A mid-latitude cyclone or extratropical cyclone is a weather phenomenon associated with atmospheric low pressure that takes place in the temperate region between the tropical and polar regions. Hence, they are also known as temperate cyclones, or more rarely, extratropical cyclones. In the northern hemisphere a cyclone rotates in the counterclockwise direction, while it rotates clockwise in the southern hemisphere. The rotation is caused by the Coriolis effect.

Mid-latitude cyclones are driven by baroclinic processes, that is the temperature contrast between warm and cold air masses.

Relation to tropical cyclones

Tropical cyclones often transition into extratropical cyclones at the end of their tropical existence. In an extratropical transition, poleward displacement of the cyclone occurs and the cyclone's primary energy source converts from the release of latent heat of condensation to a baroclinic process. While most tropical cyclones that become extratropical quickly dissipate or are absorbed by another weather system, they can still retain winds of hurricane or tropical storm force.

The reverse transition of a mid-latitude cyclone to a tropical cyclone occurs less frequently but is not uncommon. This will often involve a subtropical storm which has characteristics of both mid-latitude and tropical cyclones.

See also

• Nor'easter
• European windstorm