Why cloudy nights are warmer than clear nights? Most insolation is directly reflected off when it strikes the earth's surface and only a small quantity of insolation is absorbed. If this was not so, the earth's surface would melt due to the great heat. Thus the energy absorbed by the earth is re-radiated by it.This re-radiation is called terrestrial radiation and its wavelength is very much longer than insolation and hence cannot easily penetrate the atmosphere.Whereas insolation can easily penetrate through clouds, but terrestrial radiation cannot. Therefore terrestrial radiation heats the atmosphere.Terrestrial radiation tries to go out to space at night and it is reflected back to the Earth's surface by the clouds.That is why cloudy nights are much warmer than nights with clear skies. Ver más