NASA - National Aeronautics and Space Administration

Weather is the condition of the air at a certain place and time. We describe the weather in many ways. For example, we may talk about the temperature of the air, whether the sky is clear or cloudy, how hard the wind is blowing, or whether it is raining or snowing. The weather may be warm and sunny in one place but cold and snowy somewhere else.

Image to left: Because Earth is tilted and spins around, large bodies of air and water in the northern half of Earth tend to move in a clockwise direction. That is, they move in much the same way as the hands of a clock. In the southern half of Earth, they tend to move in the opposite direction. This affects the movements of winds and ocean currents, which bring weather changes. Credit: World Book illustration by John Sandford

The Earth has many kinds of weather conditions. The highest temperature ever measured and written down was 136 degrees F (58 degrees C) in Libya in 1922. The lowest temperature was -128.6 degrees F (-89.2 C) in Antarctica in 1983. The driest place on Earth is Arica, Chile. It hardly ever rains there. Arica once had no rain at all for 14 years.

The Earth is not the only planet that has a variety of weather conditions. Every planet except Mercury has enough of an atmosphere to have weather. An atmosphere is a layer of air around a planet. One of Saturn's moons, Titan, also has an atmosphere. However, this article is only about the weather on Earth.

Scientists who study the atmosphere and the weather are called meteorologists. These scientists forecast the weather -- they tell what the weather will be in the near future. Some meteorologists provide weather information for businesses. The best-known meteorologists are those who give weather reports on radio and TV.

The kind of weather a place usually has is called its climate. Scientists can tell what a region's climate is by looking at the plants that grow there as well as the temperatures and the precipitation. Rain and snow are two kinds of precipitation. Weather can change quickly, but changes in climate take many years.

What Causes Weather

Weather takes place in the atmosphere, the layer of air around the Earth. Air is a mixture of gases and tiny bits of dirt and ash that are too small to be seen. The gases are about three-fourths nitrogen and one-fourth oxygen, with very small amounts of several other gases. These other gases include water vapor, an invisible gas that is produced when water evaporates. Water vapor forms clouds, rain, and snow.

Most weather takes place in the lowest part of the atmosphere, called the troposphere. It extends to about 10 miles above the ground. The three main things that affect weather in the troposphere are air temperature, air pressure, and humidity.

Image to right: When a warm front moves into an area, the warm air it brings rides up and over cold air. As the warm air rises, it cools and can hold less water. As a result, clouds may form or rain may fall. Credit: World Book illustration by John Sandford

Air temperature depends partly on the sun. The rays of the sun warm the air. Thus, days are usually warmer than nights because the sun shines on the Earth during the day.

Air pressure is the weight of the air. Air that is closer to the Earth's surface is heavier than air that is farther up in the sky. Cold air is heavier than warm air, so it has higher pressure. Cold air sinks and warm air rises. This movement of the air is what we call the wind.

Humidity is water vapor in the air. Warm air can hold more water vapor than cool air can. However, air can hold only a certain amount of water vapor. When air has all the water vapor it can hold, the water vapor turns into droplets of water that form clouds. These droplets join together, and if they get big enough, they become raindrops and fall to the ground.

Meteorologists study large patterns of weather called weather systems. Some weather systems are brought by winds. Others include the movement of an air mass -- a huge area of air that has about the same temperature and humidity.

Image to left: When a cold front moves into an area, it pushes warm air upward. As warm air cools, it can hold less water. Rain or snow often follows. Credit: World Book illustration by John Sandford

The four main kinds of air masses are cold and dry, cold and humid, warm and dry, and warm and humid. When two air masses meet, a front forms. A warm front is the leading edge of a warm air mass, and a cold front is the leading edge of a cold air mass. A cold front often brings rain or snow with it.

Measuring the Weather

No single country can report on all the Earth's weather all the time. Thus, the countries of the world must all work together to watch the weather. In 1873, a group of countries formed the International Meteorological Organization to share information about the weather. The organization changed its name in 1950 to the World Meteorological Organization.

Weather information comes from many different places. Observation stations check weather conditions from the ground. They use thermometers to measure air temperatures and rain gauges to measure rainfall or snowfall. Weather balloons, airplanes, and ships provide information on the atmosphere above the Earth's surface. Satellites in space provide weather information about large areas of the world.

Image to right: Some weather stations use radar to track how fast rain clouds and other air masses are moving, and where they are going. This can point to severe weather on the way. Credit: National Center for Atmospheric Research

How to cite this article: To cite this article, World Book recommends the following format: "Weather." The World Book Student Discovery Encyclopedia. Chicago: World Book, Inc., 2005.

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