Why does the sun warm us up on earth but remains cold at higher altitudes?

Chris - If you have a gas, which is, say, in an aerosol you're going to spray into your armpit, like your deodorant, there's a gas under pressure in there.

When you spray it in your armpit, it feels very, very cold. What's happened? Well the gas has expanded. Put simply, if you imagine there's some kind of piston inside the aerosol can, when the gas expanded it effectively pushed on the piston, it's done some work, let's say.

If something has does some work, it must have less energy after it's done the work than before it did the work.

Since temperature is proportional to the energy in the particles, if something's got less energy, it's therefore going to be at a lower the temperature; so the temperature must fall and that's why we think that when a gas expands, the temperature goes down.

Dave - OK. And now this is actually very related to why mountains are cold. The temperature of things on Earth is sort of in balance between the amount of heat which is arriving either from the sun or from heat moving around the world, and the amount of heat it can lose by radiating into space.

The only things which can absorb sunlight very well tend to be on the ground. The atmosphere is transparent, so the heat is going into the ground and heating it up and then that heats up the air above it.

The tops of mountains are very, very small. So, basically, what's the temperature of the atmosphere at 30,000 feet? The reason why that's very cold is because if you have pockets of air which is being warmed up on the surface of the Earth and then it lifts up by convection, it's moved upwards, the pressure drops to about half the pressure it was before, which means that gas expands. As gases expand, they get cold, so the air gets very, very cold.

So, the air around the mountain is very, very cold and also anything which is pointy like a mountain has got lots - can emit infrared light in lots more directions and the flat thing has got more surface area compared to mountain sunlight which hits it. So it cools down better during the night and emits light into the space very much better so it tends to be very cold.