The Highs and Lows of the Westerlies

The Westerlies and the Columbus’ Return Route
The picture painted by Hadley about the origin of the trade winds had a major flaw: He missed the fact that latent heat is brought toward the equator by the converging trade winds. The cell is driven not only by the (somewhat) greater solar heating at the equator, but it (mainly) results from the gathering of heat from all over the tropics into an area known as the "Intertropical Convergence Zone." Nevertheless, Hadley's treatment can be viewed as the founding act for the of meteorology, because he attempted to explain weather and winds from physical principles.

Hadley's picture was quite realistic as concerns the trades and the doldrums. It had to be modified considerably, however, to account for the other global wind belts. The most important air currents, next to the trades, are the “westerlies,” with their complicated pattern of high and low pressure centers following each other, and their dependency on the changing position of the westerly jet stream along the polar front. The Swedish-American meteorologist Carl-Gustaf Rossby worked out the elements of this system in the 1930's and 1940's, thus founding modern meteorology.

Description of Ferrel Cells

The complete diagram summarizing atmospheric circulation. Note that rising warm, moist air results in a low pressure zone and descending cold, dry air results in a high pressure zone. These zones of high and low pressure along with the Coriolis force create the major wind belts. Note: The Hadley cell extends from the equator to about 30 degrees N and S latitude, the Ferrel cell extends from about 30 to 60 degrees N and S latitude and the Polar cell extends from 60 degrees latitude to the poles (90 degrees N and S latitude).
After the Hadley Cell, we have to consider "Ferrel Cells." Ferrel Cells connect sinking air in the arid zone to the westerlies poleward of the arid zone. They also link this same sinking air the upward motion of warm air riding up the cold air masses along the polar front, within the westward moving storms. The American meteorologist William Ferrel (1817-1891) showed how the tendency of winds to move in circles on a rotating planet gives rise to the cyclones that pull the air in from the warmer regions toward the polar front, thus driving the westerly winds. The mechanism is somewhat analogous to making the trades (in that latent heat drives the convection which sucks in the air to make winds) but in this case heat is moved poleward. Ferrel realized air pulled in toward a low pressure center (where the air rises, cools, and loses water) moves along a spiral path: a "cyclone." The westerlies can be understood as being powered by a series of large low pressure centers (located, for example, near Iceland or the Aleutians) and carrying along smaller cyclonic eddies (storms) spawned by these centers. The big pattern resulting from this can be seen in the "Rossby waves" (named after Carl-Gustaf Rossby) around the North Pole, anchored by the Iceland and Aleutian low pressure centers. These eddies moving with the westerlies, bringing rain (cyclones) or sunshine (anti-cyclones) and are marked as the familiar “L” and “H” patterns on weather maps. The L centers show where warm air has penetrated poleward into colder air masses. This moist air being sucked in brings heat. Thus, rain makes the climate mild (example: Bergen, Norway). The H centers show where tongues of colder air are moving toward low latitude. No heat is gathered (other than from the sunshine in a clear sky). So, high pressure areas in high latitudes typically have a harsh climate, with strong summer-to-winter contrast.

The Intertropical Convergence Zone

The red line on the maps demonstrates the northward movement of the ITCZ in the summer (July) and the southward movement of the ITCZ in the winter (January).
Now that we have familiarized ourselves with the main air currents, the trade winds and the westerlies, we can appreciate the chief elements of general atmospheric circulation. Moist air rises in a region called the Intertropical Convergence Zone, or ITCZ. In the northern hemisphere, the ITCZ tends to move north in summer and south in winter. Release of latent heat warms the air of the ITCZ and makes it rise. Away from the ITCZ the de-watered air sinks, heating up while doing so, and produces an arid zone. North and south of the ITCZ, air is being sucked in close to the sea surface, just as Hadley envisioned. This air, moving equatorward, picks up moisture (and heat) from the tropical ocean. The air's motion is deflected to the right on the northern hemisphere, and to the left on the southern hemisphere, as a result of the Coriolis force (see glossary).

Satellite photo demonstrating the ITCZ and its relation to the trade winds. Notice the line of clouds that form at the ITCZ as warm air rises near the equator.