A depression (also called a low-pressure system or cyclone in meteorological language) is an area where atmospheric pressure is lower than its surroundings — typically below 1013 hPa. Air converges towards the centre and rises, creating clouds and precipitation. In the Northern Hemisphere, winds spiral anticlockwise around the low; in the Southern Hemisphere, clockwise.
Depressions are classified by their origin and intensity. Extratropical depressions form along the polar front where cold and warm air masses collide, and drive most of Europe's day-to-day weather. Explosive cyclogenesis (or "weather bomb") occurs when the central pressure drops more than 24 hPa in 24 hours, generating fierce winds and waves. Subtropical depressions share features with tropical and extratropical systems, and the medicane is a Mediterranean example.
On weather maps, depressions are marked B (baja) or L (low) and surrounded by tightly packed isobars indicating strong winds. They are typically accompanied by cold fronts and warm fronts radiating from the centre. The deeper the low (lower central pressure), the more intense the wind and rain. Famous depressions include Storm Ciarán (2023, central pressure 953 hPa) and the Great Storm of 1987 (central pressure 958 hPa).