What is a Low Pressure Area?

A deep low circulation brings heavy rains to the West Coast (Nov 28, 2012)
NOAA Environmental Visualization Lab

Besides being the red capital letter "L" you're used to seeing on weather maps, a low pressure area (or "low") is an area where air pressure is lower than it is at other areas surrounding it.

As a general rule of thumb, lows have a pressure of around 1000 millibars (29.54 inches of mercury).

Lows also tend to bring stormy weather and have counterclockwise winds. Let's explore why this is.

How Lows Form

In order for a low to form, something must happen to decrease air pressure over a certain spot.

This "something" is the flow of air from one place to another. It happens when the atmosphere tries to even out a temperature contrast, like that which exists at the boundary between cold and warm air masses. This is why lows are always accompanied by a warm front and a cold front; the differing air masses are responsible for creating the low center.

Low Pressure = Stormy Weather

Air rises near areas of low pressure, and it's a general rule of meteorology that when air rises, it cools and condenses. This is why lows tend to bring clouds, precipitation, and generally unsettled weather.

(More: How air pressure affects your weather)

The kind of weather a location sees during the passage of a low pressure system depends on where it is relative to the warm and cold fronts.

  • Locations in front of the low center (out ahead of the warm front) typically see cool temperatures and steady precipitation.
  • Locations to the south and east of a low center (a region known as the "warm sector") will see warm, moist weather. Because winds flow counterclockwise around a low in the Northern Hemisphere, winds in the warm sector are generally from the south. This results in milder air being fed into the system. Showery precipitation and thunderstorms also occur here, but at the boundary of warm sector and the leading edge of the cold front.
  • Locations behind, or to the west of a low center will see cold, dry weather. This is because the counterclockwise flow of winds around the low are from a northerly direction, suggesting colder temperatures. It's also typical to see conditions clearing here (the colder, denser air is more stable).

Of course, how extreme each of the above weather conditions are ultimately depends on how strong the low pressure system is.

Some lows are weak and only produce light rain and moderate temperatures, while others may be strong enough to produce severe thunderstorms, tornadoes, or a major winter storm. If a low is unusually intense, or "deep," it can even take on the characteristics of a hurricane.

Upper level low (troughs)

Sometimes surface lows can extend upward into the middle layers of the atmosphere. When they do this they're known as toughs.

There are 2 kinds of upper level lows, or troughs. A closed low . A cut-off low: