A weather system characterized by cool, wet, and unsettled weather.

A pressure map showing a strong low pressure system over the Western U.S. Credit: Pivotal Weather

Low pressure systems are characterized by rising air, leading to cool, wet, and unsettled weather. Low pressure usually equates to snowy weather in the mountains during the winter months. There are two common types of low pressure systems in the Western U.S: troughs and closed lows.

Low pressure causes rising air, which leads to cool, wet, and unsettled weather. In contrast, high pressure is caused by subsiding air, which leads to relatively warm, dry, and fair weather. Credit: Pearson Prentice Hall, Inc.

Satellite imagery of a low pressure system centered off of the Pacific Coast. Credit: National Oceanic and Atmospheric Administration