Extratropical cyclones are large-scale low-pressure systems that develop in the temperate zones, typically between 30° and 60° latitude. Unlike tropical cyclones, which draw energy from warm ocean waters, extratropical cyclones form due to temperature contrasts between air masses, such as the collision of cold, dry polar air with warm, moist air from subtropical regions.
How do extratropical cyclones form?
Extratropical cyclones often develop along a boundary between contrasting air masses, known as a front. This process, called cyclogenesis, typically occurs in regions of strong temperature gradients, such as along the polar front.
Extratropical versus tropical cyclones
While both types of cyclones are low-pressure systems, they differ in formation and structure:
| Extratropical Cyclone | Tropical Cyclone | |
|---|---|---|
| Energy Source | Temperature contrast | Warm ocean waters |
| Location | Mid-latitudes (30°–60°) | Tropics (10°–30°) |
| Structure | Asymmetrical, with fronts | Symmetrical, no fronts |
| Size | Larger (up to 4,000 km wide) | Smaller (500–1,000 km wide) |
| Typical Weather | Frontal rain, snow, wind | Heavy rain, strong winds |
Comma-shaped cloud pattern
One of the most distinctive features of mature extratropical cyclones is their comma-shaped cloud pattern, easily observed in satellite imagery. The comma-shaped cloud pattern forms as a result of the interaction between warm and cold air masses within the cyclone. At the center of the storm, a low-pressure area drives the rotation of air and organizes the system into a spiral.
The “head” of the comma represents the densest cloud cover and often corresponds to the area of heaviest precipitation. This area typically is where the cold front overtakes the warm front, creating what is known as an occluded front.
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The “tail” of the comma extends along the cold front, where colder air pushes beneath the warm, moist air, forcing it upward. This upward motion causes condensation and the formation of clouds, which stretch along the boundary of the front. The clear region behind the tail corresponds to the drier, colder air wrapping around the cyclone’s center, sometimes referred to as the dry slot.
Where do extratropical cyclones form in the United States?
Extratropical cyclones in the United States frequently form along well-defined boundaries where contrasting air masses converge, such as the polar front.
One of the most active regions for cyclone formation is the Great Plains. Here, cold Arctic air collides with warm, moist air from the Gulf of Mexico, creating the ideal conditions for these storms to develop. The Appalachian and Rocky Mountains further shape the formation and movement of extratropical cyclones by disrupting airflow and intensifying temperature contrasts. As these powerful systems move eastward, they often bring severe weather to the Midwest, the Northeast, and the East Coast.
Extratropical cyclones also significantly impact the Pacific Northwest, bringing a distinct set of weather patterns influenced by the region’s unique geography and proximity to the Pacific Ocean. These storms frequently form over the North Pacific, where cold, dry air from the Arctic interacts with warm, moist air masses over the ocean.
