Classification of Planetary Winds | Climatology | Geography

In this article we will discuss about the classification of planetary winds.

The winds blowing almost in the same direction throughout the year are called prevailing or permanent winds. These are also called as invariable or planetary winds because they involve larger areas of the globe. On the other hand, winds with seasonal changes in their directions are called seasonal winds (e.g., monsoon winds).

On an averages, the location of high and low pressure belts is considered to be stationary on the globe (though they are seldom stationary). Conse­quently, winds blow from high pressure belts to low pressure belts. The direction of such winds remains more or less the same throughout the year though their areas change seasonally.

Thus, such winds are called permanent winds. Since these winds are distributed all over the globe and these are related to thermally and dynamically induced pressure belts and rotation of the earth and hence they are called planetary winds. These winds include trade winds, westerlies and polar winds (fig. 35.7).

i. Winds in the Tropics:

Generally, the areas extending between 30°N and 30°S latitudes are included in tropical zone. For­merly, it was believed that trade winds blow from the subtropical high pressure belts to the equatorial low pressure belt. The north-east and south-east trades converge along the equator and there are upper air anti­trades blowing in the opposite directions of the surface trade winds.

The weather conditions throughout the tropical zone remain more or less uniform. There is a belt of calm or doldrum characterized by feeble air circulation. These views of weather conditions pre­vailing in the tropics are now considered as old con­cepts and have now been refuted on the basis of new information based on numerous observations made in the upper air and near the earth’s surface during and after Second World War.

New discoveries have now put question marks against the views of zonal character and regularity of surface trade winds, uniformity of weather conditions in the tropics, and upper air antitrade winds. It has been discovered that trade winds blow with regularity only over some parts of the tropical oceans (mainly over the eastern parts).

Upper air anit-trades, contrary to earlier beliefs, are not found everywhere but are confined to certain areas only. The weather conditions in the tropics are not calm and uniform but they are frequently interrupted by atmos­pheric disturbances (cyclones, hurricanes, typhoons, sea waves etc.). Thus, the tropical zone is character­ized by doldrum, equatorial westerlies, and trade winds.

a. Doldrum and Equatorial Westerlies:

A belt of low pressure, popularly known as equatorial trough of low pressure, extends along the equator within a zone of 5°N and 5°S latitudes. This belt is called the belt of calm or doldrum because of light and variable winds. It was believed that doldrum is a regular feature all along the equator and is character­ized by strong convective instability leading to the formation of cumulonimbus clouds and copious rain­fall daily but the recent observations have now shown that the belt of doldrum is not continuous but is con­fined to certain localities only.

This belt is subjected to seasonal and spatial variations due to northward and southward movement of the overhead sun (summer and winter solstices).

In fact, the belt of doldrum shifts northward during summer solstice (when the sun is vertical (fig. 35.8) viz.:

(1) over the tropic of Cancer, 21 June) and comes back to its normal position on Sept. 23 and then shifts southward at the time of winter solstice (when the sun is vertical over the tropic of Capricorn, 22 December).

According to Flohn the doldrums ex­tend upto 200 longitudes in discontinuous manner. Crowe has identified 3 zones of doldrums e.g. Indo-Pacific Doldrum extending from the eastern coast of Africa to 180° longitude for a distance of 16,000 km and cover­ing an area of 25,800,000 km², thus, covers about one third of the total length of the equator,

(2) Equatorial Western Coastal Region of Africa, and

(3) Western Coastal Margin of Central America.

A few meteorologists and climatologists be­lieve in the existence of cyclonic fronts in the equato­rial regions while some refute this concept on the ground that fronts are formed only when two air masses of contrasting temperatures converge but such conditions are not found in the equatorial region. The equatorial or tropical fronts are called intertropical fronts (ITF) or intertropical convergence (ITC).

These fronts represent the meeting ground of north-east and south-east trade winds. The northern and southern boundaries of intertropical convergence are called north intertropical convergence (NITC) and south intertropical convergence (SITC) respectively (fig. 35.10). There is seasonal shifting in the NITC and SITC with the northward (summer solstice) and south­ward migration (winter solstice) of the sun.

On an average, there is westerly air circulation (form west to east) in the doldrums or say in the intertropical convergence. These westerly winds have been called by Flohn as equatorial westerlies (fig. 35.10) which cover 200° longitudes. According to Flohn the equatorial westerlies cover the areas extending from the western parts of Africa across the Indian Ocean to the western Pacific Ocean.

The equatorial westerlies are associated with strong atmospheric disturbances (cyclonic storms). Flohn has further maintained that south-western monsoons are, infact, equatorial west­erlies because these winds are extended upto 30-35°N latitudes over Indian subcontinent due to northward shifting of NITC at the time of summer solstice (fig. 35.9).

b. Trade Winds:

There is more or less regular inflow of winds from subtropical high pressure belts to equatorial low pressure belt. These tropical winds have north-easterly direction in the northern hemi­sphere while they are south-easterly in the southern hemisphere. These winds are called trade winds because of the fact that they helped the sea merchants in sailing their ships as their (of trade winds) direction remains more or less constant and regular.

According to Ferrel’s law (based on coriolis force generated by the rotation of the earth) trade winds are deflected to the right in the northern hemisphere and to the left in the southern hemisphere. There are much variations in the weather conditions in the different parts of trade winds.

The poleward parts of the trade winds or eastern sides of the subtropical anticyclones are dry because of strong subsidence of air currents from above. Because of the dominance of anticyclonic conditions there is strong atmospheric stability, strong inversion of tem­perature and clear sky.

On the other hand, the equator-ward parts of the trade winds are humid be­cause they are characterized by atmospheric instability and much precipitation as the trade winds while blow­ing over the oceans pick up moisture.

It may be stated that the trade winds are more regular and constant over the oceans than over the lands. At some places on the lands (e.g. S.E. Asia and southern USA) the trade winds disappear during summer season due to forma­tion of low pressure cells because of high temperature but the trade winds are more constant and regular over the continents during winter season. It may be pointed out that the zone of trade winds is called Hadely Cell on the basis of the convective model prepared by Hadley for the entire earth.

ii. Horse Latitudes and Westerlies:

a. Horse Latitudes:

The dynamically induced (due to subsidence of air currents) subtropical high pressure belt extends between 30°-35° (25°-35°) lati­tudes in both the hemispheres. Thus, this belt separates two wind systems viz., trade winds and westerlies. It is also apparent that the subtropical high pressure belt is the source for the origin of trade winds (blowing towards equatorial low pressure belt) and westerlies (blowing towards sub-polar low pressure belt) because winds always blow from high pressure to low pressure.

The air after being heated near the equator ascends and after blowing in opposite direction to the surface trade winds descends in the latitudinal zone of 30°-35°. Thus, the descent of winds from above causes high pressure on the surface which in turn causes anticyclonic conditions. This is why the anticyclonic conditions cause atmos­pheric stability, dry condition and very weak air circu­lation.

Thus, this zone (30°- 35°) is characterized by weak and variable winds and calm. This belt of calm is very popularly known as horse latitudes because of the fact that in ancient times the merchants had to throw away some of the horses being carried in the ships in-order to lessen the weight so that the ships could be sailed through the calm conditions of these latitudes.

Anticyclones are produced due to subsidence of air currents in the horse latitudes. These anticyclones are known as ‘subtropical highs’ or subtropical anticy­clones, the eastern and western parts of which are characterized by contrasting weather conditions. The eastern parts (spreading over the western parts of the continents) are marked by descent of air currents, inversion of temperature and consequent atmospheric stability and dry conditions.

This is why hot and dry tropical deserts are found in the western parts of the continents within the latitudinal zones of 20°-30° in both the hemispheres (e.g. Sahara and Kalahari in Africa, Chile-Peru desert or Acatama in South America, Arabian and Thar deserts in Asia, deserts of S. W. USA, and Australian deserts). The western parts of subtropi­cal anticyclones (covering the eastern parts of the continents and western parts of the oceans) are humid because some sort of atmospheric instability is caused due to weakening of air descent (e.g., in the areas of Caribbean Sea, Mexican Gulf and adjoining areas, eastern China, southern Japan, south-east Brazil and eastern Australia).

b. Westerlies:

The permanent winds blowing from the subtropical high pressure belts (30°-35°) to the subpolar low pressure belts (60°-65°) in both the hemispheres are called westerlies (fig. 35.7). The gen­eral direction of the westerlies is S.W. to N.E. in the northern hemisphere and N.W. to S.E in the southern hemisphere. There is much variation in the weather conditions in their poleward parts where there is con­vergence of cold and denser polar winds and warms and lighter westerlies.

In fact, a cyclonic front, called as polar front, is formed due to two contrasting air masses as referred to above and thus temperate cy­clones are originated. These cyclones move along with the westerlies in easterly direction. Thus, the general characteristic features of the westerlies are largely modified due to cyclones and anticyclones associated with them.

Because of the dominance of land in the northern hemisphere the westerlies become more com­plex and complicated and become less effective during summer seasons and more vigorous during winter season. These westerlies bring much precipitation in the western parts of the continents (e.g., north-west European coasts) because they pick up much moisture while passing over the vast stretches of the oceans.

The westerlies become more vigorous in the southern hemi­sphere because of lack of land and dominance of oceans. Their velocity increases southward and they become stormy. They are also associated with boisterous-gales. The velocity of the westerlies becomes so great that they are called roaring forties between the latitudes of 40°-50° S, furious fifties at 50°S latitude and shrieckng sixties at 60° S latitude.

iii. Polar Winds:

A low pressure belt, produced due to dynamic factor, lies within the latitudinal belt of 60°-65° in both the hemispheres. This belt of low pressure is more persistent in summer season but generally disappears in winter season. The Icelandic and Aleutian low pressure cells persist throughout the year. There is very high pressure over the poles because of exceedingly low temperature.

Thus, winds blow from polar high pressure to sub-polar low pressure cells. These are called polar winds which are north-easterly in the southern hemisphere and south-easterly in the south­ern hemisphere. The zone of polar winds shrinks due to northward shifting of pressure belts at the time of northern summer (summer solstice) in the northern hemisphere but it is extended up to 60°N latitude during northern winter (winter solstice).