°C), water freezes directly into ice, in a process called spontaneous nucleation. However, at higher temperatures the process becomes more complicated. Except for the largest droplets, water will not assume the crystalline structure of ice in the absence of freezing nuclei. Freezing nuclei are solid particles, such as clay, vegetable debris, or ice crystals themselves, suspended in the air, upon which water droplets freeze. Freezing nuclei serve a function in the formation of ice crystals similar to that of condensation nuclei in the formation of water droplets.

Freezing nuclei exist in the atmosphere in relatively small numbers and are sometimes in short supply within clouds where temperatures are below 32°F (0°C). This accounts for the presence of supercooled water, water that exists in a liquid state below the freezing point, within some clouds.

The second method of forming ice crystals, the deposition of water vapor, happens much less frequently than does the freezing of water droplets. Deposition only occurs at temperatures below −4°F (−20°C) and in the presence of special freezing nuclei called deposition nuclei. Deposition nuclei, examples of which include ash, diatoms, and spores, are relatively rare in the atmosphere.

Since air temperature generally declines with increasing altitude, ice-crystal clouds are most often found at the highest levels of the troposphere; liquid-water clouds at lower levels; and clouds containing both liquid and ice at middle levels.

What makes rain and snow fall

In order for water to fall to the ground as rain, water droplets in clouds have to become large enough—and obtain a terminal velocity great enough—to reach the ground. It takes from one million to fifteen million water droplets to form an average raindrop, which is about 0.08 inches (2 millimeters) in diameter. Whereas the terminal velocity of a water droplet is about 0.02 mph (0.03 kph), the terminal velocity of a raindrop is about 15 mph (24 kph).

Just how do cloud droplets grow to the size of a raindrop? The most obvious answer is condensation. That is, more and more water vapor molecules condense into a liquid until the drops of water become large enough to fall to the ground. This process alone, however, is quite slow and cannot possibly account for the amount and rate of rainfall experienced in the middle latitudes. Rain often starts falling just thirty minutes after a cloud begins forming. During that time condensation alone can not produce drops of water large enough to fall.

Early in this century, scientists discovered the answer: ice. Ice crystals often exist together with supercooled water droplets in clouds. Such clouds are called cold clouds.

Ice crystals grow more quickly than, and at the expense of, water droplets in cold clouds. The reason for this has to do with vapor pressure, the pressure exerted by a vapor when it is in equilibrium with its liquid or solid. Equilibrium is defined as the saturation point, the point at which the same number of molecules are entering and leaving the gaseous state.

Who's who: Alfred Wegener

Alfred Wegener (1880–1930) was a German meteorologist and geophysicist who solved the mystery of raindrops. He reasoned that supercooled raindrops often coexist with ice crystals in clouds. Ice crystals, which have a lower vapor pressure than liquid water, attract water molecules to them.

Although this discovery was remarkable, Wegener is more famous for developing the theory of continental drift in 1912. This theory states that 200 to 250 million years ago all land on Earth was joined together in one huge continent. Over the years forces deep within the Earth caused the land to break apart and the chunks to move away from one another, eventually reaching their current configuration.

           FURTHER READING ON ALFRED WEGE-NER: WITZE, ALEXANDRA. "ALFRED WEGENER." NOTABLE TWENTIETH-CENTURY SCIENTISTS. VOL. 4. ED. EMILY J. MCMURRAY. DETROIT: GALE RESEARCH INC., 1995.

Within cold clouds, vapor pressure is greater over the surface of a water droplet than it is over the surface of an ice crystal. This pressure differential creates a force that directs water vapor molecules away from the water droplets and toward the ice crystals. In the process, it lowers the pressure over the water droplets. To maintain equilibrium, more molecules then evaporate from the surface of water droplets, which, in turn, are directed toward the ice crystals. Each time the cycle repeats, the ice crystals grow larger and the water droplets grow smaller.

When an ice crystal becomes large enough, it begins to fall, attracting water molecules as it goes. Usually, during its descent, it takes on the form of a snowflake. If the air warms above the freezing point during an ice crystal's descent, the ice melts and hits the ground as rain. If the air remains below freezing, snow occurs.

Ice crystals within clouds, however, can't account for all precipitation. In the tropics there are warm clouds, clouds that are too warm to contain ice. Yet these clouds still produce plenty of rain. Scientists have concluded that in warm clouds, water droplets must collide to form bigger drops. While meteorologists are still seeking to answer the question of just how this happens, one current theory is that large droplets form around giant sea-salt condensation nuclei and that these large droplets become even larger by colliding with smaller droplets.

The water cycle

Precipitation represents one portion of the water cycle (or "hydrologic cycle"), the continuous exchange of water between the atmosphere, and the oceans and landmasses on Earth's surface. The other side of this equation is evaporation, the process by which liquid water at Earth's surface is converted to a gas and is returned to the atmosphere. Some of that water vapor then forms clouds, which return the water to Earth as rain or snow.

Most of Earth's water—about 97.2 percent—exists in the oceans. The rest, save the 0.001 of one percent that exists as water vapor in the atmosphere, is contained in the polar ice caps. All three phases of water—solid, liquid and gas—continually coexist on Earth. The water cycle is driven by the continuous conversion of water molecules among these three phases.

Between 85 and 90 percent of the moisture that enters the atmosphere comes from the oceans. The rest evaporates from the soil, vegetation, lakes, and rivers that exist on the continental landmasses. Even plants emit water through tiny pores on the underside of their leaves in a process called transpiration.

Some of the moist air above oceans is carried overland by the wind. Clouds form and swell, and drop rain and snow on the ground. When precipitation hits the ground, it either sinks into the surface or runs off, depending on the surface composition. For example, rainwater will sink into soil and sand. The excess water will form puddles on the surface or seep down into underground streams or reservoirs. Water found under Earth's surface is known as groundwater. If the water strikes a hard surface, like rock or pavement, it will either run off and flow into rivers and lakes or drip through cracks and make its way to the groundwater.

The oceans experience a net loss of water in this portion of the cycle. More water evaporates from them than returns as precipitation. This deficit is corrected when water in rivers and streams flows back into the oceans. Thus the global water budget—the volume of water coming and going from the oceans, atmosphere, and continental landmasses—is kept in balance.

Land and weather

The different properties of land and sea result in the formation of different weather patterns over each. The two major differences between land and sea can be loosely categorized as heat retention and surface features. These characteristics affect temperature highs and lows, cloud formation, and storm systems. Also, certain types of topography (physical features of land) create their own small-scale weather patterns.

Heat retention

Land heats up and cools down relatively quickly, whereas water is slower to absorb heat and slower to release it. Water absorbs and stores heat in a form called latent heat. Latent heat does not affect the temperature of water; it is the energy used to drive changes in phase from solid to liquid or liquid to gas.

The fact that land rapidly absorbs solar heat during the day, and rapidly loses heat at night, results in greater temperature extremes on land than at sea. These differences in heat retention can be experienced if one goes swimming both in the afternoon and in the evening on a summer day. When one swims in the heat of the day, the water feels cooler than the air. Yet once the Sun goes down and the air temperature drops, the water becomes the warmer medium.

Sea and land breezes

Two manifestations of the temperature differential (and corresponding pressure gradient) created between land and water throughout the day are sea breezes and land breezes. These are the breezes that one feels at the beach. The sea breeze blows toward shore during the day, when the sand is warmer than the water, and the land breeze blows toward the water during the night, when the water is warmer than the sand.

During the day, when the sand warms quickly, a low-pressure area is created over the sand. In comparison, a high-pressure area forms over the water. A gentle wind flows from the high-pressure to the low-pressure (from the water to the sand). This wind is known as a sea breeze.

At night, the sand loses heat more quickly than does the water, so the process is reversed. The breeze flows from the high-pressure area over the sand, out to the low-pressure area over the water. This wind is known as a land breeze.

Mountain weather

The peaks and valleys of a mountain range alter the behavior of a large-scale weather system as it travels across the mountains. These surface features also produce small-scale weather patterns unique to that topography. For instance, a storm slows as it crosses a mountain range and re-intensifies on the other side. When the storm travels over the mountain, the storm center is flattened between the mountaintop and the top of the troposphere. The spinning winds at the storm center are therefore forced to expand horizontally, which slows the spinning. When the storm emerges on the other side of the mountain, it has more room to stretch out vertically and spins faster again.

Mountains also produce distinct, small-scale weather patterns that are limited to the mountainous area. For instance, some clouds have unique shapes that resemble banners or disks. These clouds form at the tops of mountains and are the products of orographic lifting. Orographic lifting is the process in which a warm air mass rides upward along the surface of a mountain. As the air rises, it cools. Once it reaches the dew point, condensation occurs and clouds form.

The rain that comes from these clouds generally falls on the peak and the westward, or windward, side of the mountain, the side on which the warm air ascended. On the other side of the mountain—the eastward or leeward side—conditions are much drier. As air descends across the leeward side, it warms, causing water droplets and clouds to evaporate. The uneven distribution of precipitation across a mountain is known as the rain-shadow effect.

A rain shadow may occupy the base of a single large mountain or the entire region east of a mountain range. An example of the former is Mount Wai'-'ale'ale, on the island of Kauai in Hawaii. The windward side of this mountain is considered the rainiest place on Earth. Near the peak of the mountain, the windward side receives on average 40 feet (12 meters) of rainfall a year. The leeward side of the mountain, in contrast, is extremely dry. It receives only 20 inches (50 centimeters) of rainfall, on average, each year.

A larger-scale rain-shadow effect occurs east of the Rocky Mountains, in the high plains of the central United States. This region receives relatively little rainfall especially compared to the windward side of the Rockies. In South America, a similar situation exists in the arid region east of the Andes Mountains. Many of the world's deserts lie in the rain shadow of mountain ranges.

Oceans and weather

The oceans cover more than 70 percent of Earth's surface. They have a tremendously important role in global heat distribution. What is more, they are involved (in conjunction with the winds) in generating global weather patterns and influencing climate, the weather experienced by a given location, averaged over several deacdes. The Sun heats Earth unevenly, and the atmosphere strives to even out heat distribution. While winds are responsible for about two thirds of the world's heat distribution, ocean currents are responsible for the remaining one third.

Oceans retain heat over long periods of time, even as the amount of energy they receive from the Sun varies. While this enables ocean currents to carry heat from the equator to the poles, it also means that the temperature of the oceans is often different from that of land at similar latitudes. On the coasts, the exchange of cold air and warm air can generate wind, rain, and storms.

There are also seasonal differences between land and sea. While the temperature changes that accompany a new season take effect immediately on land, they lag behind by several weeks in the oceans. It isn't until several weeks after the first day of winter that oceans reach their lowest temperature of the year, and until several weeks after the first day of summer that they reach their highest temperature of the year. As a result, still-warm ocean air warms up some coastal regions as winter sets in. Still-cold ocean air slows the warming of some coastal areas in the spring.

Ocean currents

Ocean currents are permanent or semipermanent largescale circulations of water, at or below the ocean surface. Ocean currents are closely tied to the global circulation of winds. As the wind blows, it causes the surface layer of water to move with it. As the surface water flows, it gradually piles up and creates differences in pressure in the levels of water beneath it. The result is that deeper water moves as well. Due to the relatively high friction that exists between layers of water, ocean currents move much more slowly than the wind.

Ocean currents, like air currents, are influenced by the Coriolis effect, the rotational force of Earth. The Coriolis effect deflects the motion of both ocean currents and air currents to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This causes surface waters to blow in a direction, on average, 45 degrees different than that of the wind. That's because once the wind starts the surface waters in motion, the Coriolis effect alters the direction of the water flow.

Similar to the winds, ocean currents travel in a circular fashion around major high-pressure systems in the atmosphere above them. The large circular patterns of ocean currents are called gyres [JEE urs]. Ocean currents—like winds—travel clockwise around atmospheric high-pressure systems in the Northern Hemisphere and counterclockwise around atmospheric high-pressure areas in the Southern Hemisphere.

Upwelling

Water travels in a series of loops from the equator to the poles and back. The net effect of ocean currents is to cycle heat from the warm equatorial region to the poles. While one would expect surface water to become consistently colder as it travels north and warmer as it travels south (in the Northern Hemisphere), this is not always the case. Sometimes this trend is interrupted, due to a phenomenon called upwelling.

Upwelling is the rising of cold waters from the depths of the ocean. Upwelling occurs when surface water along a coast flows out to sea, and deep water flows in and rises to replace it. This directional flow of water is set in motion when the wind blows parallel to the coastline. An example of where this occurs is Cape Mendocino, in northern California. Due to upwelling, the waters off the coast of Cape Mendocino are cooler in the summer than are the waters off the coast of Washington State, which is farther north.

The process that is responsible for Cape Mendocino's cold waters begins with the winds in that region, which blow from north to south along the California coast, in a clockwise fashion around the Pacific High. The surface water is pushed southward by the wind and curved to the right by the Coriolis effect. Where the wind blows parallel to the coast at Cape Mendocino, the surface water flows out to sea.

Why then does deeper water flow inward toward the coast and upward to replace the surface water? Because at great enough depths of the water, something very curious happens: the water flows in a direction that's opposite that of the surface water. That is to say, where the surface water flows out to sea, the water 100 yards (110 meters) or so below flows in toward the surface.

This changing pattern of water flow along a vertical gradient is called the Ekman Spiral. It works like this: First, imagine ocean water as being made up of a series of vertical layers. Each layer exerts a frictional drag on the layer beneath it, meaning that water travels more slowly the deeper one goes. In addition, the Coriolis effect rotates each successively deeper layer of water farther to the right than the layer above it. Thus, at great enough depths, the water flow reverses direction. In the case of Cape Mendocino, as the surface water flows out to sea, deep, cold water flows toward land. It continues upward, along the ocean floor until it meets the coast.

El Niño/Southern Oscillation

The most striking example of how ocean currents can influence global weather patterns is a phenomenon known as the El Niño/Southern Oscillation (ENSO). This phenomenon begins as the annual warming of the waters off the coast of Peru. In years when this phenomenon is stronger and more persistent than usual, it can bring drought, storms, and floods to far-flung locations around the globe.

El Niño and the Southern Oscillation are actually two different but interrelated and simultaneous events. El Niño, Spanish for "child," was given its name by the residents of the Peruvian coastal area. The name refers to the Christ child, since El Niño usually occurs around Christmas.

The waters off the coast of Peru are typically quite cold and rich in nutrients, the ideal habitat for fish. The area is known particularly for its anchovy populations. Once a year, however, warm waters move in from the equatorial region. These waters are nutrient-poor and unable to sustain fish. Most years, this warming persists for only a month or so before the cold waters return. Occasionally—usually once every three to seven years—the warm waters do not leave. When they remain for a year or two, the period is called a major El Niño event.

The most immediate consequence of a major El Niño event is felt by the coastal Peruvians, whose fishing-based economy is disrupted. Since the warm water is inhospitable to marine life, dead fish, gulls, and marine plants litter the beaches. Their decomposing carcasses and resultant increase of bacteria in the water produce a foul odor. A major El Niño event even affects the poultry industry in the United States, since fish meal produced in Peru is fed to chickens here. Meteorologists did not begin to learn about the larger impact of a major El Niño event and its connection with the Southern Oscillation until the 1950s.

The Southern Oscillation is the name given to the shifting pattern of air pressure that occurs between opposite ends of the Pacific Ocean in the Southern Hemisphere. Generally, pressure is higher over the eastern Pacific, near South America, and lower over the western Pacific, near Australia. This pressure gradient drives the trade winds westward, and toward the equator. Every few years, however, this pressure differential reverses. Since surface water circulations and sea level are also driven by trade winds, the Southern Oscillation has long-range effects. Weather patterns are disrupted not only throughout the Pacific region of the Southern Hemisphere but into the Northern Hemisphere as far north as Alaska and northern Canada.

That major El Niño events and the Southern Oscillation occur in the same years (these are commonly referred to as "ENSO years") is no coincidence. The warming of the waters off the coast of Peru results in a decrease of air pressure in the eastern Pacific. As a result, the air pressure in the western Pacific rises.

At present, ENSO is being widely studied by meteorologists. Even as meteorologists learn to predict more easily when El Niño years will occur, they still find it difficult to predict how ENSO will influence the weather at different locations. Unlocking the mystery of ENSO will surely be of great value in making long-range predictions of future weather (forecasts) and climatic change.

[See AlsoClouds; Fog; Forecasting; Human Influences on Weather and Climate; Hurricane; Local Winds ]

For More Information

BOOKS

Ahrens, C. Donald. Meteorology Today: An Introduction to Weather, Climate, and the Environment. 6th ed. Belmont, CA: Brooks Cole, 2004.

Day, John A. The Book of Clouds. New York: Sterling Publishing, 2005.

Dunlop, Storm. The Weather Identification Handbook. Guilford, CT: The Lyons Press, 2007.

Lutgens, Frederick K., Edward J. Tarbuck, and Dennis Tasa. The Atmosphere: An Introduction to Meteorology. 10th ed. Upper Saddle River, NJ: Prentice Hall, 2006.

Monmonier, Mark. Air Apparent: How Meteorologists Learned to Map, Predict, and Dramatize Weather. Chicago: University of Chicago Press, 2000.

WEB SITES

Groleau, Rick. "Inside the Jet Stream." NOVA Online. 〈http://www.pbs.org/wgbh/nova/vanished/jetstream.html〉 (accessed April 15, 2007).

"National Oceanic and Atmospheric Administration Home Page." National Oceanic and Atmospheric Administration. 〈http://www.noaa.org〉 (accessed April 15, 2007).

"Planet Earth" NASA's Observatorium〈http://observe.arc.nasa.gov/nasa/earth/earth_index.shtml.html〉 (accessed April 15, 2007).

"World Meteorological Organization Homepage." World Meteorological Organization: A United Nations Specialized Agency. 〈http://www.wmo.ch〉 (accessed April 15, 2007).