Why are Clear Nights Cooler Than Cloudy Nights?

Dear Student, During the night, temperatures drop due to the emission of heat energy from the ground. The primary mechanism for the escape of heat energy from the ground is infra-red (IR) radiation. Any body with a temperature above absolute zero emits electromagnetic radiation whose intensity and frequency depend on its temperature. This radiation is also known as thermal radiation or black-body emissions. Bodies at temperatures common on Earth emit thermal radiation at micron wave lengths – infra-red (IR) radiation. This radiation is emitted to and from the ground both during the day and at night. However, during the day there is also solar radiation absorbed in the ground. Although the radiation balance is not the only mechanism that determines ground temperature, to a close approximation when the radiation absorbed in the ground is greater (less) than the radiation emitted from the ground, the temperature will rise (drop). At night, there is no contribution from solar radiation, and the temperature drops, because the radiation emitted from the ground exceeds the radiation absorbed from the air above. Clouds absorb and emit radiation. (High-altitude clouds emit very little thermal radiation as they are very cold). Therefore, if there is considerable cloud cover, a significant fraction of the radiation emitted from the ground is absorbed in the clouds and reradiated back into the ground. This is true even during complete overcast conditions because the clouds are generally colder than the air at the ground. Thus clouds in effect reduce the radiation imbalance between the upward flux from the ground and downward flux from the atmosphere, relative to that on clear nights. The cloudier the sky and the closer the cloud cover is to the ground, the more moderate will be the decrease in ground temperature. That's why cloudless nights are cooler than cloudy nights, regards