Threshold Values for Classifying Pressure Systems
Date: Fall 2011
At what pressure does a low pressure system become a high pressure system? Two sailors say 998mb and two airmen say 1013.2 Can you please clarify for us?
From this URL:
The barometric altimeter setting that will cause the altimeter to read
airfield elevation when on the airfield. In ISA temperature conditions the
altimeter will read altitude above mean sea level in the vicinity of the
The barometric altimeter setting that will cause an altimeter to read
zero when at the reference datum of a particular airfield (in general, a
runway threshold). In ISA temperature conditions the altimeter will read
height above the datum in the vicinity of the airfield.
Average sea-level pressure is 101.325 kPa (1013.25 mbar, or hPa) or 29.921
inches of mercury (inHg) or 760 millimeters (mmHg). In aviation weather
reports (METAR), QNH is transmitted around the world in millibars or
hectopascals (1 millibar = 1 hectopascal), except in the United States,
Canada, and Colombia where it is reported in inches (to two decimal places)
Anything above these values is a high pressure area, anything below it is a
low pressure area.
The definitions below came from geography.about.com:
A low pressure system, or "low," is an area where the atmospheric pressure is
lower than that of the area surrounding it. Conversely, a high
or "high," is an area where the atmospheric pressure is greater than
that of the
Saying that the pressure is high or low is therefore somewhat relative to
the characteristics of the air surrounding the air mass. You could say that
the pressure is high when it's above average and low when it's below
average, but this is an inadequate definition.
High and low pressure could be defined by saying that, on average, the air in
the Troposphere is generally rising in a low pressure system and sinking in
a high pressure system. The rising or sinking of air results in a
having a low or high pressure, respectively.
David R. Cook
Climate Research Section
Environmental Science Division
Argonne National Laboratory
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Update: June 2012