Question about low pressure areas

[seriously disabled]

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, 2008 by swansont
fixed link

[DJBruce]

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.

[swansont]

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

[CaptainPanic]

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.