From: Steve (Primus) (Avatar) 30/10/2001 5:55:21 Subject: re: STEVE'S WEATHER FAQ post id: 476432 Why does it get colder in the wee hours. Why is it coldest at dawn? Radiation from the sun is absorbed by the earth which warms up. The Earth then radiates in the infra-red range back out into space. At sunset, the Earth stops getting heated by the sun but continues to radiate into space. The earth cools down and cools the atmosphere in contact with it. This cooling continues until the sun comes up again and starts warming the Earth up. The cross-over point where the incoming radiation becomes greater than the outgoing radiation is just after sunrise, so that is the coldest part of the day. In the afternoons, the crossover point where the outgoing radiation becomes greater than the incoming radiation is mid to late afternoon - and that is the hottest time of the day. The Earth a) emits heat into space and b) receives heat from the Sun. If b-a < 0 then the Earth is cooling. If b-a > 0 then the Earth is heating up. The coldest time is when the Earth goes from cooling down to heating up. Since b=0 until after dawn, this transition cannot happen until after dawn. The sun's rays, before it has risen, hitting the upper atmosphere and warming up way up there. The troposphere is mainly heated from below. The warm earth heats the air in contact with it which, in turn, heats the air above that. Some of the outgoing longwave radiation from the earth is absorbed by gases in the troposphere which also adds to the heating. Descending air, such as in the sub tropical ridge, is heated adiabatically by being compressed as it descends. Heat in the upper atmosphere has little if any effect on the surface. Please remind me what adiabitic means? Adiabatic processes presume that changes in temperature, pressure and density within a parcel of air are caused by changes in altitude without losing energy to the outside or gaining energy from it. In real life, of course, this does not happen but it makes a useful model for describing atmospheric processes.