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Name: David
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Question:
The standard atmospheric pressure at sea level is 29.921" Hg. at 25,000 feet, 11.10" Hg, 50,000 feet, 3.436" Hg. Now on a rainy day, at sea level, it may be, say 29.0". Is it still the same differences at 25,000 feet and 50,000 feet? Perhaps 25,000 ft will change, and 50,000 ft will not change since it is above the rain bearing clouds? In other words, is a low pressure system effecting pressure at all altitudes?

Replies:
David,

This is a great question.

The standard atmosphere values are an average and rarely occur at a particular location. Whenever there is an atmospheric system in the vicinity, whether a high pressure or a low pressure system, the pressures at various altitudes can be affected by that system.

In the case of your low suggested low pressure system, the pressure at a particular altitude is affected by the strength and depth of the low pressure system. Very strong low pressure systems (which, by the way, have the lowest pressures) often extend very high up into the atmosphere and thus lower the pressure at a large range of altitudes. Weak low pressure systems have less affect on pressure with altitude.

You can tell the strength of a low pressure system by looking at the weather (or pressure) maps at the surface, 850 mb, 500 mb, and 300 mb. If you see a closed low or a deep trough all the way up through 300 mb (particularly if the center of the low at those levels is in the same location), you can assume that it is a strong and active low pressure system.

The presence of clouds and precipitation does not always reflect an affect on pressure with altitude. But it is likely that if it is raining as a result of a low pressure area being present, the pressures at most altitudes will be lower than the standard atmosphere pressures.

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

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