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The Flavour of Lows - Page 2

© Keith C. Heidorn

Extratropical Lows, aka mid-latitude Lows, are the most familiar lows to those of us living in the middle latitudes, from about 37 to 60 degrees of latitude, North and South. They generally form along the Polar Front, the hemispheric boundary region that separates subtropical air from polar air. It is in this tempestuous zone where the cold, drier air of the polar regions collides with the moist, warm air of the subtropics. As the frontal boundary separating these airs begins to twist, the twist often becomes a swirl and the swirl, a closed low pressure region. Extratropical Lows may remain along the Polar Front, showing a characteristic warm-front/cold-front wedge as a member of a cyclone family, or break away to live a singular life. Since cold air is important in the formation of these Lows, they are called cold-core systems.

Low Pressure Cells forming along Polar Front

Certain areas of the globe harbour favoured spawning grounds for extratropical Lows. In the Northern Hemisphere, the two most prolific producers of Lows are the semi-permanent Alaskan Low in the Gulf of Alaska and the Icelandic Low, around Iceland.

Subtropical Lows exhibit characteristics of both their tropical and extratropical cousins and often form from a melding of two such Lows. The infamous Perfect Storm of 1991 arose when a strong extratropical Low merged with the remnants of Hurricane Grace off the Atlantic Coast.

Polar Lows are small, shallow cyclones which form mainly in winter over the open waters of high-latitude seas within a polar or arctic air mass. They are spawned by the contrast between the cold air above a relatively warmer sea surface. The warm air rising from the sea starts the convection/convergence process. Polar lows are small in size but can be quite windy.

Thermal Lows, also called heat Lows, are similar to polar Lows. They form in regions where the surface is very hot compared to the overlying air. Very hot air at the surface rises rapidly since it is less dense and therefore more buoyant.

Thermal Lows are common over deserts or arid regions, particular during the height of summer. For example, thermal Lows often form over the US Southwest or the dry regions in eastern British Columbia/eastern Washington during the peak of summer. Thermal lows generally remain where they are born and often totally dissipate overnight when the sun no longer heats the land.

Lows given names such as the Colorado Low, Hatteras Low, etc. are often extratropical Lows that form in a specific geographical location with some degree of regularity before storming out into new territory.

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