What competing factors determine the thickness of the atmosphere? What causes atmospheric pressure?

Question

Thanks in advance!

Answer 1

temperature (kinetic energy of molecules) tends to expand it, gravity tends to compress it; hence pressure!

Answer 2

The temperature of the air and the height of the column of air above the point where the pressure is being measured.

If you could look at the top of the atmosphere you would see that it is not flat but appears to be expansive waves, same as the top of a body of water.