DEFINITION OF EXTRATROPICAL CYCLONES

Also known as wave cyclones—

hurricane-strength storms that form in middle and high latitudes

at all times of the year. Examples of these strong storms

include the famous “nor’easters” of New England in the

United States; storms along the east slopes of the Rockies and

in the Gulf of Mexico, and smaller hurricane-strength storms

that form in arctic regions. These storms develop along polar

fronts that form semi-continuous boundaries between cold

polar air and warm subtropical air. Troughs of low pressure

can develop along these polar fronts, and winds that blow in

opposite directions to the north and south of the low set up a

cyclonic (counterclockwise in the Northern Hemisphere)

wind shear that can cause a wave-like kink to develop in the

front. This kink is an incipient cyclone and includes (in the

Northern Hemisphere) a cold front that pushes southward

and counterclockwise, and a warm front that spins counterclockwise

and moves to the north. A comma-shaped band of

precipitation develops around a central low that develops

where the cold and warm fronts meet, and the whole system

will migrate east or northeast along the polar front, driven by

high-altitude steering winds.

The energy for extratropical cyclones to develop and

intensify comes from warm air rising and cold air sinking,

transferring potential energy into kinetic energy. Condensation

also provides extra energy as latent heat. These storms can

intensify rapidly and are especially strong when the cold front

overtakes the warm front, occluding the system. The point

where the cold front, warm front, and occluded front meet is

known as a triple point and is often the site of the formation of

a new secondary low-pressure system to the east or southeast

of the main front. This new secondary low often develops into

a new cyclonic system and moves east or northeastward, and it

may become the stronger of the two lows. In the case of New

England’s “nor’easters,” the secondary lows typically develop

off the coast of the Carolinas or Virginia, then rapidly intensify

as they move up the coast, bringing cyclonic winds and moisture

in from the northeast off the Atlantic Ocean.

See also HURRICANE.

extrusive rocks See VOLCANO.

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