6. Give reason, why:
(i) Cloudy nights are warmer than the clear nights.
(ii) Sunlight is hot while moonlight is cool.
Answers
(i) That is very simple. All the light absorbed by earth is radiated back to the atmosphere as infrared rays and nearly escape (in north India people throw cold water on bricks laid floor-British idea- to cool down the surface-during the summer evenings); otherwise the Earth would have become so hot that we will not exist today. Even the short waves (ultra-violet and the visible absorbed by the Earth is radiated back to the outer space as infrared. Clouds, on the other hand absorb most of the infra-red radiated by Earth and sends back. Snow and ice on top of hills reflect back instantly the sunlight. Refer “albido” the reflecting power of different materials. A cloudy night will be warm. A clear sky will bring “Fogs” if enough moist air is there and no surface wind.
Cloud cover is the most demonstrative form of water vapor in the atmosphere.
When the sky is overcast, it blocks to a great degree the radiational cooling from the earth, including your body.
Without cloud cover, infra-red radiation transmits into space. If one has an infra-red thermometer or camera, one can demonstrate by taking a picture of the sky on a cloudy and a clear night. On a clear night, at least on my FLIR camera, the infinity default is -40C. even on a warm night. On a cloudy night, it is similar to air temperature.
It is also known that in the desert, even when it may be hot during the day, the lack of water vapor in the atmosphere allows deserts to become cold at night, by this same effect.
(ii). Sun is a planet. The rays of sun fall on the Moon, later it reflects its rays towards Earth. Sun's rays directly fall on Earth, so these are very hot where as the Moon reflects the Sun's rays which they fall on the Moon, so these are cool.