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How is an air mass formed?


Warm air masses originate to the south and cold air masses originate to the north.

Cold air masses typically form because there is less solar radiation where they form, near the Earth's poles. This usually occurs when there are no clouds; energy is lost from the Earth's surface at night, cooling the air. In the winter, snow cover near the poles reflects sunlight away during the day, thereby not allowing the Earth to warm. If this is repeated for several days and nights in a row, the air can cool considerably over a large area.

Warm air masses tend to form in areas nearer the equator or over extensive desert areas such as the southwestern USA, where there is greater solar radiation. In clear to partly cloudy conditions the Sun warms the Earth, and the energy is radiated to the atmosphere, warming it. In desert areas the air temperature can become very high, expanding the warm air mass tremendously. Many of our western and plains states droughts are caused by this process.

At the equator the temperature can also become very high, partially because of high solar radiation, but also because the relative humidity is high; the large amount of water vapor in the air absorbs much of the energy radiated by the Earth, warming the air.

David R. Cook
Climate Research Section
Environmental Science Division
Argonne National Laboratory

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