Understanding Pressure

An area of high pressure (sometimes called a high) is typically represented by a blue H on a weather map. This signals a region where the sea-level pressure is high compared to the surrounding area. The airflow around a high-pressure system is clockwise. This causes the air to be pushed outward from the center of the high pressure, called divergence. The air from the upper atmosphere sinks to replace the air moving away. As it does, the air dries out leaving cloudless skies and fair weather.

An area of low pressure (sometimes called a low) is typically represented by a red L on a weather map. This shows a region where the sea-level pressure is low compared to the surrounding area. The airflow around a low-pressure system is counter-clockwise. This causes the air to be pulled into the center of the low pressure, called convergence. The air has nowhere to go but up, and as it rises, the water vapor in the air cools and condenses, producing clouds and precipitation.