Atmospheric Circulation

One of the implications of the Ferrell hypothesis is that there should be relatively little surface wind near the equator. In this region, surface winds should be flowing toward the equator from the Hadley cells and, when they meet, rising upward into the upper atmosphere. Equatorial regions would be expected to be characterized, therefore, by relatively low pressures with weak surface winds.

But these conditions are exactly what mariners have observed for centuries. Indeed, they long ago gave the name of the doldrums to the equatorial seas. For centuries, ship captains have feared and avoided equatorial waters because winds are so weak and unreliable there that they could easily become stranded for days or weeks at a time.

A second region of calm on the earth's surface, according to the three-cell model, would be around latitude 30°. In this region, air moving downward from both the Hadley and Ferrell cells collides as it reaches the earth's surface, producing regions of high pressure. As in the doldrums, the regions around latitude 30° are characterized by weak, unpredictable winds.

Again, such regions have long been feared and avoided by sailors, who have given them the name of the horse latitudes. The origin of this name comes from the fact that ships bringing horses to the Americas often became becalmed in the waters around 30°N latitude. As supplies ran low, ships were forced to throw their horses overboard. Many stories are told of the waters in these latitudes being littered with the carcasses of the unfortunate animals.

The regions between the horse latitudes and the doldrums (between 0° and 30° latitude) are those in which surface winds flow toward the equator. That flow is not directly from north to south or south to north, of course, because of the Coriolis effect. Instead, winds in these regions tend to blow from the northeast to the southwest in the northern hemisphere and from the southeast to the northwest in the southern hemisphere. Since the winds tend to be strong and dependable—the sorts of wind upon which sailing ships depend—these winds have long been known as the trade winds.

The intersection of the Ferrell and polar cells around latitude 60° is another region at which surface flows of air meet. One, from the Ferrell cell, consists of relatively warm air flowing toward the poles. The other, from the polar cell, consists of much colder air flowing toward the equator. The point at which these two systems meet is called the polar front and is characterized by some of the world's most dramatic storms.

The prevailing direction of surface winds with the Ferrell and polar cells is determined by the Coriolis effect. In the former cell, winds tend to blow from the southwest to the northeast in the northern hemisphere and from the northwest to the southeast in the southern hemisphere. To residents of North America, these prevailing westerlies are well known as the mechanism by which weather systems are carried across the continent from west to east.

In the polar cell, the predominant air movements are just the opposite of the prevailing westerlies: from northeast to southwest in the northern hemisphere and from southeast to northwest in the southern hemisphere.