why is it that temperature falls as we go up in the atmosphere?

Answer 1

pressure.

Temperature and pressure are directly related. If one goes up, the other goes up. If one goes down, the other goes down.

As you get closer to the surface, the air pressure raises. This is because gravity is "pulling" all the air molecules down and the ones closer to the surface have more molecules above them "pushing" them down.

So this higher pressure near the surface means higher temperature as well.

Answer 2

Our atmosphere is almost tansparent to the incoming short wave solar radiation.This means that the atmosphere is not directly heated by the solar radiation.The short wave solar radiation heats the earth's surface first and then heats the atmosphere above the earth's surface by conduction as well as by its reflected long wave radiation.Thus the atmosphere is heated from the bottom.That is why temperature decreases as you go up.

Answer 3

Light from the sun does not turn into heat until it is absorbed by something, usually the surface of the earth gets 95% of it. Part of the heat is then re-radiated as infra-red which can be absorbed by the atmospheric GHGs and here the higher density of the air at lower altitudes does play directly.
Convection and Conduction do the brunt of the work and both of these are surface up effects.
So you it is the earth that is hot, not the sun. So it makes sense that it is hotter at the surface than at altitude.

Answer 4

The heat in the atmosphere comes from the ground not the sun. The sun heats the ground which then heats the air above it. The heat is passed along up the atmosphere by transferring the heat through molecular collisions. As you go up the number of air molecules gets less and less so there are fewer collisions translating to less heat transfer.

Answer 5

Pressure. Let me try to make it clearer.

As the air moves up the pressure gets less and the air expands to a larger volume. That absorbs energy and makes the air cooler.