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Name: Julie
Status: Educator
Age: 20s
Location: N/A
Country: N/A
Date: N/A 


Question:
Why does temperature decrease as one goes higher in the atmosphere?



Replies:
Dear Julie-

...your question about how the atmosphere is heated is a good one, because the correct answer is not quickly apparent...

You would surmise, that since the sun is the source of the heat for the earth and atmosphere, that the atmosphere should be warmer at points closer to the sun, like at the TOP of the atmosphere, instead of the reverse.

But the atmosphere is not warmed directly by the sun...it is warmed by the EARTH...! Most of the energy from the sun arrives at the earth in the form of light energy, or short-wave energy. The atmosphere is transparent to short-wave energy, and most of the solar energy reaches the surface of the earth, where it is absorbed. This energy warms the surface of the earth, which, in turn, warms the atmosphere with long-wave energy. The type of energy radiated by a star primarily depends on the temperature. So the energy from the sun arrives at the earth as short-wave (in the visible part of the spectrum) energy, and leaves the earth as long-wave (in the infra-red part of the spectrum) energy.

The atmosphere is not transparent to long-wave radiation, and absorbs the heat energy from the earth. That part of the atmosphere closest to the earth gets the most energy and is the warmest.

Because the sun is nearly overhead in the latitudes nearest the equator, those locations receive more solar energy per unit area, and become warmer than those locations near the poles. This difference in the amount of energy received and subsequent heating of the atmosphere and evaporation of water from the lakes and oceans, is what drives the "weather machine" on the earth.

I hope this has answered your question, Julie. If not, please let me know, and I'll provide you additional information.

Wendell Bechtold, Meteorologist
Forecaster, National Weather Service
Weather Forecast Office, St. Louis, MO



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