That capital “L” on weather maps stands for low-pressure system, and it usually means cloudy or stormy weather is at hand. A low-pressure system is an area of weather in which the barometric pressure is lower than the surrounding air.

When the barometric pressure is lowered and a low-pressure system forms, it brings in air, usually from a cold front to the north. The counter-clockwise circulation brings the air in and as the cooled air rises, it forms clouds and precipitation. A low-pressure system forms based on the interaction between the atmosphere and large bodies of water.

A low-pressure system generally comes in conjunction with, or as a crossroads for a front, or area of markedly warmer or cooler air. The air near the low-pressure system often becomes unstable and if other conditions are favorable, severe weather can result from a low-pressure system and the passage of a cold front.

Tornadoes, in fact, always have low air pressure inside their funnels. A strong low-pressure system in winter may bring snow and ice to the areas it covers. This kind of low-pressure system is often seen as a comma-shaped cloud mass on a satellite photograph, due to the cyclonic cloud flow around it. A tight, well-defined comma usually indicates a well-organized storm system.

A low-pressure system over warm water may begin to organize and can form a tropical wave, which can turn into a hurricane. A hurricane usually has extremely low barometric pressures, and the lower the pressure, the more intense the storm. When a hurricane moves inland, in fact, it loses its tropical characteristics and ends up as a wet, windy low-pressure system.

Meteorologists use computer programs specifically designed to chart weather systems to help them predict the weather. One function of these programs is to estimate how a low-pressure system will move across the country, and what kind of weather it may bring with it. Even a day’s notice of bad weather can help save lives and property.