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USGS CMG InfoBank: Subtropical Deserts

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Comment: 04:14 - 05:45 (01:31)
Source: Annenberg/CPB Resources - Earth Revealed - 22. Wind, Dust and Deserts
Keywords: orbit, Sun, tropic, Equator, evaporation, condensation, cloud, rainfall, air, humid

Our transcription: As the Earth orbits the sun from season to season, the sun always shines directly overhead somewhere in the tropical latitudes.
Therefore, the sun's rays strike the Equator more directly than that rest of the globe.
As the air heats up, water evaporates.
The warm, wet air rises, and as it rises it expands and cools.
The vapor it contains condenses into clouds, many of which release rain.
The cool air is now dry, but more and more warm air is rising up beneath it.
This displaces the cool air away from the Equator pushing it north and south toward the subtropics.
Because of Earth's spheroidal shape air currents begin to crowd together as they move into higher latitudes.
This causes the air to grow dense and heavy, so that it descends earthward about 30 degrees north and south of the Equator.
The air is compressively heated upon reaching low altitudes.
The result is a mass of warm, dry air, few clouds, and low humidity.
This encourages evaporation.
The result is the stark, parched landscape of most of the world's deserts.

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