Pressure and wind belts
Atmospheric pressure, also called barometric pressure, force per unit area exerted by an atmospheric column (that is, the entire body of air above the specified area). Atmospheric pressure can be measured with a mercury barometer (hence the commonly used synonym barometric pressure), which indicates the height of a column of mercury that exactly balances the weight of the column of atmosphere over the barometer. Atmospheric pressure is also measured using an aneroid barometer, in which the sensing element is one or more hollow, partially evacuated, corrugated metal disks supported against collapse by an inside or outside spring; the change in the shape of the disk with changing pressure can be recorded using a pen arm and a clock-driven revolving drum.
The atmosphere is held on the earth by the gravitational pull of the earth. A column of air exerts weight in terms of pressure on the surface of the earth. The weight of the column of air at a given place and time is called air pressure or atmospheric pressure. Atmospheric pressure is measured by an instrument called barometer. Now a days Fortin’s barometer and Aneroid barometer I are commonly used for measuring air pressure.
Atmospheric pressure is measured as force per unit area. The unit used for measuring pressure is called millibar. Its abbreviation is ‘mb’. One millibar is equal to the force of one gram per square centimetre approximately. A pressure of 1000 millibars is equal to the weight of 1.053 kilograms per square centimetre at sea level. It is equal to the weight of a column of mercury which is 76 centimetre high. The international standard pressure unit is the “pascal”, a force of one Newton per square meter. In practice atmospheric pressure is expressed in kilopascals, (one kpa equals 1000 Pa).
Wind belts on earth
Inter-Tropical Convergence Zone (Doldrums)
Since air is heated and rises at the equator, a zone of low pressure is formed. This zone is referred to as the equatorial trough. Air moves towards the equatorial trough, where it converges and moves aloft as a part of Hadley cell. Convergence occurs in a narrow zone, called the inter-tropical convergence zone (ITCZ).
It is the belt of equatorial calms and winds lie over the equatorial trough of low pressure. Average location of doldrums is 5°N and 5°S from the equator and this belt lies between two trade winds. As the horizontal pressure gradient is weak, therefore winds are light and variable.
Because of the convergence of winds, convective activity is dominant. Convection becomes vary strong in the late afternoons carrying warm moist air, often forming huge cumulus clouds, which results in heavy thunderstorms.
Because of the enormous amount of latent heat released by these clouds, the atmosphere becomes hot, oppressive and sultry. Since it is the meeting zone of north easterly and south easterly trade winds, it is also called Inter Tropical Convergence Zone (ITCZ) or doldrums.
Trade Wind Pattern
This belt extends roughly from 5° to 30° N & S of equator. Here, at the surface wind flows from poles towards the equator and in the upper atmosphere, flow is towards the poles. These trade winds originate because of the pressure gradient force from sub-tropical high to equatorial low. In the northern hemisphere, the trades are north-easterly and in the southern hemisphere, these are south easterly. These winds are regular (steady) and flow in constant direction.
The trade wind belt is also called Hadley cell after the name of the scientist as it resembles the convective model used by Hadley for the whole earth. The energy to derive this cell is believed to come from the release of latent heat during the formation of cumulonimbus clouds in the equatorial region.
The poleward moving winds in the upper atmosphere in this cell begin to subside between 20°-35°N & S latitudes. The subsidence over here may be due to the radiational cooling, because at upper levels it makes the air heavy and at the same time it begins to converge at higher levels over the middle latitudes around 30°. This convergence (piling up) of air aloft increases the mass of air above the surface.
Because of accumulation of air mass at higher levels, it starts descending around 30° latitude in both the hemispheres. This zone of descending air produces sub-tropical high pressure belts and is also known as ‘horse latitudes’ where like the doldrums the winds are light and variable.
The descending air over the sub-tropical high, is dry and warm. As a result, subsiding air produces clear sky and high temperature. Major deserts of the world like Sahara are located in this region.
Subtropical High Pressure (Westerlies Belts)
These lie between 30° and 60°N & S latitudes in both the hemispheres. The winds move from poleward margins of sub-tropical high pressure belts. While moving to higher latitudes, these winds are deflected and become south-westerlies in northern hemisphere and north- westerlies in southern hemisphere. The westerlies of middle latitudes are more variable than trades both in direction and intensity.
These westerly winds are frequently over-powered by polar air masses and cells of cyclones and anti-cyclones are formed in these areas. The surface flow of the westerlies may be interrupted by storms and irregular winds blowing from different directions, but in the upper atmosphere these are steady and blow in westerly direction.
The westerlies prevail throughout the year, but are stronger in winter season, especially over North Atlantic and North Pacific oceans. This is because of the steep pressure gradient from the Aleution islands and icelandic low pressure areas towards the extremely cold continental interiors where the pressure is very high.
These two semi-permanent lows are the cause of a number of cyclonic storms moving along the westerlies across the globe. In the southern hemisphere, between 40° & 60° latitudes, westerlies are persistent and powerful over water, sailors call them roaring forties, furious fifties and screening sixties.
Polar easterlies are the winds which move from polar highs towards sub-polar lows. Winds blowing from north pole are not regular. Because the polar high is not considered as quasi (semi)-permanent feature of arctic circulation. However, there are prevailing outflowing winds from the green land.
In winter, the easterly winds are observed from anticyclones of Siberia and Canada. The winds in these areas generally blow from various directions and these are largely controlled by local weather disturbances. But on the poleward side of the depressions (cyclones) that form in the northern Atlantic and northern Pacific, the easterly winds do occur.
The easterly winds in southern hemisphere are well defined and are coherent (semi-permanent) and regular. Easterly winds blow from the anti-cyclonic systems formed over the plateau of eastern Antarctica. The Indian ocean near the Antarctica experiences such easterly winds.
Little is known about the atmospheric motion at upper levels in high latitudes beyond 70° or 75° (i.e. 70, 80 or 90°) in both the hemispheres due to lack of the meteorological information.