Tuesday, November 5, 2013

Understanding Marine Weather – Global Circulations


Global Circulations explain how air and storm systems travel over the Earth’s surface. The global circulation would be simple if the Earth did not rotate, the rotation was not tilted relative to the sun, and had no water.


In a situation such as this, the sun heats the entire surface, but where the sun is more directly overhead, it heats the ground and atmosphere more. The result would be the equator becomes very hot with the hot air rising into the upper atmosphere.

That air would then move toward the poles where it would become very cold and sink, then return to the equator. One large area of high pressure would be at each of the poles with a large belt of low pressure around the equator.


However, since the earth rotates, the axis is tilted, and there is more landmass in the northern hemisphere than in the southern hemisphere, the actual global pattern is much more complicated.


Instead of one large circulation between the poles and the equator, there are three circulations:

  1. Hadley cell – Low latitude air movement toward the equator that with heating, rises vertically, with pole ward movement in the upper atmosphere. This forms a convection cell that dominates tropical and sub-tropical climates.
  2. Ferrel cell – A mid-latitude mean atmospheric circulation cell for weather named by Ferrel in the 19th century. In this cell, the air flows pole-ward and eastward near the surface and equator-ward and westward at higher levels.
  3. Polar cell – Air rises, diverges, and travels toward the poles. Once over the poles, the air sinks, forming the polar highs. At the surface, air diverges outward from the polar highs. Surface winds in the polar cell are easterly (polar easterlies).
Usually, fair and dry/hot weather is associated with high pressure; rainy and stormy weather associated with low pressure. You can see the results of these circulations on a globe. Look at the number of deserts located along the 30°N/S latitude around the world. Now, look at the region between 50°-60° N/S latitude. These areas, especially the west coast of continents, tend to have more precipitation due to more storms moving around the earth at these latitudes.


Between each of these circulation cells are bands of high and low pressure at the surface. The high-pressure band is located about 30° N/S latitude and at each pole. Low-pressure bands are found at the equator and 50°-60° N/S.


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