DEFINITION OF POLAR LOW

Hurricane or gale-strength storms that form

over water behind (poleward) the main polar front are

known as polar lows. They may form over either the Northern

or Southern Hemisphere oceans but are a larger menace

to the more-populated regions around the North Atlantic,

North Sea, and Pacific Oceans, as well as the Arctic Ocean.

Most polar lows are much smaller than tropical and midlatitude

cyclones, with diameters typically less than 620 miles

(1,000 km). Like hurricanes, many polar lows have spiral

bands of precipitation (snow in this case) that circle a central

warmer low-pressure eye, whereas other polar lows develop a

comma-shaped system.

Most polar lows develop during winter months. In the

Northern Hemisphere they form along an arctic front, where

frigid air blows off landmasses and encounters relatively

warm current-fed ocean water, resulting in a rising column

of warm air and sinking columns of cold air. This situation

sets up an instability that induces condensation of water

vapor in the rising air, along with the associated release of

latent heat that then warms the atmosphere. The warming

lowers the surface pressure adding convective updrafts to the

system and starting the classical spiral cloud band formation.

Polar lows may attain central barometric pressures

comparable to hurricanes (28.9 inches or 980 mbars) but

tend to dissipate more quickly when they move over the cold

polar landmasses.

See also EXTRATROPICAL CYCLONES.

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