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Name: Keith
Status: educator
Grade: 9-12
Location: FL
Country: N/A
Date: 6/21/2005


Question:
What barometric number is considered to be a "low pressure" indicator?


Replies:
Dear Keith-

"Low Pressure" is a relative term, and the same atmospheric pressure at a given location can be "low" or "high," depending on the pressure at adjacent locations. The air pressure at a point is constantly changing, due to diurnal variations, and the movement of weather systems. Seasonal variation plays a part also. Air pressure shows a bigger range or variability in the wintertime than in summer. Again, a given air pressure value might be considered high in summer, and low in winter.

The standard sea-level atmospheric pressure is 29.92 inches of mercury. So you might say that any value less than that would be considered "low," and vice-versa. But many low-pressure weather systems have the central low pressure higher than 29.92 inches, so again, absolute numbers are not useful in determining "low" or "high" pressure...

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


"Low pressure" is really arbitrary, but I suppose that an indicator would be the unit of 1 atmosphere, denoted, 1 atm. The conversion to other measures of pressure is:

1 atm = 760 mm Hg = 101325.01 Pascals (Pa) = 101325.01 newtons / meter^2 = 14.69595 pounds / in^2.

Vince Calder


Keith,

High and Low pressure are relative terms. There is no particular threshold number that separates the two. The barometric high pressure in summer can be much higher than the barometric high pressure in winter. Conversely, the winter barometric low pressure can be significantly lower than that in the summer. Weather maps show adjacent centers of high and low pressure, which are high and low in relative terms, when comparing them.

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


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