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When you see the red capital letter "L" on a weather map, you're looking at a symbolic representation of a low-pressure area (or "low"). A "low" is an area where air pressure is lower than it is in other areas surrounding it. As a general rule of thumb, lows have a pressure of around 1000 millibars (29.54 inches of mercury). Low air pressure tends 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. That's because the temperature is higher in the upper part of the atmosphere. As water vapor condenses, it creates clouds, precipitation, and generally unsettled 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 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 (the colder, denser air is more stable).
While it's possible, in general, to say "low pressure = stormy weather," every low-pressure area is unique. 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, or "deep," it can even take on the characteristics of a hurricane.
Sometimes surface lows can extend upward into the middle layers of the atmosphere. When they do this they're known as troughs. Troughs are long areas of low pressure that can lead to rain, wind, and other weather events.