Tuesday, October 27, 2009


PLANETARY WINDS The general, permanent circula­tion of surface winds throughout the world is.denoted by the term 'planetary winds'. The wind belts are basically controlled by the latitudinal pressure belts and by the forces produced by rotation of the earth.

Doldrums Doldrums is the name for the equatorial I of low pressure lying between 5° South and 5° Nc latitude. This zone has no pressure gradients to induc persistent flow of wind.

Trade Winds The word 'trade' comes from the Sa) word 'tredon' which means to tread and follow a reg\ path. Moving north and south of the equator, the m wind belts are trade winds, covering roughly the Zl between 5° and 30° North and South. They blow from subtropical high pressure area (Horse latituaes) towa equatorial low pressure areas (doldrums). Under the in ence of the Coriolis force, they blow from the north-E in the northern hemisphere (north-east trades) and fr the south-east in the southern hemisphere (SOUth-E trades). They are also called tropical easterlies.

Like pressure belts which cause them, trade winds and doldn; have a seasonal shifting tendency through several degI of latitudes. In summer these belts shift farther nortl1 the northern hemisphere than they shift in the south direction during winters because of the longer landmas: the northern hemisphere. Trade winds are upset in Indian Ocean due to the proximity of the great Asi landmass. They are best developed over the Pacific j Atlantic oceans. Trade winds are, however, not
favourable for navigation and flying. Though very const in strength and direction, they sometimes contain stn depressions.

Horse Latitudes
The subtropical belts of variable wi and columns that lie between the latitudes 25° and 35° Sc and North are called Horse Latitudes. They coincide v the sub-tropical high-pressure belts. (The high pressur probably caused by the rising air of equatorial latitu which descends here.)

Westerlies These winds blow from subtropical t pressure areas (Horse Latitudes) to subpolar low pres~ areas and lie between 35° and 60° Nand S latitudes. Vari, in direction and strength, westerlies contain depressiom the northern hemisphere, land masses cause considen disruption to the westerly wind belt. But between 40°
60° S lies the almost unbroken ocean belt. Westerlies strong and persistent here, giving rise to the marin expressions-"roaring forties", "the furious fifties" and' screaming sixties". In earlier times, this belt was extensi1 used for sailing vessels travelling eastward from Sc Atlantic ocean to Australia, Tasmania and New Zealand the Southern Pacific Islands.

It was easier to conti eastwards around the world from these places thar return to European ports. Today the westerlies are iml tant not in shipping but in long distance trans-oceanic transcontinental flights; in the easterly direction, tl require less fuel and a shorter time than the west... flights.

Polar Easterlies
These constitute the wind syst characteristic of the arctic and polar zones. They blow f] polar high pressure areas to subpolar low pressure ar In fact, winds in these regions take a variety of direction as dictated by local weather disturbances. Deflected to left in the southern hemisphere, the radial winds mov anticlockwise direction, producing a system of south-E erly winds. Perhaps in Antarctica, an ice-capped landn surrounded by vast oceanic expanse, the outward spiral flow of polar easterlies is a valid concept.


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