1.4. Air Pressure and Winds

The distribution and characteristics of the winds over a region are determined by several global and local factors. The principal determinants are the seasonal global distribution of air pressure, the rotation of the earth, the daily variations in heating and cooling of land and sea and the topography of the given region and its sur­roundings.

The pressure belts and regions

Over each hemisphere of the earth's surface there are belts and centres of high and of low atmospheric pressure. Some of these are permanent while others only exist for part of the year.

Two high-pressure belts surround the earth at the subtropical latitudes between 20° and 40° in the two hemispheres, and are shifted in summer towards the poles and in winter towards the equator. During the winter, both are continuous round the earth and the pressure is higher over the continents than over the oceans. In summer low-pressure centres (depressions) develop over the continents, interrupting the continuity of the belts. The polar regions are zones of permanent high pressure but at a lower pressure than in the subtropical belts.

The equatorial belt is the principal region of low pressure which is maintained throughout the year. In the summer of each hemisphere there is a shift towards higher latitudes, particularly over the great continents. Thus, in July-August the region covers mainly the northern tropics extending from North-East Africa to Central-East Asia, and its centre is in the Persian Gulf. In January-February the region covers mainly the southern tropics. Other low-pressure centres exist in the higher latitudes forming a belt in the southern hemisphere approximately over the Antarctic. In the northern hemisphere the distribution of the low-pressure areas is complicated by the presence of large land masses around the Arctic Ocean. Because of this, high- and low-pressure areas occur at about the same latitude and constantly move eastwards, so that any one place is subjected to successive periods of high and low air pressure.

The main cause of the pressure belts and centres is the uneven distribution of solar radiation over the earth and the resulting variation in surface heating. A large quantity of radiation is received by the equatorial region and here the air is heated above the level in adjacent regions. This air expands and is lifted upwards, leaving a belt of low pressure towards which air flows from the surrounding regions still at a higher pressure.

As the centre line of the maximum radiation zone in each hemi­sphere is shifted in summer towards the subtropics, the low pressure belt moves accordingly. The distribution of land masses and oceans in these latitudes determines the summer position of the maximum heating zone, especially in the northern hemisphere. There the highest temperatures are obtained over the land area of South-West Asia, shifting the centre of the zone to the Persian Gulf.

The air mass which is lifted to form the equatorial low-pressure belt divides in the upper atmosphere and flows towards the poles; it sinks back to earth between the latitudes 20° and 40° in winter, and in the 30-40° zone in summer according to the position of the sun. This elevates the air pressure in these regions, creating the sub­tropical high pressure zones. The polar high pressure regions result from the chilling of the lower air layers over the ice surface. The pressure difference between centres of high and low pressure also varies over the earth. According to Ashbel [1.1], the difference in winter between the low over southern Greenland and the high over eastern Russia is about 70 mm Hg while the low over the Mediter­ranean only differs from the high over Russia by 40 mm Hg.

Air flows from higher pressure to lower pressure zones. The permanent and semi-permanent regions of high pressure are sources of large masses of air which, because they remain for some time in a region under relatively constant conditions, acquire specific physical characteristics, considerably uniform within the mass, according to the nature of the surface on which they rest. The main regions of sufficient size and uniformity of conditions to generate such air masses are:

a. The polar regions (throughout the year)

b. The cold land masses of Asia, North Africa, Australia and North America (in winter)

c. The subtropical high pressure belt, particularly over the oceans (in summer).

The bodies of air may be therefore classified as "Polar" or "Tropical", and as "Continental" or "Maritime".

Air masses migrate from their high pressure sources towards the regions of low pressure. However, the flow is not in the direction of the greatest pressure gradient, at right angles to the lines of equal pressure (isobars), but is deflected by a phenomenon known as the Coriolis force, resulting from the rotation of the earth. For example, an air mass over the equator has the rotational velocity of the earth at this latitude, which is about 1670 km/h eastwards. When the air is set in motion due north this velocity is maintained, apart from a slight reduction by friction with the earth's surface. But as the rotational speed of the surface of the earth decreases with circum­ference towards the poles (at 30° latitude it is only 1120 km/h), the equatorial mass moves eastwards faster than the land beneath it, and this air appears to come from the south-west. Thus the original northerly direction undergoes an easterly deflection relative to the earth. This applies to all air masses moving northwards away from the equator. Similarly air moving towards the equator will undergo a relative westerly deflection.

Relative to the true air direction the deflection due to Coriolis force in the nothern hemisphere is to the right and in the southern hemisphere to the left. Thus, the pattern of divergence from high pressure centres is clockwise in the north and anticlockwise in the south; the convergence at low pressure centres follows the reverse paltern. The Coriolis force is zero at the equator and increases towards the poles; its magnitude is proportional to the sine of the latitude.

The wind systems

There are three global belts of winds in each hemisphere: the trade winds, the westerlies and the polar winds. In addition there are wind systems known as the monsoons which are the result of annual differences in heating of land and sea areas.

Local wind patterns occur over mountains and valleys and there are day and night breezes along shorelines [1.4].

The trade winds

The trade winds originate in the sub-tropical high pressure regions of the two hemispheres and converge at the inter-tropical front forming the low-pressure equatorial belt. They flow to the south-west in the northern hemisphere and to the north-west in the southern hemisphere. The characteristics of the trade winds depend on the surface over which they travel. Over most of the oceanic surfaces the two air streams have a similar temperature and humidity so that only small disturbances are produced when they meet: here they travel with a constant direction and speed (15-35 km/h and up to 45 km/h). But over the Indian Ocean and the south-west Pacific the directions are reversed by the monsoons in the summer of each hemisphere. Complicated patterns and diversions are also obtained over land masses.

The westerlies

The westerlies also have their origin in the sub-tropical regions, but flow towards the sub-arctic low-pressure regions. Along the polar fronts the westerlies and the polar winds converge and because of the vast differences between the temperatures of the two air masses the fronts are very stormy. In the winter of the northern hemisphere the winds vary greatly in speed and direction, and form systems of travelling depressions. In summer, however, they are less variable and generally the air flow is to the north-east. In the southern hemi­sphere, which has much smaller land masses, the winds are more regular but here also the pattern at any one place is complicated by the travelling depressions.

The polar winds

The polar winds are formed by the cold air masses spreading out from the polar and arctic high-pressure regions. Their general direction is to the south-west in the northern hemisphere and north­west in the southern hemisphere.

The monsoon winds

The difference between the annual mean temperature patterns over land and sea produces the winter land winds and summer sea winds known as the monsoons. Their effect is greatest in the region of the Indian Ocean bounded by Australia, South Asia and East Africa. In this area the extremes of pressure are on the continents and air Hows from the high pressure in one hemisphere to the low pressure