Semipermanent Pressure Regions

Near the 30° and 60° latitude lines, both in the northern hemisphere and in the southern hemisphere, the surface winds converge from, or diverge in, opposing directions. This gives the air an impetus to spin clockwise at 30°N and 60°S, counterclockwise at 60°N and at 30°S. The air also spins around the poles, clockwise in the arctic and counterclockwise in the Antarctic. The eddies near the 60th parallels tend to pull surface air inward, and the eddies near the 30th parallels and the poles tend to push surface air outward.

Because of the tendency of the air to rotate near the 30th and 60th parallels and the poles, we find persistent cyclonic (inward-spiraling) and anticyclonic (outward-spiraling) atmospheric zones. The cyclonic zones are semipermanent low-pressure systems, and the anticyclonic zones are semipermanent high-pressure systems.

Over the oceans, semipermanent low-pressure regions are found near the Aleutian islands in the North Pacific, Iceland in the North Atlantic, and just off the coast of Antarctica in the South Pacific, South Atlantic, and Indian Oceans. Semipermanent high-pressure systems are found near Hawaii in the North Pacific, Bermuda in the North Atlantic, Tahiti in the South Pacific, between South America and Africa in the South Atlantic, and between Australia and Africa in the Indian Ocean.

Over land masses, the semipermanent high and low pressure systems are not as stable as they are over the oceans. This is because temperature variations are greater over land masses, and temperature changes affect atmospheric circulation. Nevertheless, certain land regions, notably deserts, are usually dominated by semipermanent high pressure. Sometimes, a high- or low-pressure system can stall over a land mass for a considerable period of time. This brings droughts or wet spells to such regions.

A semipermanent low-pressure region, called the intertropical convergence zone (ITCZ), exists near the equator in a sinuous band that completely circles the globe. Semipermanent highs are found near the poles. The ITCZ, in particular, is associated with major weather events, particularly hurricanes, that develop over warm ocean waters at certain times of the year.

PROBLEM 2-2

Are Hadley cells, along with semipermanent highs and lows and prevailing winds, unique to the earth?

SOLUTION 2-2

No. Similar patterns are observed on other planets that have atmospheres. On Jupiter and Saturn, each hemisphere has multiple cells. The atmosphere of Jupiter is an especially good example, because the belts and zones of prevailing winds show up as light and dark cloud bands. These bands can be seen through a good telescope on a clear night.

Weather Systems

Semipermanent oceanic highs were well known to mariners who sailed between Europe and the New World or the Orient. Seamen, becalmed in the hot, fair weather near 30°N or 30°S, sometimes remained motionless for days. Rations ran short. The men got hungry and ate the food intended for the horses they had brought with them, and threw the horses overboard to drown. Sometimes the hapless sailors even ate the horses themselves! This, according to some legends, is how the regions became known as the horse latitudes.

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Responses

  • Mary Goetz
    Why is the equator a semipermanent low?
    6 years ago
  • hildifons
    What is the semi permanent low over europe?
    6 years ago
  • aatifa
    What is a semi permanent pressure region?
    6 years ago
  • Peregrin
    What causes high pressure systems to stall over land masses?
    6 years ago
  • Pimpernel Underhill
    Where are semi permanent pressure cells located?
    6 years ago

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