Weather

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Weather

Solar energy

Humidity, clouds, and precipitation

Atmospheric pressure and winds

Terrestrial characteristics

Topographic factors

Weather and climate

Resources

Weather is the condition of the atmosphere at any given time and place. Weather conditions are determined by six major factors: air temperature, air pressure, humidity of the air, amount and kind of cloud cover, amount and kind of precipitation, and speed and direction of the wind. Weather condition patterns for a region or the entire planet can be charted on a weather map containing information about all six of these factors. This information often can be used to produce a weather forecast, which is a prediction of weather conditions at some future time for some given region.

The study of weather is known as meteorology. No exact date can be given for the beginnings of this science, because humans have studied weather conditions for many centuries. Indeed, the word meteorology itself goes back to Meterologica, a book written by the Greek natural philosopher Aristotle in about 340 BC. Many scholars date the rise of modern meteorology to the work of a Norwegian father and son team, Vilhelm and Jakob Bjerknes. They were the first to develop the concept of masses of air moving across Earth’s surface, affecting weather conditions as they moved. They also created the first widespread system for measuring weather conditions throughout their native Norway.

The six factors determining weather conditions result from the interaction of four basic physical elements: solar radiation, Earth’s atmosphere, Earth itself, and natural landforms on Earth’s surface.

Solar energy

The driving force behind all meteorological changes taking place on Earth is solar energy. The outer portions of Earth’s atmosphere receive an average of 2 calories per square cm per minute. This value is known as the solar constant. Although the solar constant changes over very long periods of time, it does not vary enough to affect the general nature of Earth’s weather over short periods of time.

Thirty percent of all solar energy is lost to space by scattering and by reflection off clouds and Earth’s surface. Another 19% is absorbed by gases in the atmosphere and by clouds. About a quarter of it (25%) reaches Earth’s surface directly; another quarter (26%) eventually reaches the surface after being scattered by gases in the atmosphere.

An important factor in determining the fate of solar radiation is its wavelength. Shorter wavelengths tend to be absorbed by gases in the atmosphere (especially oxygen and ozone) whereas radiation of longer wavelengths tends to be transmitted to Earth’s surface.

Solar radiation that reaches Earth’s surface is absorbed to varying degrees, depending on the kind of material on which it falls. Because dark and rough surfaces absorb radiation better than light and smooth surfaces, soil tends to absorb more solar radiation than water.

Solar energy that reaches Earth’s surface is re-radiated back to the atmosphere as heat, also referred to as infrared radiation. Infrared radiation consists of much longer wavelengths. This re-radiated energy is likely to be absorbed by certain gases in the atmosphere such as carbon dioxide and nitrous oxide. This absorption process, the greenhouse effect, is responsible for maintaining the planet’s annual average temperature.

Humidity, clouds, and precipitation

The absorption of solar energy by Earth’s surface and its atmosphere is directly responsible for most of the major factors making up weather patterns. For example, when the water in oceans, lakes, rivers, streams, and other bodies of water is warmed, it tends to evaporate and move upward into the atmosphere. The amount of moisture found in the air at any one time and place is called the humidity. Humans are very sensitive to this characteristic of weather.

Water that has evaporated from Earth’s surface (or escaped from plants through the process of transpiration) rises to an altitude in the atmosphere at which the air around it is cold enough to cause condensation. When moisture condenses into tiny water droplets or tiny ice crystals, clouds are formed.

Clouds are an important factor in the development of weather patterns. They tend to reflect sunlight back into space. Thus, an accumulation of cloud cover may contribute to a decrease in heat retained in the atmosphere.

Clouds give rise to various types of precipitation. As water droplets or ice crystals collide with each other, they coalesce and form larger particles. Eventually, the particles become large enough and heavy enough to overcome upward drafts in the air and fall to Earth as precipitation. The form of precipitation that occurs (rain, snow, sleet, hail, etc.) depends on the atmospheric conditions through which the water or ice falls.

Atmospheric pressure and winds

Solar energy also is responsible for different atmospheric pressures across the planet’s surface, and the winds that result from these different pressures. Because Earth’s surface differs in color and texture from place to place, some locations will be heated more intensely by solar radiation than others. Warm places usually heat the air above them, setting convection currents into movement that carry masses of air upward into the upper atmosphere. Those same convection currents then carry other masses of air downward from the upper atmosphere toward Earth’s surface.

In regions where warm air moves upward, the atmospheric pressure tends to be low. Downward air movements are associated with higher atmospheric pressures. These higher or lower atmospheric pressures can be measured by a barometer. Barometers provide information not only about current pressures but about possible future weather patterns as well. The existence of areas with different atmospheric pressures accounts for the movement of air, which is felt as wind.

Terrestrial characteristics

The sun provides the energy by which weather patterns can develop, and certain features of Earth itself determine the precise forms in which those patterns may be exhibited. One example has already been provided above. Earth’s surface is highly variable, ranging from oceans to deserts to cultivated land to urbanized areas. The way solar energy is absorbed and reflected from each of these regions is different, accounting for variations in local weather patterns.

Other characteristics of the planet account for more significant variations in weather patterns. These characteristics include such features as the tilt of Earth on its axis in relation to its plane of revolution, and the variations in Earth’s distance from the sun.

The fact that Earth’s axis is tilted at an angle of 23 1/2° to the plane of its orbit means that the planet is heated unevenly by the sun. During the summer, sunlight reaching the Northern Hemisphere strikes more nearly at right angles than it does in the Southern Hemisphere. In the winter, the situation is reversed.

The elliptical shape of Earth’s orbit around the sun also affects weather conditions. At certain times of the year the planet is closer to the sun than at others. This variation means that the amount of solar energy reaching the outer atmosphere will vary from month to month depending on Earth’s location in its path around the sun.

Earth’s rotation on its own axis also influences weather patterns. If Earth did not rotate, air movements on the planet would probably be relatively simple: air heated along the equator would rise into the upper atmosphere, travel northward toward the poles, be cooled, and then return to Earth’s surface at the poles.

Earth’s rotation causes the deflection of these theoretically simple air movements. Instead of a single overall equator-to-poles air movement, global winds are broken up into smaller cells. In one cell warm air rises above the equator, moves northward in upper

KEY TERMS

Humidity —The amount of water vapor contained in the air.

Meteorology —The study of Earth’s atmosphere and the changes that take place within it.

Orographic —A term referring to effects produced when air moves across a mountain range.

Solar constant —The rate at which solar energy strikes the outermost layer of Earth’s atmosphere.

Solar energy —Any form of electromagnetic radiation that is emitted by the sun.

Topography —The detailed surface features of an area.

altitudes, is cooled, and returns to Earth in the regions around 30° north and south latitude. A second cell consists of air that moves upward in the regions around 60° north and south latitude, across the upper atmosphere, and then downward at about 30° north and south latitude. The final cell contains winds traveling upward at 60° north and south latitude and then downward again at the poles.

Topographic factors

Irregularities on Earth’s surface affect weather. A mountain range can dramatically affect the movement of approaching air masses. Suppose that a mass of warm moisture-filled air is forced to ascend one side of a mountain range. As the air is pushed upward it cools off and moisture begins to condense out, first in the form of clouds then as precipitation. Thus, the windward sides of mountains typically receive more rain and snow than the leeward sides.

As the air mass continues over the top of the mountain range, it does so without its moisture. The winds that sweep down the far side of the range will tend to be warm and dry. The term orographic is used to describe changes in weather patterns like these induced by mountain ranges.

Weather and climate

The terms weather and climate often are used in conjunction with each other, but refer to different phenomena. Weather involves atmospheric conditions that currently prevail or that exist over a relatively short period of time. Climate refers to the average weather pattern for a region (or for the entire planet) over a much longer period of time. Many authorities use a time period of at least three decades to distinguish between weather and climate.

Changes in weather patterns are easily observed. It may rain today and be clear tomorrow. Changes in climate patterns are much more difficult to detect. If the summer of 1997 is unusually hot, there is no way of knowing if that fact is part of a general trend towards warmer weather or a single variation that will not appear again for some time.