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

01 November 2009

Question

Why is it that the sun warms us up on Earth while it remains cold above us despite the fact that above us is really closer to the sun and at some point closer to the sun it’s probably really hot so at what point does it become freezing cold?

Answer

Chris - If you have a gas, which is, say, in an aerosol, says the contain you're going to spray into your armpit with 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. I thought about 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 somebody's got less energy, is therefore going to 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. Okay. The temperature of things on Earth is sort of balance between the amount of heat which is getting either from the sun or from heat moving around the world and the amount of heat it can lose by radiating into space or radiating anything at room temperature, radiating heat and infrared really quite well. And so, basically the only things which can absorb sunlight very well tend to be at 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. And now, the tops of mountains are very, very small. So basically, what's the temperature of the atmosphere at 30,000 feet? 10,000 meters, or 9,000 meters? And 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 about half the pressure it was before, which means that gas expands and 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.

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