Higher Doesn't Always Mean Colder

Is it possible for something to be very hot, yet produce little heat? Surprisingly, the answer is yes.

Temperature is a measure of the velocity at which molecules of a substance vibrate. The molecules of ice water vibrate more slowly than the molecules in a pan of boiling water. Hot air molecules vibrate more rapidly than cold.

You would certainly not want to immerse your hand in a pan of boiling water, but what if your hand were to come into contact with only a few (or a few thousand) water molecules at boiling temperature? You wouldn't even notice them. This is why something can be very hot, yet produce little heat. The density of the vibrating particles in a given volume of space determine the actual amount of heat that is given off.

This produces one of the most bizarre of earth's many peculiarities. We are all accustomed to thinking that it gets colder as elevation increases, and this is true as long as we are still within the realm of an appreciable atmosphere. However, when we begin to enter the ionosphere, where the air is too thin and the molecules too widely dispersed to carry even sound, the temperature actually begins to increase. This is because the sun's radiation adds energy to the molecules and causes them to vibrate faster, while at the same time, there are fewer other molecules around for them to bump into and slow them down. In fact, the temperature of a molecule at an altitude of 400 miles approaches 2300°F, and this temperature gets even hotter up to about 600 miles where the earth's gravitational field can no longer hold onto the now highly-energetic particles and they escape into space.

If an astronaut at this altitude were to stick his hand out of the window, he would not feel heat because the molecules are so widely dispersed. He would encounter instead a very bitter cold, unless the hand was in the sunlight, in which case he would get a very bad sunburn because the "air" is too thin to shield the sun's rays.