Basic atmospheric physics
Luke Wilson, of Balclutha Primary School, asks:
If hot air rises, why do mountains have snow at the top? Is it because the air is thinner?
Gavin Fisher, an atmospheric physicist with the National Institute of Water and Atmospheric Research (NIWA), responded:
The distance from the earth to the sun is roughly 150,000,000 kilometres, which is enormously larger than the 10km height of even the tallest mountain. It is not much of a difference really.
So why is it colder as you go higher?
It all comes down to the basic physics of our atmosphere. Sunlight contains quite a lot of energy, about 1000 watts per square metre. Most of this is absorbed by the earth’s surface and warms things up. Air near the ground gets warm because it is near these things. This warm air rises and mixes up, making the air around us the temperature that it is. This is called convection.
But, the heating of the air does not work all the way up through the atmosphere. The air is kept near the earth because it is held down by gravity, just like you are. The force due to gravity gets weaker the higher you go and the air gets thinner.
Convection can only carry the heat a certain distance up in the atmosphere. Usually this is just a fewhundred metres. So the temperature of the air is just about always hottest right near the earth’s surface, and cools off as you go higher. It cools at about 10degC for every kilometre higher you go.
Even on a nice sunny day, the temperature at the height of jet aeroplanes can be 50degC.
Usually it doesn’t get quite this cold on mountains, because the sun is heating up the surface of the mountain. But it is just about always colder as you go higher in the atmosphere.
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