Part of the show Science Q&A Show
Sanda in the Netherlands asked:
My girlfriend and I were talking about flying in a plane and the fact that the higher you go, the colder it gets. How can this be seeing as sunshine and radiation are everywhere?
The atmosphere of the earth is pretty much transparent to the heat coming from the sun, which hits the surface of the earth and heats it up to about 1 kW per metre squared. This heat then gets irradiated back up into the air, warms the air close to the planet - and of course, you. But then the hot air rises, goes up into the atmosphere, expands and gasses get colder when they expand (this is why an aerosol gets cold when you use it). That's why as you go further up in the atmosphere, things get colder.