Why does it get cooler as we climb higher if we are getting closer to the sun?

We put this question to physicist Caroline Steel...

Caroline - Firstly, the sun is 150 million kilometres away. So actually, climbing up a mountain or sort of moving a little bit further away from sea level is really quite negligible in comparison - there's 150 million kilometres. So that in itself shouldn't really have an effect on temperature. But it does get colder as we increase in altitude. There's lots of ways to look at this but one way to think about it is, as air rises, it feels less pressure the higher up it rises. So it expands. And to expand, the gas must do work and to do work, it must lose energy, and therefore, the individual gas particles have slightly less energy. And the definition of temperature is sort of the average energy of these molecules. So, if the molecules have less energy at a higher altitude, there's a lower temperature.

Kat - So basically, temperature, something that's hot, it's all the molecules that are squished together and then moving a lot and that's really hot. And then if you're going up into space, the air just kind of goes wooh!

Caroline - Yeah, exactly. It spreads out and by spreading out, you have less molecules with less energy and therefore, lower temperature.

Kat - David?

David - Yeah. What Caroline is describing is for lower atmospheres. This is probably what the questioner is asking about. But you get above the troposphere into the stratosphere, it starts to get warmer again because that's where ozone molecules are absorbing solar ultraviolet energy so it does get warmer. But there's another layer beyond that called the thermosphere where the temperature is officially thousands of degrees, but you wouldn't feel hot if you were there because it's almost a vacuum. It's just the energy of the individual molecules. It's absorption of solar photons that shake at the molecules.

Kat - Ah, so they're really going as we're up there.

David - They're going for it but they are so few and far between. There's high temperature but almost no heat.

Kat - And then when you get out of the atmosphere then you're really in trouble.

Caroline - So what would you sort of feel if there's a thermometer there? Would it have a high temperature or a low one, or does it sort of depend where it is - is it near the molecules or not?

David - What a thermometer is filled with mercury or something? I don't know. It depends how you define temperature. The molecular temperature is hot, but you would feel cold. You would need to be wrapped really, really warm apart from being almost in a vacuum.