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Name: Tina R.
Status: student
Age: 30s
Location: N/A
Country: N/A
Date: 2001


Question:
Is a heavy air mass the same thing as heavy air pressure? Why does heavy air pressure produce dry weather?


Replies:
A heavy air mass is associated with high pressure areas. Air has a certain capacity to hold water vapor that varies roughly with the density of the air. You can think of it as the "solubility" of water in air. The denser the air mass (high barometric pressure) the greater the amount of water it can hold without forming liquid water (clouds).

Vince Calder


Dear Tina-

I am assuming you mean by "heavy air mass," something more commonly known as "high pressure." Air circulating around high pressure systems is usually subsiding, that is, moving from a higher altitude to a lower one. This type of air movement tends to dissipate clouds and hold precipitation to a minimum.

Wendell Bechtold, meteorologist
Forecaster, National Weather Service
Weather Forecast Office, St. Louis, MO


Tina, Although I have not heard of the term "heavy air mass", I have heard people say that the air is heavy with humidity, meaning that it is very humid and usually very warm. This can happen when the air pressure is high (or heavy, as you say), but sometimes the air pressure can be high even when it is not at all humid. High pressure is an indication of air being piled on top of an imaginary air column through the atmosphere. The more air there is above the surface, the greater it's weight and thus the greater the pressure, as the air is compressed by the greater weight. The air is falling (very slowly) in a high pressure system, thereby suppressing cloud formation and precipitation. In that way it is "dry". However, I think that the dry high pressure systems that you are thinking of are those from the polar regions of Canada that drop south into the United States; these high pressure systems are dry because of their origin near the pole, where water vapor cannot build up in the initially cold air over the pole.

David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory


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