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Barometric Low Pressure Standard
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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Update: June 2012
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