Numerous readers have written in to say that the answers given to this question in the issue of 16 December 1995 missed out the most important part of the argument. Indeed, they most certainly did. Owing to an oversight during the pre-Christmas rush the key letters which explained how expansion of a gas leads to cooling were omitted, leaving only some which were intended as a supplement to the main answers. We apologise to everyone who may have been misled. The Sun heats the Earth which is a better absorber, and the Earth then heats the air close to it.
Hot air rises because it is less dense than surrounding cooler air at the same pressure.
As it rises, a mass of hot air expands because the ambient pressure is less. This expansion cools the air, so the temperature of a thermal bubble decreases with increasing altitude until it reaches equilibrium. The explanations given previously all miss the fundamental reason, which is based on the gas laws. If air at a low altitude rises it moves into an area of lower pressure. If there is no heat transfer its temperature falls.