The following wind belts are found on the earth’s surface in each hemisphere

If there were no land areas to modify the belted arrangement of pressure zones, the general circulation pattern at the earth’s surface would be like that. However, the general scheme of atmospheric circulation as shown in the aforesaid figure is disturbed because of the uneven distribution of land and water and many other factors which will be discussed at a later stage.

The following wind belts are found on the earth’s surface in each hemisphere:

(1) Doldrums.

(2) Trade wind belt.

(3) Prevailing westerlies.

(4) Polar easterlies.

(1) Doldrums:

The doldrums is the equatorial belt of calms and variable winds lying over the equatorial trough of low pressure. Average location of the doldrums is roughly between 5°S and 5″N latitudes. This belt of calms lies between the two trade winds.

There being no strong pressure gradient, the wind is light and variable. Mostly there are vertical movements in the atmosphere in this zone. In the late afternoon, there is strong convection which brings about heavy thundershowers.

Because of the prevailing calms, the atmosphere in the doldrums is hot, oppressive and sticky. Since this zone is the meeting place of the two trade winds, it is also called the inter-tropical convergence zone.

(2) Trade wind belt:

On both the sides of the equatorial trough of low pressure lie the trade wind belts extending roughly from 5° to 30° of latitude. Here the surface flow is equator-ward, and the flow in upper troposphere is pole-ward.

Trade winds originate because of the pressure gradient from the subtropical belts of high pressure to the equatorial belt of low pressure. In the northern hemisphere the prevailing winds are northeasterly and are called the northeast trades.

In the southern hemisphere the prevailing winds moving equator-ward are southeasterly and are termed the southeast trades. These winds are steady and flow from a constant direction.

It was held that the trades were overlaid by the anti-trade winds. However, the existence of the anti-trades is now a point of controversy. As more and more upper-air data are made available, controversies regarding the so-called anti-trades are likely to be resolved.

The zone of trade winds is also called the Hadley cell, because it resembles the convective model used by Hadley for the whole earth. The energy to drive this cell is believed to come from the latent heat released during the formation of cumulonimbus clouds in the equatorial region.

The pole-ward moving winds in the upper troposphere in this cell begin to subside in a zone between about 20 and 35 degrees latitude. The general subsidence in this zone may be attributed to two factors. First, away from the equator the radiational cooling makes the air aloft cold and heavy.

This causes subsidence. Secondly, since the Coriolis force becomes progressively stronger with increasing distance from the equator, the pole-ward upper winds are deflected into a nearly east-west flow at about 25° latitude.

Thus, the pole-ward flow is partly restricted which results in the accumulation of air aloft. This also contributes to the descent of air in the zone lying between 20° and 35° latitudes. The subsiding air is relatively dry because of its adiabatic heating during its descent.

Besides, it has already released much of its moisture in the equatorial region in the form of precipitation. That is why all the tropical deserts of the world are located in this zone.

This zone of descending air currents is popularly known as the horse latitudes where like the doldrums, wind is light and variable. Further details of the subtropical high pressure belt are given in the chapter on air pressure and winds.

(3) Prevailing westerlies:

The belt of prevailing westerlies lies between 30° and 60° latitude in each hemisphere. The westerlies are winds that move out from the pole-ward margins of subtropical high-pressure belts.

These winds while moving into higher latitudes are deflected and become the southwesterly and the northwesterly winds in the northern and the southern hemispheres respectively.

The prevailing westerlies of the mid-latitudes are relatively more variable than the trade winds both in direction and intensity.

There are frequent invasions of polar air masses along with the travelling cyclones and anticyclones. These moving cells of low and high pressures largely affect the path of movement of westerlies.

However, all the cyclones and anticyclones found in these regions travel eastward along with the prevailing westerlies.

It may be mentioned that even though the surface flow of the westerlies is subject to many interruptions by storms and irregular winds blowing from different directions, at the cirrus cloud level they are steadier and blow from a westerly direction.

In the northern hemisphere, because of the larger percentage of land areas with their high mountains and plateaus and changing seasonal pressure systems, the westerly flow is nearly obscured. But in the southern hemisphere, where there is preponderance of water over land, the westerlies are stronger and retain their directional persistency.

The westerlies blow throughout the year, but are stronger in the cold season, particularly in the North Atlantic and North Pacific. This is because of the steep pressure gradients from the Aleutian and Icelandic low pressure areas towards the extremely cold continental interiors during winter.

These two semi- permanent lows are described as statistical averages that represent the large number of cyclonic storms moving along the westerlies across the globe, and which converge in these areas.

In the southern hemisphere, between the latitudes 40° and 60^HS there is all water. Here the westerlies are so powerful and persistent that the sailors used such expressions as ‘roaring forties’, ‘furious fifties’ and ‘screaming sixties’ for these high-velocity winds in the latitude belts of forties, fifties and sixties respectively.

(4) Polar easterlies:

The polar easterlies are those winds which move out of the polar high towards the sub-polar low pressure belt. There are no regular winds blowing from the North Polar Region. Moreover, the polar high is not considered to be a quasi-permanent feature of arctic circulation.

However, there are prevailing out flowing winds from the Greenland. In winter, the easterly winds are observed from the anticyclones in Siberia and Canada. But on the pole-ward sides of the depressions that form over the North Atlantic and North pacific the easterly winds do occur.

The winds in Polar Regions generally blow from various directions, and they are largely controlled by local weather disturbances.

The polar easterlies of the southern hemisphere are rather more coherent and well-defined. Easterly winds blow out from the anticyclone formed over the high plateau of eastern Antarctic. The Indian Ocean near the Antarctic coast line experiences such easterly winds.

Needless to say that little is known about the atmospheric motion in high latitudes. Our knowledge about the atmospheric conditions hardly extends beyond 70° or 75° N latitude in the northern hemisphere and 60°S latitude in the southern hemisphere. Whatever we know about the meteorological conditions in high latitudes is very sketchy.