On striking the earths surface,most of the insolation is directly reflected off and only a small fraction is absorbed. If thir was not so, the earths surface melts due to greater heat. The energy, thus absorbed by the earth ,is re-rediated by it this re-rediation,called terrestrial radiation,is of very much longer wave length than insolation and hence cannot easily penetrate the environment.
Terrestrial radiation ,therefore heats the atmosphere .whereas insolation can easily penetrate through clouds ,terrestrial radiation cannot. That is why cloudy nights are much warmer than nights with clear sky.
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