The atmospheric greenhouse effect explains how clouds influence the temperature experienced at the surface. All substances emit electromagnetic radiation. When the sun goes down, there is no longer a heat source to warm the ground. The ground is continuously emitting electromagnetic radiation just as all substances do. A greater emission from the ground of electromagnetic radiation than is being absorbed results in the ground cooling. At temperatures experienced on Earth’s surface, the electromagnetic radiation emitted will be longwave radiation. When no clouds are present and when the air is dry, longwave radiation that is emitted from the surface will escape to space. Thus, under similar conditions except for a difference between cloud cover/water vapor and dry air, the night with clear skies and dry air will be cooler. Lack of cloud cover at night is a cooling agent. When clouds are present, some of the longwave radiation emitted by the ground will be absorbed by water vapor and clouds. This results in a warming of the water vapor and clouds. The water vapor and clouds are also emitting longwave radiation. Since the ground is not just emitting longwave radiation, but is also receiving it, the ground will not cool off as much. Thus, cloud cover and abundant water vapor is a warming agent. It will be warmer than it otherwise could have been. The diagram below shows a comparison of longwave radiation emission at night without clouds and with clouds. When clouds are present, the ground will receive more longwave radiation than it otherwise would have and this produces a warming. During the day, instead of clouds contributing to causing the temperature to be warmer, clouds act as a cooling agent. This is because clouds are highly reflective of solar radiation from the sun. Since less solar radiation reaches the surface, the ground does not warm up as much. On a clear day, more solar radiation will reach the surface and this contributes to warmer temperatures. Why doesn’t the atmospheric greenhouse effect produce warmer temperatures even during the day since the clouds are emitting longwave radiation that the ground absorbs? It is because solar radiation is of the shorter wavelength variety and shorter wavelengths have more energy. Thus, solar warming is more effective at warming the ground than is the atmospheric greenhouse effect. Since clouds reflect a high percentage of the shortwave radiation back into space, the ground is cooler on a cloudy day as compared to if that same day would have been clear.  |