If air flows away only in the upper troposphere, why does it leave less mass on the surface?

 

http://www.weatherquestions.com/What_causes_low_pressure.htm

Edited August 23, 200817 yr by swansont
fixed link

I might be wrong but I believe the atmosphere is at or very close to equilibrium between the gases floating away and the earths gravity pulling them to the ground.

Less mass per unit area. The expansion pushes air out of the way, so overall it is slightly less dense.

If you remove some air at the top, then less air is pushing down... which is exactly what happens in a low pressure. (Low pressure, less pushing down).

 

Low pressure means lower density.