Precipitation

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Precipitation

Rain
Drizzle
Virga
Showers
Freezing rain
Snow
Snowflakes
Snow grains and snow pellets
Intensity of snowfall
Blizzards
Avalanches
Ice
Ice pellets
Hailstones
For More Information

Precipitation is defined as any form of water that originates in the clouds and falls toward the ground. By this definition, precipitation includes rain, snow, and ice. Each of the three main forms of precipitation can be broken down into specific categories, according to the temperature of the air layers through which the precipitation passes, the size of the individual water particles, and the intensity with which it falls.

Most precipitation (except in the tropics) originates in clouds as ice crystals. As an ice crystal descends through a cloud, it grows by collecting water vapor and droplets of supercooled water, which remains in the liquid state below the freezing point. In the process, the ice crystal takes on the shape of a snowflake, a lump of snow or ice, or in severe thunderstorms, hail. What happens next depends on the air temperature at various heights throughout the ice crystal's descent.

If the air temperature remains below freezing throughout the entire descent, the precipitation will reach the ground in the frozen state as snow. If the ice crystal passes through a layer of air above the freezing point of water, the ice crystal will melt and fall as rain. However, if the melting raindrop passes through a freezing layer, it will refreeze and reach the ground as ice pellets, or frozen raindrops. Finally, if the ice crystal melts, only to reenter freezing air just at ground level, it will strike the ground as freezing rain.

Rain

Each raindrop is made up of a million or so microscopic cloud droplets. The average raindrop measures 0.04 to 0.24 inches (0.10 to 0.61 centimeters) in diameter. Raindrops that are larger than average become unstable and tend to break apart into smaller raindrops. "Rain" is defined as liquid water that falls from the sky. A rain water drop must be larger than 0.02 inches (0.05 centimeters). Precipitation consisting of anything smaller than that is called drizzle.

The main sources of rain are thick clouds with high bases, namely nimbostratus and cumulonimbus clouds. Occasionally rain falls from a thick layer of altostratus or tall cumulus clouds.

Some raindrops form in warm clouds, clouds that are too warm for ice crystals to form. Although raindrops also collide with water droplets as they descend through a cloud, the collisions are less likely to result in coalescence (the process by which an ice crystal grows larger). Hence, water drops do not grow as large as their ice-crystal counterparts. Raindrops from warm clouds are usually less than 0.08 inches (0.2 centimeters) in diameter.

The exception to this rule are raindrops that form in towering cumulus or cumulonimbus clouds in the tropics. In those clouds, where strong updrafts (columns of air blowing upward, inside a vertical cloud) exist, a water drop may be blown from the bottom to the top of the cloud several times, growing larger each time, until it finally falls to the ground. Raindrops up to 0.32 inches (0.8 centimeters) in diameter have been found falling from such clouds in Hawaii.

Visibility in the air improves following a rainfall because precipitation has a cleansing effect on the air. Water droplets in the air form around condensation nuclei, tiny particles of dust and debris. When precipitation falls, it removes these particles from the air.

Drizzle

Drizzle is precipitation made up of drops that are between 0.008 inches (0.02 centimeters) and 0.02 inches (0.05 centimeters) in diameter. These drops are just barely large enough to overcome the upward force of air resistance. Once they do, they slowly drift downward, sometimes taking over an hour to travel from the cloud to the ground.

Drizzle is produced in two ways. It falls from stratus clouds, which are low and exist in shallow layers less than 1.5 miles (about 2.5 kilometers) thick. In stratus clouds, water drops have far less opportunity to grow by coalescence than they do in rain-producing clouds. Drizzle can also form as it begins as rain, descends through dry air, and partially evaporates. By the time the raindrops reach the ground, they have been reduced to the size of drizzle.

Weather report: The shape of a raindrop

Contrary to popular belief, raindrops are not tear-shaped or pear-shaped. Raindrops vary in shape between a sphere and a lump, depending on their size.

Small raindrops, less than 0.08 inches (0.2 centimeters) in diameter, are nearly spherical. Raindrops with diameters greater than 0.08 inches look saucer-shaped, flattened at the bottom and rounded on the top. They are wider than they are tall. Large raindrops have been said to resemble falling parachutes, mushroom caps, and hamburger buns.

The shape of a raindrop is caused by surface tension and air resistance. Surface tension is the attraction between water molecules at the surface. Surface tension forces molecules into the configuration with the smallest surface area, which is a sphere. In larger raindrops, this shape is distorted by the effect of air pressure (the pressure exerted by the weight of air over a given area of Earth's surface). Air pressure is felt most strongly on the bottom of the drop and most weakly on the sides of the drop. It pushes the bottom upward, flattening it, while allowing the sides to expand.

If a raindrop becomes larger than 0.25 inches (0.6 centimeters) in diameter, it will flatten out even more. At the same time it will become pinched on the top, into a bow-tie shape. The two halves of the bow tie will bulge and become more pronounced until the drop divides into two smaller spherical drops.

Drizzle may fall constantly, even for an entire day. The heaviest drizzle is produced where warm, moist air rises along the side of a mountain and forms mountain-wave clouds. Drizzle that falls from these clouds can produce 0.4 inches (1 centimeter) of water per day.

The drops in drizzle fall very close together, which reduces visibility. In a heavy drizzle, it may be possible to see only five-sixteenths of a mile (0.5 kilometer) ahead.

Virga

When humidity is very low, rain or snow may completely evaporate into the air during its descent. This creates streaks of falling water, called virga. Virga looks like dark fringes extending from the base of a cloud.

Virga often sets the stage for heavier precipitation. It does this by increasing the humidity of the air into which it evaporates. Thus, water or ice that falls may subsequently make it to the ground. Virga can also provide additional condensation nuclei for another cloud.

Showers

A shower is a spell of heavy, localized rainfall that occurs only in warm weather. Showers fall from towering cumuliform clouds, which are produced by strong convection currents (the circular movements of gas or liquid between hot and cold areas). Convection is the rising of pockets of warm air that occurs when Earth's surface is heated.

A shower occurs only while the shower-producing cumuliform cloud is overhead. It can last anywhere from two minutes to a half hour, depending on wind speed and the size of the cloud. In an area where a series of cumuliform clouds exist, several showers may occur, separated by dry, even sunny, periods.

The area being showered at any given time is no larger than 4 to 5 square miles (10 to 13 square kilometers). Rain, in contrast, can fall on an area larger than 100 square miles (260 square kilometers) at a time and can last all day.

Towering cumuliform clouds give rise to showers because they are able to generate large quantities of raindrops quickly. Ice crystals or water drops bounce between the top and bottom of the cloud numerous times, growing larger by coalescence on each trip. In most cases, by the time an ice crystal reaches the bottom of the cloud, the air is warm enough to cause the ice to melt.

When a water drop becomes too large, it breaks apart, forming smaller drops. These drops, in turn, get blown to the top of the cloud, and the process of coalescence is repeated. A chain reaction ensues, producing more and more drops. This reaction continues until the updrafts weaken or change direction. Then a sudden, heavy shower falls.

The heaviest showers are called cloudbursts. To qualify as a cloudburst, precipitation must fall at a rate of 4 inches (10 centimeters) or more per hour. The heaviest showers occur in the tropics, where the air is warm and moist, and powerful convection currents produce huge thunderstorm clouds.

Freezing rain

Freezing rain, as its name suggests, is rain that freezes on the ground. Freezing rain begins its journey to the surface as snow. As the snow descends, it encounters a layer of warm air and melts. Then, just above the ground, it travels through a shallow layer of subfreezing air. The raindrops don't have time to refreeze but remain in the liquid state at temperatures below freezing. In other words, they become supercooled.

When the supercooled liquid makes contact with a cold surface, it spreads out and then freezes. It forms a layer of clear, smooth ice called glaze. If drizzle becomes supercooled and then freezes on the ground, it is known as freezing drizzle. Freezing rain creates a beautiful, but hazardous, coating of ice on trees, power lines, and roads.

Freezing rain occurs most often in the winter, following a cold night in which the ground rapidly loses heat to the atmosphere through radiational cooling. As a result, an inversion is produced. An inversion exists when a layer of cold air is found next to the ground and a warmer layer of air lies above.

A heavy downpour of freezing rain is called an ice storm. While the layer of glaze deposited by freezing rain is usually less than an inch thick, it can be much thicker in ice storms. For instance, an ice storm in northern Idaho in January 1961 produced an 8-inch-thick (20-centimeter-thick) layer of glaze. It has been estimated that during a severe ice storm, a 50-foot-tall (15-meter-tall) evergreen tree with an average width of 20 feet (6 meters) may be loaded down with five tons of ice.

A key reference to: Measuring the intensity of rainfall

The intensity of precipitation is the amount of water that falls over a given period of time. For instance, a cloudburst is the most intense form of rainfall because it releases a large amount of water to the ground in a very short period of time. On the other hand, a steady rain may produce the same amount of water as a cloudburst, but since it falls over a longer period, it has a lower intensity.

The following are commonly accepted definitions for categories of rainfall, based on intensity:

Heavy rain: greater than 0.3 inches (0.75 centimeters) of water falls per hour. Heavy rain appears to fall in sheets and greatly reduces visibility.
Moderate rain: between 0.1 and 0.3 inches (0.25 and 0.76 centimeters) of water falls per hour. While the rain does not fall in sheets, it still falls too fast to see individual raindrops.
Light rain: less than 0.1 inches (0.25 centimeters) of water falls per hour. Individual raindrops can be seen.
Trace: rainfall is too light to measure.

The intensity of drizzle, which produces very small quantities of water, is measured in terms of visibility.

Heavy drizzle: visibility is restricted to 0.31 miles (0.5 kilometers).
Moderate drizzle: visibility is between 0.31 and 0.62 miles (0.5 and 1 kilometer).
Light drizzle: visibility is greater than 0.62 miles (1 kilometer).

In an ice storm, it is common for tree limbs and power and telephone lines to be knocked down. The ice is very difficult to drive on and causes traffic accidents. Over 85 percent of the deaths that occur in ice storms are traffic related.

One of the most severe ice storms on record struck a huge area, from Mississippi to New England, in January 1983. That storm resulted in an estimated 25 deaths and cut off power to more than 250,000 people.

Freezing rain is common in hilly or mountainous areas, where cold air sinks into the valleys. In the Appalachian mountains of Pennsylvania and West Virginia, freezing rain can fall on localized areas for long periods of time. The ice deposited by freezing rain usually melts within a few hours, although occasionally it can persist for days. The record for the longest-lasting glaze was set in 1969. In that year, ice remained on the trees for six weeks in Connecticut.

Snow

Snow is precipitation that is common during the winter in middle latitudes (the regions between 30° and 60°, north and south) and year-round on mountaintops. Its basic unit is the snowflake, which consists of many ice crystals joined to each other. Like raindrops, snow-flakes come in various shapes and sizes. Like rain, snow is categorized by the intensity with which it falls: from the lightest form, flurries, to the heaviest form, a blizzard.

In order to qualify as snow, precipitation must remain frozen when it reaches the ground. This does not mean, however, that it can snow only when the surface air temperature is at or below freezing.

Rather, snow can remain frozen at temperatures above freezing for distances up to 1,000 feet (300 meters) without melting. The exact air temperature at which snow turns to rain depends on the humidity of the air.

At the beginning of a snowfall, when temperatures are above freezing, snow may turn to rain as it falls. If air is dry, rain rapidly evaporates into it. The process of evaporation (in which water changes from a liquid to a gas) draws heat from the air, leaving the air cooler than before the evaporation began.

Weather report: Rainmaking

People throughout the ages have sought ways to bring rain to moisture-deprived areas. In the 1800s and early 1900s people tried, unsuccessfully, to produce rain by ringing church bells or firing cannons into the air. In the mid-1940s, scientists began testing an experimental method called cloud seeding. Cloud seeding involves injecting particles into a cloud, which act as freezing nuclei. Cloud droplets adhere to the particles and fall to the ground as precipitation.

A requirement of cloud seeding is that clouds are already present. Those clouds must be tall enough so that their upper portions extend into regions where temperatures are below freezing. In other words, they must be cold clouds. That is because the droplets that stick to the injected particles must be supercooled, meaning they exist in the liquid state at temperatures below freezing. Supercooled water droplets are found only in cold clouds.

The earliest cloud seeding experiments, performed in 1946 by atmospheric scientist Vincent Schaefer, involved dropping crushed dry ice (carbon dioxide) pellets into the top of a cloud from an airplane. Since dry ice is extremely cold (?108°F or ?78°C), it cools the air around it, which produces more condensation. The dry ice pellets also act as freezing nuclei. In one experiment, Schaefer dropped three pounds of ground dry ice into an altocumulus cloud. Five minutes later, snow began falling from that cloud. The following year one of Schaefer's colleagues, Bernard Vonnegut, discovered that silver iodide makes a better cloud seeding agent than dry ice.

The 1950s saw a boom of rainmaking operations in drought-stricken areas around the world that experienced extended periods of abnormal dryness. These operations failed, however, to produce a significant increase in precipitation. Furthermore, they provoked the objections of many people who were concerned with the broader implications of manipulating the atmosphere. Would cloud seeding irreversibly alter the Earth's water cycle? Would it lead to uncontrollable flooding or prolonged droughts?

Despite these concerns, at the end of the 1950s a U.S. federal research program was launched on cloud seeding and other forms of weather modification. Government agencies and private companies conducted many experiments, with inconclusive results.

Cloud seeding was the center of controversy more than once in the 1970s. In 1972, a spate of cloud seeding was followed by a flash flood (a sudden, intense, localized flooding caused by persistent, heavy rainfall or the failure of a levee or dam) in Rapid City, South Dakota, in which more than two hundred people lost their lives. While the cloud seeding and the flash flood may or may not have been linked, the tragedy was enough to deter people from further rainmaking experiments there. Cloud seeding was also a questionable practice used by the U.S. military during the Vietnam War. After the flooding of the Ho Chi Minh Trail, which supposedly came about as a result of cloud seeding by the U.S. military, the U.S. Senate called the practice into question.

Does cloud seeding really work? More than sixty years after the first cloud-seeding experiment this question is still hotly debated. Some studies suggest that, under particular circumstances, cloud seeding can increase precipitation by 5 to 20 percent.

Some of the most impressive results have been obtained by seeding cumulus clouds. As droplets freeze to the injected particles, they release latent heat (the energy that is either absorbed by or released by a substance as it undergoes a phase change), which fuels the upward expansion of the clouds. When a cumulus cloud develops vertically, it lasts longer and is more likely to produce precipitation. Another way in which cloud seeding seems to be effective is by seeding winter clouds that are already producing precipitation. This practice has been shown to increase the amount of snow falling from those clouds.

On the flip side, by overseeding a cloud, it's possible to reduce precipitation. When too many freezing nuclei are present, they remain too small to fall to the ground—even after all available supercooled droplets have frozen onto the freezing nuclei. The moisture in the cloud will then evaporate. For this reason, overseeding is used at airports to dissipate thick fog (clouds that form near the ground).

The practice of cloud seeding continues to the present day. Current expectations of what cloud seeding can produce, however, are quite humble compared to the expectations of researchers in the 1940s and 1950s. In the United States, cloud seeding is performed mainly in California. There, the goal of cloud seeding is to slightly increase the amount of rainfall or snowfall produced by storms.

As snow continues to fall from the cloud, it encounters lower temperatures, although temperatures may still be above freezing. If the snow enters above-freezing temperatures, it begins to melt. Water from the edge of a snowflake rapidly evaporates into the air, further cooling the air, as well as cooling the snowflake.

If cooled below the freezing point by the evaporative process, the snowflake will reach the ground intact. If not, it will melt, and some of the water will evaporate, further lowering the air temperature. As long as air remains unsaturated (with less than 100 percent relative humidity, a measure of humidity as a percentage of the total moisture a given volume of air, at a particular temperature, can hold), the cooling process will continue. However, once air becomes saturated (has 100 percent relative humidity), no net evaporation will occur, and cooling will cease.

Snowflakes

The first person to study the intricate detail and uniqueness of snowflakes extensively was an American farmer named Wilson Bentley (1865–1931). Beginning in 1880, when he was just fifteen years old, Bentley placed snowflakes under a microscope and photographed them. He continued this work for fifty years, making thousands of photographs. In 1931 he published a book with W. J. Humphreys, entitled Snow Crystals, which contained more than 2,300 photos of snow and frost.

One thing that all snowflakes have in common is a hexagonal (six-sided) configuration. The structure of the ice crystals that make up a snowflake is also hexagonal. In fact, this basic shape can be traced back to water molecules. Because of the electrical attraction between water molecules, they take a hexagonal shape when they freeze.

A snowflake begins its existence as an ice crystal within a cold cloud. As it bounces between the bottom and top of the cloud, it grows by coalescence with supercooled water drops or by deposition, the freezing of water vapor molecules directly onto the ice crystal. As the ice crystal grows, it bonds with other ice crystals and assumes the shape of a snowflake (also called a snow crystal). When the snowflake becomes heavy enough, it descends to the surface.

Weather report: Ice and aircraft

Freezing rain and supercooled water droplets within clouds pose major hazards to aircraft. As an aircraft travels through clouds where the temperature is between 10 and 32°F (?12 and 0°C), it encounters a mixture of ice crystals and supercooled droplets. Where supercooled raindrops or large droplets in clouds come in contact with an aircraft, they spread out and form an even coating of ice called glaze. Where the small droplets strike an aircraft, they freeze immediately without spreading, trapping air bubbles. The second type of icy coating, which appears white and weighs less than glaze, is called rime.

A layer of glaze, and to a lesser extent a layer of rime, affects an aircraft in several ways. First, it makes the plane heavy—sometimes so heavy that it is literally pulled from the sky. Second, if ice forms on the plane's wing or fuselage, it provides wind resistance and alters the plane's aerodynamics. If the engine's air intake opening is iced over, it can lead to a loss of power. Ice can also cause the failure of brakes, landing gear, or instruments.

Ice poses the greatest danger to small, single- or twin-engine planes. Ice affects jet airliners to a lesser extent since those planes spend most of their time above the clouds. Nonetheless, airline pilots are given warnings of, and strive to avoid, clouds that may contain supercooled droplets. As an added precaution, the wings of an aircraft are usually de-iced, meaning they are sprayed with a type of antifreeze before take-off.

Snowflakes can exist in flat, platelike forms; long, six-sided columns; or needles that are two hundred times longer than they are wide. They may also form starry shapes, called sector plates. When a sector plate accumulates moisture, it may develop feathery branches on its arms. In this way, the most distinctive and most common snowflake, the dendrite, is formed. As dendrites travel through the cloud, they may combine with other dendrites, forming a wide array of complex patterns.

The shape of a snowflake depends upon the air temperature within the cloud where it is formed.

Below ?8°F (?22°C), snowflakes are hollow columns.
Between ?8 and 3°F (?22 and ?16°C), they are sector plates.
Between 3 and 10°F (?16 and ?12°C), they are dendrites.
Between 10 and 14°F (?12 and ?10°C), they are sector plates.
Between 14 and 21°F (?10 and ?6°C), they are hollow columns.
Between 21 and 25°F (?6 and ?4°C), they are needles.
Over 25°F (?4°C), they are thin hexagonal plates.

Why are dendrites the most common form of snowflake, given that they only form within a narrow range of temperatures (10 and 14°F or ?12 and ?10°C)? The reason is that dendrites form more rapidly than do other snowflake types.

In the dendrite-forming temperature range, the difference in vapor pressure (the pressure exerted by a vapor when it is in equilibrium with its liquid or solid) between water droplets and ice crystals is greatest. Vapor pressure is the pressure exerted by a vapor when it is in equilibrium with its liquid or solid. Vapor pressure is greater over the surface of a water droplet than it is over the surface of an ice crystal. Similar to air, water molecules migrate from an area of high pressure to an area where pressure is lower. Thus, when a water droplet comes in contact with an ice crystal, water molecules leave the water droplet and freeze onto the ice crystal. The greater the difference in vapor pressure between water and ice, the more rapidly ice crystal growth occurs.

Question: Why do we salt icy roads?

Salt is applied to snowy, icy roads because it melts ice and prevents the water from refreezing. Compared to other materials that are used to combat slippery roadways, such as sand and cinders, salt is relatively cheap and easy to apply. For these reasons, salt has been the road de-icing agent of choice since the 1960s.

The chemical composition of salt used on roads is sodium chloride (NaCl). When it comes in contact with water molecules, NaCl breaks down into one positively charged sodium ion (NA+2) and two negatively charged chloride ions (Cl–). A water molecule consists of one oxygen atom (O?2) and two hydrogen atoms (H+). The positively charged hydrogen atoms are drawn to the negatively charged chloride atoms. Negatively charged oxygen atoms are drawn to the sodium atoms.

Sodium chloride thus causes the components of individual water molecules to disassociate. It bonds with the hydrogen and oxygen atoms so that hydrogen and oxygen are not free to recombine into water. The sodium and chloride ions also draw water molecules away from one another. Salt both prevents liquid water molecules from forming ice crystals and breaks up existing ice crystals.

Sodium chloride lowers the freezing point of water from 32°F (0°C) to 20°F (?7°C). At temperatures below 20°F, salt is no longer effective at melting ice.

Whereas salt provides an efficient means of melting snow and ice on roads, and has greatly contributed to highway safety, it also has a downside. Salt is bad for the environment. It kills vegetation along the side of the road and can seep down into wells, making the water undrinkable. Salt also causes vehicles to rust and bridges to corrode. For these reasons, salt is only applied in the minimum quantities necessary to get the job done.

The size of a snowflake depends upon the temperature of the air as the snowflake descends. When a snowflake falls through air in which the temperature is above freezing, it melts around the edges. A film of water forms that acts like glue. It causes snowflakes that come in contact with one another to stick together, producing large, soggy snowflakes, 2 to 4 inches (5 to 10 centimeters) or larger in diameter. These snowflakes stick to surfaces and are heavy to shovel.

In contrast, snowflakes that fall through very cold dry air do not readily stick together. They are small and powdery when they hit the ground. This snow makes for ideal skiing conditions.

Snow grains and snow pellets

Snow grains are the frozen equivalent of drizzle. They are small, soft white grains of ice that form within stratus clouds and fall to the ground in only small amounts. Snow grains are elongated and generally have diameters less than 0.04 inches (0.1 centimeters). Because they are so light, they fall very slowly and land gently, without bouncing or shattering.

In contrast to snow grains, snow pellets fall rapidly and bounce high off the ground. These white pieces of icy matter, also called graupel or soft hail, measure between 0.08 and 0.2 inches (0.2 and 0.5 centimeters) in diameter. Snow pellets fall in showers and feel brittle and crunchy underfoot.

Snow pellets form within towering cumuliform clouds, where the atmosphere is very unstable. This instability occurs when air temperature drops rapidly with height. The top of the cloud, where temperatures are lowest, is inhabited mostly by ice crystals. The ice crystals grow by the deposition of water vapor onto them and take on the shape of snowflakes.

As a snowflake travels downward into the warmer, middle region of the cloud, it encounters supercooled water droplets. In what is known as riming, the droplets freeze to the snowflake, trapping numerous air pockets in the process. If riming occurs to a great enough extent, the snowflake will be transformed into a lumpy, white pellet of snow called graupel. It is in this form that precipitation reaches the surface.

Intensity of snowfall

The lightest form of snowfall is flurries. Flurries are brief and intermittent and originate in cumuliform clouds. While they produce very little accumulation, flurries may interfere with visibility.

A heavier and more persistent snowfall, which most people think of as snow, comes from nimbostratus and altostratus clouds. This snowfall may continue steadily for several hours.

A brief but heavy snow shower is called a snow squall. Snow squalls, like flurries usually originate in cumuliform clouds. Snow squalls can be compared in intensity to summer rain showers and are accompanied by strong surface winds.

Heavy snow is defined as that which reduces visibility to 0.3 miles (0.5 kilometers). Heavy snow, on average, yields 4 inches (10 centimeters) or more in a twelve-hour period or 6 inches (15 centimeters) or more in a twenty-four hour period. However, the amount of snow accumulation deemed heavy varies from one geographic area to another. For instance, in places where accumulations of 4 inches during a twelve-hour period are common, snow may not be considered heavy until more than 6 inches have accumulated during that period. On the other hand, where any accumulation of snowfall is rare, an accumulation of 2 to 3 inches (5 to 8 centimeters) in a twelve-hour period may be considered heavy.

Snow can also be classified by how it behaves on the surface. For instance, drifting snow is loose snow that has been swept by strong winds into large piles, or drifts. Blowing snow is snow that has been lifted off the surface by the wind and blown about in the air. Blowing snow may reduce visibility in a manner similar to that which occurs in a heavy snowfall. A ground blizzard is the condition that results when snow continues to drift and blow after a snowfall has ended.

Blizzards

A blizzard is the most severe type of winter storm. It is characterized by strong winds, large quantities of snow, and low temperatures. The National Weather Service defines a blizzard as a snowstorm with winds of 35 mph (56 kph) or greater. The temperature is generally 20°F (?7°C) or lower. The falling and blowing of fine powdery snow greatly reduces visibility, often to less than a quarter of a mile and sometimes to just a few yards.

When a blizzard strikes, it can bring traffic to a standstill, strand motorists, and shut down entire cities. Prolonged exposure to a blizzard can cause frostbite (the freezing of the skin), hypothermia (a condition characterized by a drop in core body temperature from the normal 98.6°F to below 95°F), and even death. Some people have actually suffocated to death during blizzards by choking on fine, powdery snow.

A severe blizzard is a blizzard in which wind speeds exceed 45 mph (72 kph), the snowfall is heavy, and the temperature is no higher than 10°F (?12°C). When falling, drifting, and blowing snow reduce visibility to almost zero, the condition is called a whiteout. Everything appears white, making the ground and sky indistinguishable. People stranded in such conditions can easily become disoriented and lose their way.

Can two snowflakes be alike?

To answer this question, a definition of "alike" is required. "Alike" can mean that two snow-flakes are the same, molecule for molecule, throughout. By this definition, the answer is no, two snowflakes cannot be alike. First of all, a snowflake contains over 180 billion water molecules. These molecules come together under many different conditions, making it all but impossible for any two snowflakes to have an identical configuration. In addition, water molecules are constantly freezing to and evaporating from snowflakes, meaning that snowflakes are constantly changing at the molecular level.

An alternate definition of "alike" is "identical in appearance." By this definition, the answer is yes, two snowflakes can be alike. This fact was discovered in 1989 by Nancy Knight, a cloud physicist with the National Center for Atmospheric Research. Knight collected snow samples while on board a research aircraft at a height of 20,000 feet (6 kilometers) over Wausau, Wisconsin. She discovered two hollow-column snowflakes, both 250 microns long and 170 microns wide (a micron is one-millionth of a meter—by way of comparison, a human hair is about 100 microns in diameter), proving that snowflakes can be identical in appearance. To this day, however, there is no record of identical dendrites.

Avalanches

An avalanche is the cascading of at least 100,000 tons of snow down a steep slope. It occurs when stress is placed on a weak layer of snow. For an avalanche to occur, snow on the ground must be layered in such a way that it is structurally unstable. For instance, a loose layer of snow may be sandwiched between two more compact layers.

Every time new snow falls, it places additional weight and pressure on the existing snow. At some point, this pressure may give rise to a fracture (break) across the blanket of snow, down to the weakest layer. As soon as any additional stress is added, a slab of snow breaks off and goes hurtling down the slope, The additional stress may take the form of more snow, a strong gust of wind, the weight of a skier, or even a loud noise.

Weather report: Lake-effect snow

Lake-effect snow is the name given to the heavy snowfalls that occur along the shorelines of the Great Lakes. The process that gives rise to lake-effect snow begins when dry, polar air masses, informally called Alberta Clippers, sweep down from Canada. As this air travels across the Great Lakes, evaporation in the form of steam fog raises the humidity of the air considerably. Clouds form and deposit heavy snowfall on the land downwind of the Great Lakes.

In January 1959, 51 inches (130 centimeters) of lake-effect snow fell during a sixteen-hour period on Bennetts Bridge, New York, on the southern shore of Lake Ontario. Lake-effect snow fell on Buffalo, New York, for forty straight days during the winter of 1976–77.

Avalanches are extremely destructive. They bury everything in their path, even cities, in a matter of seconds. The largest avalanches occur in the Andes, the Himalayas, and the mountains of Alaska. However it is in the Alps, where valley regions are heavily populated, that avalanches pose the greatest danger to humans.

A key reference to: Winter storm alerts and safety procedures

The National Weather Service issues winter storm alerts whenever snowfall is anticipated to be heavy enough to create dangerous travel conditions. There are four different types of these alerts based on the seriousness of the storm, defined as follows:

A winter weather advisory states that snow, sleet, freezing rain, or high winds may be on the way. It advises people to exercise caution when traveling.
A winter storm watch states that at least 6 inches of snow and an ice storm may be on the way. It advises people to limit their travels and to exercise great caution if they must venture onto the roads.
A winter storm warning states that a storm, including heavy snow and possibly ice, has already begun or will soon begin. It advises people not to travel, except in an emergency.
A blizzard warning states that a blizzard—including heavy snow, low temperatures, and winds of at least 35 mph (56 kph)—is on the way. The combination of heavy snowfall and low clouds make it appear that the ground and sky are a continuous white sheet (called a whiteout), making travel nearly impossible. This warning advises people to remain indoors.

Safety procedures:

If you live in an area affected by winter storms, it is wise to take the following precautions at the start of the season:

Store extra blankets and warm clothing and boots at home for every member of the family.
Put together a supplies kit for your home containing first aid materials, a battery-powered flashlight, a battery-powered radio, extra batteries, nonperishable food, a nonelectric can opener, and bottled water.
Store a similar supplies kit, plus the following equipment, in the trunk of your car: a shovel, a bag of sand, tire chains, jumper cables, and a piece of brightly colored cloth to tie to your antenna.
Keep your car's gas tank full to prevent the fuel line from freezing.

If you must go outside during a winter storm, follow these rules:

Wear several layers of lightweight clothing, gloves, a hat, and a scarf covering your mouth.
Walk carefully over icy ground.
When shoveling, take frequent breaks to avoid overexertion.
If you must drive, inform someone of your route, destination, and expected time of arrival.

If you get stranded in your car during a winter storm:

Stay with your car. Tie the brightly colored cloth to your antenna so rescuers can spot you. Don'tattempt to walk away. It's easy to become disoriented and lose your way in a snowstorm.
Only start the car and turn on the heater for ten minutes each hour. When the car is running, leave on the inside light so you can be spotted. When the car is not running, periodically check the tailpipe and clear it of snow, if necessary. If your tailpipe is blocked, dangerous fumes can back up into the car.
Move your arms and legs continuously to stay warm and maintain your blood circulation.
Let in fresh air by slightly opening the window that's opposite the direction of the blowing wind.

In the United States, between twelve hundred and eighteen hundred avalanches are reported each year. Most of these occur in the western states. When taking into account avalanches that go unnoticed or unreported, the actual number is much higher.

Colorado has the highest death rate caused by avalanches, with six to eight fatalities a year. Most of the people killed are skiers or snow-mobilers. The best way to stay out of an avalanche's path is to avoid snow-covered slopes at angles steeper than 30 degrees.

Ice

Ice pellets and hailstones are the two forms of precipitation that fall to the ground as hard, mostly transparent pieces of ice. Ice pellets and hailstones have little else in common besides their composition.

Ice pellets

Ice pellets are frozen raindrops. They are formed by precipitation that passes first through a warm layer of air and melts, then reenters a layer of freezing air and refreezes. The precipitation reaches the ground as tiny pellets of ice.

Ice pellets differ from freezing rain in the depth of the layer of freezing air through which they pass. Ice pellets encounter the freezing air at a higher elevation than does freezing rain. Thus, ice pellets have time to freeze before they hit the ground, while freezing rain freezes only on contact with the cold ground.

In the United States, ice pellets are also referred to as sleet. Sleet is a confusing term, however, since this word is used in Australia and Great Britain to refer to a mixture of rain and wet snow. Even in the United States, sleet is often used by the news media to describe slushy precipitation. For this reason, this text will use the term "ice pellets" instead of "sleet."

Weather report: Snow rollers

On rare occasions, Mother Nature gives us a hand in building a snowman by creating snow rollers. A snow roller is a lumpy, spherical or cylindrical mass of snow, generally less than 1 foot in diameter. It is created only under a very particular set of conditions. First, there must be a layer of smooth, hard, crusty old snow on the ground. Then, a light layer of new snow falls on top of the old snow. Finally, a strong warm wind blows in, rapidly raising the temperature.

This wind literally lifts up a patch of snow and rolls it along the surface snow, which is warm and sticky. This process continues until the accumulated snow becomes too heavy to be rolled any farther.

One place where snow rollers have been witnessed is in Boulder, Colorado. They are created by the strong, warm Chinook winds, or dry, warm winds that blow down the eastern slopes of the Rocky Mountains.

Ice pellets measure only 0.2 inches (0.5 centimeters) in diameter, are irregular in shape, and bounce when they hit a surface. Ice pellets can also be identified by the ping sound they make when they strike a glass or metal surface. An accumulation of ice pellets can create hazardous driving and walking conditions.

Hailstones

Hailstones are a larger and potentially much more destructive form of frozen precipitation than ice pellets. They have either a smooth or jagged surface and are either totally or partially transparent. While most hailstones are pea-sized, they may reach the size of softballs.

Large hailstones have been responsible for destroying crops, breaking windows, and denting cars. They have also caused numerous human and animal deaths. The largest single hailstone on record was about the size of a cantaloupe. It measured 7 inches (18 centimeters) in diameter and 18.75 inches (48 centimeters) in circumference, with a weight of just under 1 pound (.45 kilograms). It fell on Aurora, Nebraska, on June 22, 2003. The heaviest hailstone on record in the United States measured 5.5 inches (14 centimeters) in diameter and 17 inches (43 centimeters) in circumference, with a weight of 1.7 pounds (.77 kilograms). It fell on Coffeyville, Kansas, on September 3, 1970.

WORDS TO KNOW

accretion:: the process by which a hailstone grows larger, by gradually accumulating cloud droplets as it travels through a cloud.
accretion:: the process by which an ice crystal grows larger. The ice crystal collides and sticks together with water droplets as the ice crystal travels down through a cloud.
condensation:: the process by which water changes from a gas to a liquid.
condensation nucleus:: a tiny solid particle around which condensation of water vapor occurs.
convection:: the upward motion of an air mass or air parcel that has been heated.
deposition:: the process by which water changes directly from a gas to a solid, without first going through the liquid phase.
downdraft:: a downward blast of air from a thunderstorm cloud felt at the surface as a cool gust.
drizzle:: precipitation formed by raindrops between 0.008 inches and 0.02 inches in diameter.
freezing nuclei:: a tiny particle of ice or other solid onto which supercooled water droplets can freeze.
supercooled water:: water that remains in the liquid state below the freezing point.
updraft:: a column of air blowing upward inside a vertical cloud.

Hailstones are formed within cumulonimbus clouds during intense thunderstorms. A hailstone forms around a small particle, called an embryo. Objects that can act as embryos include ice crystals, frozen raindrops, graupel, dirt, or even insects. There have also been reports of larger organisms, such as frogs, being swept up in a tornado (a rapidly spinning column of air that extends from a thunderstorm cloud to the ground) and returning to Earth with a hailstone formed around them.

As an embryo travels through a cloud, it is coated by cloud droplets and grows larger by accretion (gradual accumulation). On reaching the bottom of the cloud, the developing hailstone is blown back up to the top by powerful updrafts. It repeats its journey down and up through the cloud many times. In the case of very strong updrafts, this process may last several minutes. When the hailstone becomes heavy enough to overcome the force of the updraft, it falls to the ground.

Thunderstorms, and hence hailstones, are warm-weather phenomena. As a hailstone descends toward the surface and encounters warm air, it begins to melt. Small hailstones may melt completely in the air. Hailstones that are large enough, however, melt only partially and reach the ground in the frozen state. In the tropics, where the air is very warm, hail always melts before reaching the ground, turning to rain.

When sliced in half, a hailstone resembles an onion, with a pattern of concentric rings. The number of rings is equal to the number of trips the hailstone made through the cloud. Up to twenty-five rings have been counted in large hailstones.

Layers of a hailstone alternate between clear ice and white ice, called rime. The clear layers are formed when the hailstone is in warmer air, in the lower portion of the cloud. There, supercooled water droplets are plentiful. They form a layer of water around the hailstone, which slowly freezes when the hailstone returns to the colder, upper portion of the cloud.

Weather report: Hail alley

Hail alley is the region of North America where hail frequently damages crops. It covers a north-south belt from Alberta, Canada, to Texas. It extends westward to the Rockies and eastward to the Mississippi River. Hailstorms occur with the greatest frequency in the Great Plains states. There, hail falls accompany 10 percent of all thunderstorms.

A hailstorm can flatten an entire field in minutes. Hail damage to crops in the United States alone tops $700 million per year. To guard against the chance that a single storm can wipe out an entire year's earnings, farmers in hail alley spend large sums of money on hail insurance. Illinois farmers top the list of insurance buyers, purchasing over $600 million worth of liability coverage annually.

The milky white layers are formed in the upper, freezing portion of the cloud, where supercooled droplets are scarcer and freeze directly onto the hailstone by the process of riming. In a manner similar to the process by which a graupel is formed, the droplets trap air bubbles when they freeze to the hailstone.

The accumulation of hailstones during a thunderstorm can be considerable. One of the largest hail falls on record reached a depth of 18 inches (46 centimeters) and occurred in Selden, Kansas, in June 1959. Occasionally, snowplows must be taken out of summer storage to clear roads of hailstones. This was the case in September 1988, in Milwaukee, Wisconsin, when hailstone drifts reached 18 inches. In August 1980, snowplows were required to remove hailstone drifts 6 feet (2 meters) deep in Orient, Iowa.

The hailstorms that have caused the greatest toll in terms of human life have occurred in Asia. In 1888, in northern India, hail killed 246 people. In 1932, in southeast China, a hailstorm claimed about two hundred lives and injured thousands. In 1986, in Bangladesh, a storm that produced some very unusual hailstones weighing more than 2 pounds (1 kilogram) each killed ninety-two people. In the United States, only two deaths have been attributed to hail in the last century.

If you are caught in a thunderstorm, watch for these warning signs of hail: a green tinge develops at the base of the cloud or the rain begins to take on a whitish color. If you witness either of these signs, it is time to collect your family and pets and move indoors.

[See AlsoClouds; Forecasting; Weather: An Introduction ]