Why as we go higher the atmospheric pressure decreases?

Answer 1

I think it's a little odd that I haven't been able to find any internet reference to respond to this question. The answer is pretty obvious.

If you think about it, atmospheric pressure is nothing besides the weight of the air overhead. The odd little item is that damp air is actually lighter than dry air. This is because of Avogadro's Principle, which says roughly, "All gases at same pressure and temperature contain almost the same numbers of molecules," and because water vapor molecules (H2O) are significantly lighter (a little less than half as dense) than dominant dry air molecules (N2 and O2). Avogadro's Principle doesn't apply strictly here, because of course damp air yields generally lower pressures due to its lighter weight than dry air, but the basic principle still applies.

Large masses of air move around over Earth's surface, and they are generally not well mixed for a bunch of reasons. Sometimes those air masses move over oceans, or damp forests, or dry deserts. They exchange water with the surface of the Earth various ways, depending on all the ground conditions wherever they are and their own relative humidity. Air masses with higher water content will be less dense than the dry ones, meaning they weigh less and will thus exert less atmospheric pressure at given altitudes than drier air. So atmospheric pressure is nothing much more than a simple way to gauge the weight, and thus inversely the water content, of the large (and tall!) mass of air overhead in one's vicinity. And where there's water in the air above, there's likely to be water falling out of it. So that's why lower atmospheric pressure generally indicates damp weather, while higher pressure generally means clear, dry weather.

Edson C. Hendricks 12/29/07 18:08 PST

San Diego, California

Atmospheric Water Vapor is on thermodynamic equilibrium based on a force (pressure) which is affected by temperature and elements concentration normally located over an evaluating point. Therefore, if temperature increases then water vaporizes pushing away (displacing) other molecules suspended in the air and therefore diminishing atmospheric pressure. NOTE: Earth Poles have a poorly Water vapor content.

Jose R. Castellanos 10/26/2009

Minneapolis. MN