Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne 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

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory