Cold mountains, hot air: it's all about pressure

A climate scientist explains how the sun heats the Earth, which warms the air that rises and immediately begins to cool as it loses pressure.

- By James Renwick

That is an excellent and thoughtful question, Ollie - why indeed?

You're right, when air is warmed, it rises. This is what gives us the "thermals" gliders can use to soar upwards and large birds of prey such as the South American condors use to help them stay aloft for hours at a time.

But there are lots of other things influencing air temperature. When air rises, it expands because air pressure decreases with height. The energy in the air gets spread out over greater volumes and its temperature goes down.

This effect wins out over warm air rising. The warm air in a thermal will cool as it rises, until it reaches the temperature of the air around it and is no longer buoyant.

But why do we have rising air at all?

That's because the air around us is heated from below, from Earth's surface.

When the sun is shining, it doesn't heat the air in the lowest few kilometres of the atmosphere (the troposphere) as there are very few gases in that air to absorb sunlight.

The sun's rays heat Earth, not the air. The air is then warmed from below, from the ground, just as water in a pot on a stove is warmed from the bottom of the pot.

Earth’s greenhouse