Threshold Values for Classifying Pressure Systems ``` Name: James Status: other Grade: other Country: Morocco Date: Fall 2011 ``` Question: 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? Replies: James From this URL: http://en.wikipedia.org/wiki/Atmospheric_pressure QNH 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 airfield QFE 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) of mercury. Anything above these values is a high pressure area, anything below it is a low pressure area. Sincere regards, Mike Stewart James, 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 pressure system, or "high," is an area where the atmospheric pressure is greater than that of the surrounding area. 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 weather system having a low or high pressure, respectively. David R. Cook Meteorologist Climate Research Section Environmental Science Division Argonne National Laboratory Click here to return to the Weather Archives

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