In Meteorology, What Is a Low-Pressure Area?

Does low air pressure always mean stormy weather?

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

When you see a red capital letter "L" on a weather map, you're looking at a symbolic representation of a low-pressure area, also known as a "low." A low is an area where air pressure is lower than it is in the areas surrounding it. As a general rule of thumb, lows have a pressure of around 1,000 millibars (29.54 inches of mercury).

Here is how these low-pressure systems form and how they affect the weather.

How Low-Pressure Areas Form

In order for a low to form, the flow of air must move from one place to another, decreasing the air pressure over a certain spot. This 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 low-pressure areas are always accompanied by a warm front and cold front; the differing air masses are responsible for creating the low center.

Low Pressure Typically Equals Unsettled Weather

It's a general rule of meteorology that when air rises, it cools and condenses. This is because the temperature is higher in the upper part of the atmosphere. As water vapor condenses, it creates clouds, precipitation, and generally unsettled weather. Because air rises near areas of low pressure, this type of weather often occurs in lows.

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

  • Locations in front of a 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, which results in milder air being fed into the system. Showery precipitation and thunderstorms also occur here, but they are specifically at the boundary of a 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 as the colder, denser air is more stable.

While it's possible to generalize and say that low pressure automatically means stormy weather, every low-pressure area is unique. For instance, mild or extreme weather conditions develop based on the strength of the low-pressure system. 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, it can even take on the characteristics of a hurricane.

Sometimes surface lows can extend upward into the middle layers of the atmosphere. When this happens, they are known as "troughs." Troughs are long areas of low pressure that can also lead to weather events like rain and wind.