What is a low pressure system?

Dateline article| David Murphy|

by David Murphy

A low pressure system is usually the center of cloudy, wet and stormy conditions on any weather map.

Also known as an extratropical cyclone or simply a low, it marks an area where large volumes of air have been lifted from the surface, leaving behind less air and lower barometric pressure. Recall that air pressure is basically a measure of the amount of air molecules pushing down on any given spot on the earth. Remove some of these molecules and the pressure becomes less. A low has less pressure because air at its center is rising away from the surface, reducing the amount of molecules available to exert pressure.

The reason the low is the center of precipitation is precisely because of all that rising air. As stated elsewhere in Weather Class, rising air leads to condensation, clouds and precipitation. At the center of a low, great volumes of air are often rising, so the potential for making rain or snow is often very great.

The air within a low pressure center flows relatively quickly, when compared to other areas of higher pressure. This can make for turbulent, windy weather at the surface beneath a low.

Another unique feature of most lows is that they are usually the source of cold fronts and warm fronts. As two different air masses bump against each other, kinks develop. As these kinks (or curves) grow more and more pronounced, low pressure centers begin to form and begin to rotate, thanks to various atmospheric forces like pressure and gravity. This rotation pulls colder air down around one side of the developing low and warm air up around the other side. You now have fronts (the cold front marking the leading edge of that moving cold air, the warm front marking the leading edge of the moving warm air).

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