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Global wind systems

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From Earth Science, Tarbuck & Lutgens.

Imagine Earth as a non-rotating sphere with uniform smooth surface characteristics. Assume that the sun heats the equatorial regions much more than the polar regions. In response to this, two huge convection cells develop.

On a hypothetical non-rotating planet with a smooth surface of either all land or all water, two large thermally produced cells would form, as shown here: The heated air at the equator would rise until it reached the tropopause—the boundary between the troposphere and the stratosphere.

The tropopause, acting like a lid, would deflect this air toward the poles.

Eventually, the upper-level airflow would reach the poles, sink, spread out in all directions at the surface, and move back toward the equator. Once at the equator, it would be reheated and begin its journey over again.

This hypothetical circulation system has upper-level air flowing toward the pole and surface air flowing toward the equator.

single cell atmospheric convection in a non-rotating Earth

Figure 7.5 in The Atmosphere, 8th edition, Lutgens and Tarbuck, 8th edition, 2001.

If the effect of rotation were added to the global circulation model, the two-cell convection system would break down into smaller cells. The next figure illustrates the three pairs of cells that would carry on the task of redistributing heat on Earth.

The polar and tropical cells retain the characteristics of the thermally generated convection described earlier. The nature of circulation at the middle latitudes, however, is more complex.

Near the equator, rising air produces a pressure zone known as the equatorial low—a region characterized by abundant precipitation.

As shown ihere, the upper-level flow from the equatorial low reaches 20 to 30 degrees, north or south latitude, and then sinks back toward the surface. This sinking of air and its associated heating due to compression produce hot, arid conditions.

The center of this zone of sinking dry air is the subtropical high, which encircles the globe near 30 degrees north and south latitude. The great deserts of Australia, Arabia, and the Sahara in North Africa exist because of the stable dry conditions associated with the subtropical highs.

– – – –
We now allow the earth to rotate. As expected, air traveling southward from the north pole will be deflected to the right. Air traveling northward from the south pole will be deflected to the left.

However, by looking at the actual winds, even after averaging them over a long period of time, we find that we do not observe this type of motion.

In the 1920ís a new conceptual model was devised that had three cells instead of the single Hadley cell. These three cells better represent the typical wind flow around the globe.

Idealized, three cell atmospheric convection in a rotating Earth.
“Three cell” being either three cells north or south of the equator.
The deflections of the winds within each cell is caused by the Coriolis Force.

Idealized, three cell atmospheric convection in a rotating Earth

Figure 7.5 in The Atmosphere, 8th edition, Lutgens and Tarbuck, 8th edition, 2001

Where does wind come from?

Jet Streams



Animated satellite data of the jet stream, specifically showing a polar vortex affecting part of Canada and the northeastern United States.


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