Weather is the state of the atmosphere at some place and time. We describe the weather in many ways. For example, we may refer to the temperature of the air. We report whether the sky is clear or cloudy. We describe how hard the wind is blowing. We say whether it is raining or snowing.
At any given time, the weather is fair in some places, while it rains or snows in others. In some places it is warm. In others, it is cold.
Earth is not the only planet with a variety of weather conditions. All of the planets in the solar system larger than Mercury have enough of an atmosphere to support weather systems. In addition, the moons Titan and Triton have such an atmosphere. The remainder of this article discusses the weather on Earth.
The weather affects our lives every day. For example, it can have an impact on what type of clothing we wear. It can influence how we spend our free time. Weather also affects agriculture, transportation, and industry. Freezing temperatures can damage crops in citrus-growing regions, causing a rise in the price of oranges. Winter snows often create hazardous driving conditions.
Thick fog may slow traffic on the roads. Fog may also cause delays at airports. Our use of air conditioning during heat waves and heating during cold weather means that utility companies must supply more power at those times. Severe weather, such as tornadoes, hurricanes, and blizzards, can damage property and take lives.
Scientists who study the atmosphere and the weather are called meteorologists.
Because of weather's importance, meteorologists have developed ways to forecast weather conditions.
Forecasts for the next 12 to 24 hours are correct more than 80 percent of the time. Long-range forecasts for the next week or month are less accurate. These forecasts indicate general trends.
For example, they may say whether temperatures are expected to be warmer or colder than normal.
Closely related to weather is climate. Climate is the weather of a place averaged over a length of time. Scientists determine a region's climate by examining its average monthly and annual temperature and precipitation. They also study a region's vegetation. Earth's surface is a patchwork of climate zones. For example, in various parts of Earth, we find deserts; tropical rain forests; prairies; and forests of cone-bearing trees. We also find frozen, treeless plains; and coverings of glacial ice. Unlike changes in the weather, which can occur in minutes, climate changes take many years.
What causes weather
Weather takes place in the atmosphere, the layer of air that surrounds Earth. Air is a mixture of gases and tiny suspended particles.
The most plentiful gas is nitrogen. Nitrogen accounts for about 78 percent of the air we breathe.
The second most plentiful gas is oxygen, at 21 percent. The remaining 1 percent consists of a variety of gases. In spite of their low concentrations, some of these gases play vital roles. For example, the atmosphere contains little water vapor.
This vapor is an invisible gas produced

when water evaporates. Yet, without water vapor there would be no clouds, no rain, no snow, and no plants or animals.
Most of the tiny particles floating in the atmosphere are too small to be visible. They are solid or liquid and come from a number of sources. Sources of particles include the wind erosion of soil, volcanic eruptions, and the release of pollutants by smokestacks and automobile tailpipes.
Most weather occurs in the lowest portion of the atmosphere, called the troposphere.
The troposphere extends from Earth's surface up to an altitude of about 6 to 12 miles (10 to 19 kilometers). Three key factors that determine the weather in the troposphere are air temperature, air pressure, and humidity.
Air temperature is a measure of the energy of motion of the air's gas molecules. The factors most responsible for the heating and cooling of the atmosphere are radiation arriving from the sun and radiation flowing from Earth.
The sun continually sends energy into space as electromagnetic radiation. One kind of solar radiation is visible light. The other forms of solar electromagnetic radiation are invisible to human beings. They include infrared (heat) rays and ultraviolet rays. About 30 percent of the solar radiation that reaches the atmosphere is reflected back into space, mostly by clouds. The atmosphere and Earth's surface absorb the remaining 70 percent, becoming warmer.
The warmed Earth cools by radiating infrared rays. Some of this radiation travels directly into space. The atmosphere absorbs almost all the remainder as it streams off the surface of the planet. This absorption of radiation is known as the greenhouse effect.
It makes the air near
Earth's surface about 59 Fahrenheit degrees (33 Celsius degrees) warmer than it would be otherwise.
The atmosphere also sheds heat energy by radiating infrared rays. Some of this infrared radiation flows down to the surface. The remainder travels out into space.
Air temperature varies from day to night and from season to season because of changes in the amount of radiation heating Earth's atmosphere. For example, days usually are warmer than nights. The reason is that Earth receives the heating rays of the sun only during the day. At night, infrared radiation from the planet streams off into space, and the air temperature drops.
Air temperature also changes with the seasons. Except near the equator, summers are warmer than winters. In the summer, the sun is higher in the sky. When the sun is higher above the horizon, the intensity of the sunlight striking Earth's surface increases. Days are longer. More hours of sunlight in summer also mean more solar heating. At the equator, however, temperatures remain constant the year around.
Altitude also affects air temperature. Within the troposphere, the air temperature generally drops
3.5 Fahrenheit degrees per 1,000 feet of elevation (6.5 Celsius degrees per 1,000 meters of elevation). Thus, it is usually colder on top of a mountain than in the surrounding lowlands.

Air pressure is the weight per unit of area of a column of air that reaches to the top of the atmosphere. As you move higher, there is less and less air above you. For this reason, air pressure decreases with increasing altitude. Air pressure is, on average, highest at sea level. It drops to about half its sea-level value at an average altitude of about 18,000 feet (5,500 meters).
Air pressure also changes from place to place across Earth's surface. Part of this change is due to differences in land elevation. Most of the remainder is caused by changes in air temperature.
Cold air is dense. That is, it has more air molecules per unit volume. As a result, it exerts high pressure. Warm air is less dense and exerts low pressure.
Regions where air pressure is high experience fair weather. Regions where air pressure is low experience cloudy, stormy weather. Generally, the weather stays fair or improves if air pressure rises. If the air pressure falls, however, the weather may turn cloudy and rainy or snowy.
Air moves from areas where the air pressure is high toward areas where the air pressure is low.
This movement of air is what we call wind.
Humidity is a measure of the amount of water vapor in the air. There is an upper limit to this amount. Air that contains its maximum amount of water vapor is described as saturated.
The amount of water vapor the air can hold increases as the air temperature rises. The capacity decreases as the temperature falls. Thus, saturated warm air has more water vapor than saturated cold air.
Weather reports describe the amount of water vapor in the air in terms of the relative humidity.
Relative humidity compares the amount of water vapor in the air with the amount of water vapor at saturation. It is expressed as a percentage. If the relative humidity is 50 percent, the amount of water vapor in the air is half of what it would be if the air were saturated. Lowering the air temperature increases the relative humidity.
If the relative humidity is 100 percent, the air is saturated. When air becomes saturated, water vapor begins to condense into droplets of water. Condensation is the opposite of evaporation. It is a change from a gas to a liquid. If the air is cold enough, at saturation the water vapor develops into tiny ice crystals.
If condensation occurs on a cold surface, such as the outside of a window at night, dew or frost forms. Dew and frost do not fall from the sky like rain or snow. Rather, they form when air in contact with a cold surface is chilled to saturation. The same process occurs when small drops of water appear on the outside of a cold soft-drink can on a hot day. The temperature to which air must be cooled to reach saturation and produce dew is known as the dew point.
When saturation occurs within the atmosphere, water vapor condenses into droplets or into ice crystals that form clouds. A cloud in contact with Earth's surface is known as fog. Most clouds occur within the troposphere. Because air temperature drops with increasing altitude within the troposphere, high clouds are cold. They consist mostly of ice crystals. The crystals give these clouds a fuzzy appearance. Low clouds are warmer. They generally are composed of droplets.
These clouds appear to have sharper edges.

Clouds form where air moves upward. As air ascends, it encounters lower pressure. Air responds to lower pressure by expanding. Whenever gases expand, they cool. As air cools, its relative humidity increases until it reaches saturation and clouds form.
Where air moves downward, clouds usually do not develop. Descending air is compressed. It warms up. Its relative humidity decreases. Saturation is not possible, and so clouds do not form.
Weather systems
Meteorologists classify weather systems according to their size and how long they last. The two largest and longest-lasting types of systems are planetary-scale systems and synoptic-scale systems.
Planetary-scale systems are the belts of winds that circle the globe. They may blow in the same direction for weeks at a time. Synoptic-scale systems cover a portion of a continent or ocean. They last up to a week or so. The term synoptic comes from a Greek word meaning a general view.
Two briefer and smaller types of systems are mesoscale systems and microscale systems.
Mesoscale systems may last an hour or less. They are so small they may affect the weather of only part of a city. Examples include thunderstorms and sea breezes. Microscale systems, such as tornadoes, last only minutes. They affect an area not much larger than a few football fields.
Planetary-scale systems.
Suppose that Earth did not rotate and that the noon sun was always directly above the equator. Air temperatures would be highest at the equator. Temperatures would decrease toward the poles. Cold air is denser than warm air. Thus, air pressure would be higher at the poles and lower at the equator. Because air moves from areas of high pressure to areas of low pressure, cold air would sweep toward the equator. There it would push the warm air upward. In the upper atmosphere, the warm air would move toward the poles, cool, and sink over the poles. Thus, the planetary-scale circulation of wind would consist of two huge cells, one in each hemisphere.
The real Earth rotates. Rotation of Earth on its axis causes winds that blow thousands of miles or kilometers to shift direction gradually. This shift is known as the Coriolis effect.
The Coriolis effect causes winds in the Northern Hemisphere to shift to the right. It causes winds in the
Southern Hemisphere to shift to the left. In the Northern Hemisphere, for example, winds blowing southward shift to the west. Winds blowing northward shift to the east.
Rotation of the planet also causes winds near Earth's surface to split into three belts in each hemisphere. These three belts are the trade winds, the westerlies (winds from the west), and the polar winds. The trade winds blow near the equator, between 30° north latitude and 30° south latitude. The westerlies blow in the middle latitudes between 30° and 60° north and south of the equator; The polar winds blow in the Arctic and Antarctic, from 60° latitude toward the poles.
Trade winds north of the equator blow from the northeast. South of the equator, they blow from the southeast. The trade winds of the two hemispheres meet near the equator, causing air to rise.
Rising air cools. Its relative humidity therefore increases. Thus, a band of cloudy, rainy weather circles the globe near the equator.

Westerlies blow from the southwest in the Northern Hemisphere. They blow from the northwest in the Southern Hemisphere. Westerlies and trade winds blow away from the 30° latitude belt.
Over broad regions centered at 30° latitude, surface winds are light or calm. Air slowly descends.
Air warms as it descends. Its relative humidity decreases, making clouds and precipitation unlikely. As a result, fair, dry weather characterizes much of the 30° latitude belt.
Polar winds are easterlies (winds from the east). They blow from the northeast in the Arctic and from the southeast in the Antarctic. The boundaries where cold polar easterlies meet the mild westerlies are known as the polar fronts.
A front is a narrow zone of transition, usually between a mass of cold air and a mass of warm air. Storms develop and move along the polar fronts. They bring cloudiness, rain, or snow.
Seasonal changes take place in Earth's wind patterns. Wind belts shift toward the poles in spring and toward the equator in fall. For example, during the fall, the polar front in the Northern
Hemisphere often moves from Canada down to the continental United States.
Planetary-scale winds control the direction of movement of smaller-scale weather systems. For example, in the tropics, trade winds steer hurricanes and other weather systems from east to west.
In middle latitudes, westerlies move weather systems from west to east. The westerlies are particularly vigorous near the top of the troposphere just over the polar fronts. There, they form corridors of strong winds known as jet streams.
The jet stream over a polar front supplies energy to developing storms. Then it moves them rapidly along the front.
Synoptic-scale systems include air masses, fronts, lows, and highs. The movement of these systems causes the day-to-day changes in the weather of Europe, the continental United States, and other regions in the middle latitudes.
How air masses affect the weather
Air masses.
An air mass is a huge volume of air that is relatively uniform in temperature and humidity. An air mass covers thousands of square miles or kilometers. The properties of an air mass depend on where it forms. Air masses that develop at high latitudes are colder than air masses that form over low latitudes. Air masses that form over the ocean are humid. Those that form over land are dry. The four basic types of air masses are cold and dry, cold and humid, warm and dry, and warm and humid.
Across North America, warm air masses move north and northeastward. Cold air masses move south and southeastward. Maritime polar air is cool and humid. This air mass forms over the
North Pacific and North Atlantic. It brings low clouds and precipitation to the Pacific Northwest,

New England, and eastern Canada. Continental polar air dry and cold in winter and dry and mild in summer. This air mass forms in north-central Canada. Arctic air is dry and much colder than continental polar air. it forms over the snow-covered regions north of about 60° latitude in the
Northern Hemisphere. The movement of Arctic air to the south causes the bone-numbing cold waves that sweep across the Great Plains and Northeast in winter.
Most of the maritime tropical air that invades North America originates over the Gulf of Mexico and the tropical Atlantic. This warm, humid air mass brings summer heat waves to areas east of the Rocky Mountains. Continental tropical air forms over the deserts of Mexico and the southwest United States. In summer, this hot, dry air mass surges over Texas and other parts of the American Southwest.
As an air mass travels, its temperature and humidity can change. For example, air over the
Pacific Ocean west of North America is mild and humid. If that air mass moves eastward, it is forced up the slopes of the coastal mountain ranges. Air temperature drops. The relative humidity increases to saturation. Clouds form, and rain or snow develops. As the air travels down the opposite slopes of the mountain ranges, the air temperature rises. The relative humidity decreases. Clouds thin out or vanish.
This process repeats with each mountain range the air mass encounters as it moves eastward. By the time it reaches the Western Plains, the air mass has become drier and milder. This modified air mass is known as Pacific air. It brings mild, dry weather to much of the central and eastern regions of the United States and Canada.
Fronts form where air masses meet. A front is a narrow zone of transition between air masses that differ in density. In most cases, the air masses differ in temperature. The fronts are either warm or cold.
Formation of warm fronts
A warm front is the leading edge of an advancing warm air mass. Warm air is less dense than cold air. For this reason, warm air advances by riding up and over the retreating cold air. As the warm air ascends, its temperature drops. Relative humidity increases. Clouds and perhaps precipitation form. Clouds can extend hundreds of miles or kilometers ahead of a warm front.
Rain or snow is usually light to moderate. It may last 12 to 24 hours or longer.
Formation of cold fronts

A cold front is the leading edge of an advancing cold air mass. Cold air is denser than warm air.
For this reason, cold air advances by moving under and pushing up the retreating warm air. As warm air ascends, its temperature falls. Relative humidity rises. Clouds and often precipitation develop. Clouds associated with a cold front typically form a narrow band along the front. Rain or snow falls in brief showers. If the cold front is fast-moving and well-defined by considerable temperature contrast across the front, thunderstorms are likely. Some of these thunderstorms could become severe. Severe storms might produce hail, torrential rains, or strong winds.
Tornadoes also may develop from severe thunderstorms.
A front that stalls is known as a stationary front.
The weather along a stationary front often consists of cloudiness and light rain or snow.
Cold fronts move faster than warm fronts. As a result, a cold front may catch up to and merge with a warm front. The warm air is lifted off Earth's surface. The merged front is known as an occluded front.
An occluded front sometimes moves slowly and causes several days of cloudiness and light precipitation.
Lows are areas of low air pressure. The winds in a low pressure system bring contrasting air masses together to form fronts. For this reason, lows are sometimes described as the chief weather-makers of regions in the middle latitudes. Scientists use the term cyclone to refer to a synoptic-scale low-pressure area. They also use the term to mean a hurricane in some parts of the world.
Viewed from above in the Northern Hemisphere, surface winds in a low-pressure area blow in a counterclockwise and inward direction. In the Southern Hemisphere, they blow clockwise and inward. Surface winds converging in the low cause air to rise, cool, and reach saturation. Clouds and precipitation develop. Air ascends mostly along fronts that develop as winds in the low bring cold and warm air masses together.
In North America, lows generally travel from southwest to northeast. They may complete a journey from Colorado to New England in three or four days. As a rule, temperatures are lower to the left (north) of the path followed by the low-pressure area and higher to the right (south). In winter, the heaviest snows usually fall about 90 to 150 miles (150 to 250 kilometers) to the north and west of the moving low-pressure area.
Highs are areas of high air pressure. They are also known as anticyclones.
A high, which brings fair weather, often follows in the wake of a low. Viewed from above in the Northern
Hemisphere, surface winds in a high blow in a clockwise and outward direction. In the Southern
Hemisphere, they blow counterclockwise and outward. As winds blow out and away from a high, air descends near the center of the system. Descending air warms. The relative humidity decreases.
Highs are either warm or cold. Warm highs form in the latitudes below the polar fronts. They are characterized by high temperatures and low relative humidities. Such highs are massive weather systems that extend from Earth's surface to the top of the troposphere. In the summer, a warm

high sometimes stalls over land. If the high remains stationary for several weeks, it can cause a drought.
Cold highs form in the latitudes above the polar front. They are shallow masses of cold, dry air.
They develop mostly in the winter over snow-covered regions, such as northern Canada, Alaska, and Siberia. Cold highs that move into the middle latitudes can bring fair but cold weather.
Mesoscale and microscale systems result from the development and movement of synoptic and planetary-scale systems. Mesoscale systems may last an hour or less and affect only part of a city. They include thunderstorms and sea breezes. A tornado is an example of a microscale system. A microscale system is the smallest and briefest of significant weather systems.
How people affect the weather
Human activities affect the weather both intentionally and unintentionally. For example, the construction of cities creates areas that are drier and warmer than the surrounding countryside.
Cities are drier because they have storm sewer systems that carry off rainwater and snowmelt.
Cities are warmer for several reasons. The use of storm drainage systems means that less solar radiation is used to evaporate water. Instead, more radiation heats the city surfaces and air. The brick, asphalt, and concrete surfaces of city buildings, sidewalks, and streets readily transmit the heat they absorb. As a result, they raise urban air temperatures even more. In addition, cities themselves generate heat from a number of sources. These sources include motor vehicles and heating and air conditioning systems.
Large urban areas also affect the weather in the areas downwind of them. Smokestacks and automobile tailpipes in cities emit water vapor and tiny particles that stimulate the formation of clouds. Heat energy rising from a city also spurs the growth of clouds. Thus, the weather downwind from many large urban areas is cloudier and rainier than the weather upwind from those same areas.
Urban and industrial areas also produce air pollutants. These pollutants include carbon monoxide, nitrogen oxides, and hydrocarbons. Although improved controls on factories and motor vehicles have reduced the amount of these gases released into the atmosphere, air quality problems persist. For example, many large cities still have problems with smog. Smog is a mixture of gases and tiny particles that reduces visibility and poses health hazards.
Smog and other air pollution problems are particularly serious in areas where winds are light and a temperature inversion occurs in the lower atmosphere. In a temperature inversion, warm air overlies cold air. As a result, the air temperature rises with increasing altitude. That is the opposite of the usual situation in the troposphere. Under such circumstances, smokestack and tailpipe emissions do not rise and disperse. Emissions may build up to unhealthy concentrations.
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Contributor:
• Joseph M. Moran, Ph.D., Professor Emeritus, Department of Earth Science, University of
Wisconsin, Green Bay; Associate Director, Education Program, American Meteorological
Society.
How to cite this article:
To cite this article, World Book recommends the following format:
Moran, Joseph M. "Weather." World Book Advanced . World Book, 2011. Web. 14 June 2011.
© 2011 World Book, Inc. All rights reserved. WORLD BOOK and the
GLOBE DEVICE are registered trademarks or trademarks of World Book, Inc.
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