The General Circulation

If the Earth didn’t rotate, the general circulation would be simple. Hot air would rise at the equator and spread to the Poles. There it would cool, sink, and flow back to the equator along the surface. Because the Earth rotates, the straightforward equator-to-pole-to-equator circulation loop gets split into the three smaller loops, or cells. The surface winds in each cell alternate direction:

• between the equator and 30 degrees, the surface winds in the Hadley Cell become the easterly trade winds,
• between 30 and 60 degrees, the surface winds in the Ferrel Cell become the mid-latitude westerlies,
• between 60 and 90 degrees, surface winds in the Polar Cell become the polar easterlies.

In addition to the alternating direction of the surface wind belts, surface air pressure alternates at the boundaries between the cells:

• rising air around the equator is associated with the low pressure, Inter-tropical Convergence zone, or “the doldrums,”
• sinking air around 30 degrees is associated with the sub-tropical high pressure zones, or “the horse latitudes,”
• rising air around 60 degrees, where the westerlies collide with the polar easterlies, is associated with the sub-polar lows,
• sinking air at the poles is associated with the polar highs.

Because the surface of the Earth is not uniform, the pressure belts are not uniform either. Differences in the rate of heating and cooling of ocean and land, for example, cause the high-pressure belts in the horse latitudes to become localized into large, semi-permanent highs that change position and intensity throughout the year.

 The Race is On