High or Low pressure


Just a question about barometer pressure…

On the weather charts, among the isobars forecasters highlight areas as “High” and “Low” pressure. My question - Is there a definitive boundary value that defines where Low pressure ends and High pressure starts? You never hear forecaster refer to “medium” pressure.
E.g. my lowest pressure over the past few months has been 1000mb and the highest 1031mb. So what is 1015mb - High pressure or Low pressure?

thanks for any ideas


I long had this question and finally asked a met at my local NWS office. According to NWS guidelines, “high” and “low” pressure are relative, with no specific boundary between the two. If there’s an area of seemingly high pressure - say 1015 mb, with a focused area with a definitive drop to 1013 mb, the area of 1013 might be shown as a “low” as it deviates considerably from its surroundings.

Some say anecdotally that pressure above 29.92 (1013.2 mb) is high pressure and below that is low, but that is contrary to NWS, UK Met Office, and WMO practice.

I like to think of the highs and lows as being the summits of mountains and the lowest points of valleys. As Gary has said it’s all relative…

  1. Imagine having a small mountain…it still has a summit.

  2. Imagine a valley half way up a very high range of mountains…the lowest point of the valley may be much higher than the small mountain in (1).

The important thing to remember is that air moves (often in complicated ways) from places with high(er) pressure towards places with low(er) pressure.

Pressure is such a big issue in the UK, because we have weather, therest of the globe enjoys a climate. The hill tops and valleys is a nice model to use, but you have to also think about kinetic energy (energy of motion). Air is made up of the molecules and atoms that compose our atmosphere. Warm air is less dense than cooler air because the gas molecules in warm air have a greater velocity and are farther apart than in cooler air,therefore they exert less pressure than cooler air.

The standard air pressure is 14.7 pounds per square inch or 1013.25 mb (29.92 inches of mercury). World wide, the pressure ranges from 970 to1040 mb (28.64 to 30.71 inches of mercury) on the barometer. Another factor that affects air pressure is humidity. As the temperature of a column of air increases, so does its capacity to hold more water vapor. The more water vapor that is held in a column of air, the less pressure it will exert when compared to the same volume of air at a cooler temperature.

So medium pressure is the front, above the UK, where all that wet warm air collides with the cold dry air, and falls down, sometimes sideways, onto little old England. :roll:

Air rotation is also a factor when describing these features. Cyclone and anticyclone. If you are an aviator in light aircraft in particular, to assess your potential drift you relate this to your altitude.

also curvature of the isobars around intense highs, can increase the wind to stronger than otherwise expected
and long straight isobar lines can give stroner than expected winds too

And intense lows!