Why is thin mountain air colder than the other kind?

Question

Why is thin mountain air colder than the other kind?

Answer 1

Well the answer above could not be more wrong.

Temperature is really a measure of the amount of kinetic energy that air molecules have at a specific altitude and temperature.  When its "hotter" outside, the sun is providing more energy to the air molecules, which causes them to move around more sporadically.  This in turn causes them to impact your skin, and transfer energy to your body (making you feel warmer), as well as the sun's direct radiation making you warm.

As you go up in altitude, the air becomes thinner and thinner (less dense).  Therefore, the number of air molecules available to transfer energy to your body becomes less and less.

Answer 2

It's not about the density of the air it's about the altitude. The farther away from the ground (sea level) you are, the less heat there is reaching you. That's why it's colder at higher elevatations.

Think about it this way: space is a vacuum, and a very cold vacuum at that. The higher up you are, the closer to the cold you are. Also, the atmosphere is like insulation for the earth. The higher up you are, the less air there is separating you from space, and less insulation, so you lose more heat.