[Catryn Collins]

Catryn Collins asked the Naked Scientists:
   
Hot air rises above cold air in the house. Why is it that if you go up high in an aeroplane or up a mountain, it gets colder and not warmer?

Catryn age 10

What do you think?

[graham.d]

Hi Catryn. Good question. Air is transparent so the sun's rays don't heat it very much directly. The sun does heat the ground and the sea though, and the ground (and the sea) then heats the air above it. It does this mainly by convection. That is the air is in contact with the ground, it warms up and becomes less dense. This is why it then will rise and displace cooler air above it. The cooler air has to go somewhere though so it swaps places with the warmer air and sinks to the ground. Everywhere on a sunny day this will be happening. 

Hot air rises because it expands and becomes less dense (the same weight of hot air is much bigger than the same weight of cold air). But air also gets less dense as you go upwards because there is less weight of air above it pushing down on it and squeezing it. If you go high enough it gets thinner and thinner so that eventually there is no air at all and you would be in space. You could think of the thin hot air only rising to a height where the air is just as thin, then it won't rise any more. It will just stay there and mix with the colder air. So even though hot air rises, it only does this a certain amount. So generally it is always colder higher up. There can be special cases with funny weather where the tops of mountains are warmer than the ground (this is called an inversion) but it not the normal thing to happen.