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Name: Marcel T.
Status: other
Grade: other
Country: Canada
Date: Summer 2012


Question:
As an air mass rises in the atmosphere does the pressure change faster in warm or cold air?


Replies:
In cold air, because it is denser at the same pressure. Pressure within an unconstrained fluid is from the weight of fluid above it. In cold air, weight and thus pressure decreases more quickly with increasing altitude.

Richard E. Barrans Jr., Ph.D., M.Ed. Department of Physics and Astronomy University of Wyoming


It is difficult to make a generalization. Consider the ideal gas law: PV = nRT --> d = (n/V) = P/RT where “d” is the molar density. So the lower the density the faster the air will rise. However, the density is proportional to “P” and inversely proportional to “T” (using consistent sets of units). But “P” and “T” can change independently. So it is the ratio of P/T that determines the density.

Vince Calder


Marcel,

As an air parcel rises, it cools as it expands, as it is rising into cooler air. The faster the decrease in temperature with height, the more quickly the parcel expands. The pressure in the parcel decreases as it rises into decreased atmospheric pressure with height.

So, the temperature of the air surrounding the parcel, whether warmer or colder, is not the determining factor. The most important factors are the rates of decrease of pressure and temperature with height. The faster the parcel rises, due to the extent of decreasing pressure and temperature above it, the faster the pressure in the parcel will decrease.

David R. Cook Meteorologist Climate Research Section Environmental Science Division Argonne National Laboratory


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