11.11: Cyclones

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Did you go out on Halloween in 2011?

If you did, you probably don't live in the northeastern United States. In much of the region, Halloween was postponed in 2011. A large and early nor'easter struck the area. The massive storm dropped as much as 32 inches of snow. It caused over three million people to lose power. In all, 39 people died.

Mid-Latitude Cyclones

Cyclones can be the most intense storms on Earth. A cyclone is a system of winds that rotate around a low pressure center. The swirling air rises and cools. This creates clouds and precipitation.

Mid-latitude cyclones form at the polar front. Two very different air masses blow past each other in opposite directions. The Coriolis effect causes winds to strike the polar front at an angle. Warm and cold fronts form next to each other.

Most winter storms in the middle latitudes, including most of the United States and Europe, are caused by mid-latitude cyclones (Figure below).

A hypothetical mid-latitude cyclone affecting the United Kingdom

A hypothetical mid-latitude cyclone affecting the United Kingdom. The arrows point the wind direction and its relative temperature; L is the low pressure area. Notice the warm, cold, and occluded fronts.

The warm air at the cold front rises and creates a low pressure cell. Winds rush into the low pressure. This creates a rising column of air. The air twists, rotating counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Since the rising air is moist, rain or snow falls.

Mid-latitude cyclones form in winter in the mid-latitudes. The storms move eastward with the westerly winds. These two- to five-day storms can reach 1,000 to 2,500 km (625 to 1,600 miles) in diameter. They can produce winds up to 125 km (75 miles) per hour.

Nor'easters

Mid-latitude cyclones are especially fierce in the mid-Atlantic and New England states. They are called nor’easters because they come from the northeast. About 30 nor’easters strike the region each year (Figure below).

The 1993 “Storm of the Century” was a nor’easter that covered the entire eastern seaboard of the United States

The 1993 “Storm of the Century” was a nor’easter that covered the entire eastern seaboard of the United States.

Summary

A cyclone is a system of winds rotating around an area of low pressure.
A mid-latitude cyclone forms at the polar front. This occurs when the temperature difference between air masses is very large.
Nor'easters are mid-latitude cyclones that come from the northeast.

Review

What is a cyclone?
What are the motions of air in a mid-latitude cyclone?
What is a nor'easter?