General Studies: Geography Topic Wind System for Competitive Exams

Wind System

The pressure difference is the major causes of the genesis of the wind system. The air moves from high pressure to low pressure. The slope of pressure from high to low is called pressure gradient, which is also called as barometric slope. The imaginary line joining the points having same pressure is called isobars. The direction of air movement should be perpendicular to the isobars because the direction of pressure gradient is perpendicular to the isobars, but the direction is deviated from the expected one due to Coriolis force caused by the rotation of the Earth.

Thus the factors that control the air motion are

• Pressure gradient
• Rotation of Earth and Coriolis force
• Friction force
• Centrifugal action of wind

The winds blowing parallel to the isobars generally at the height of 600 meters is called Geo-strophic wind.

Wind Directions

The Coriolis force generated due to the rotation of Earth acts as a deflective force to the wind direction. Because of the Coriolis force, all the winds are deflected to the left in the Southern hemisphere with respect to the rotating Earth.This is referred to as Farrel's Law. The Coriolis force is absent along the equator, but increases progressively towards the poles.

Classification of Winds:

Winds are classified into Three types by the direction of wind system, They are

• 1) Primary Movement:  Permanent Winds
• 2) Secondary Movement : These are classified into Three types

                ( i ) Cyclone : Tropical and Temperate, Thunderstorms and Tornadoes

                ( ii ) Anti Cyclone

                ( iii ) Seasonal Wind i.e., Monsoon

• 3) Tertiary Movement: Local Winds

1) Primary Wind Movement:

These winds are include trade wind, westerlies and polar winds.

i) Trade Wind: These are steady currents of air blowing form the sub tropical high pressure belt towards the equatorial low pressure belt. Under the influence of the Coriolis forces they flow from the North-East in the Northeren hemisphere and from South-East in the Southern hemisphere.

ii) Westerlies: The permanent winds blowing from the subtropical high pressure belt to the sub polar low pressure belt in both the hemisphere is called Westerlies. The general direction of the westerlies is South-West to North-West in the Northern hemisphere and North-West to South-East in the Southern hemisphere. The Westerlies become more rigorous in the Southern hemisphere because of the lack of land and dominance of oceans. Their velocity increases Sothward and they become stormy so they are called roaring forties between latitudes 40°. South latitudes, furious fifties at 50° South latitude and shrieking sixties at 60° South latitudes.

iii) Polar Winds: Polar winds blows from polar high pressure belt to sub-polar low pressure belt. They are North-Easterly in Northern hemisphere and South-Easterly in the Southern hemisphere.

2) Secondary Wind Movement: 

Seasonal Winds: The winds, which reverse its direction completely every 6 months is called Seasonal Winds. The best example is Monsoon winds.

i ) Cyclone: Cyclones are the centres of low pressure surrounded by closed isobars having increasing pressure outward and closed air circulation from outside towards the central low pressure in such a way that Air blows inwards in clockwise direction in the Southern hemisphere. Air blows inwards in clockwise direction in the Southern hemisphere.

Cyclones are mainly of two types;

• Tropical Cyclone
• Temperate Cyclone

ii ) Anticyclone: They are the wind system, which has the highest air pressure at the centre and lowest at the outer margins surrounded by circular isobars where wind blows. They are high pressure system and common in subtropical belts and practically absent in the equator region. They are generally associated with rainless fair weather and that's why they are called as weather less phenomena. 

3) Tertiary Movement 

Tertiary Movement is also called Local Winds. Some local winds are mentioned in the below table

Tertiary Movement (Local Winds)
Wind	Nature	Region
Land Breeze	Warm	Land breeze blows from land to sea
Sea Breeze	Cold	Sea breeze blows from sea to land
Chinook (snow eater)	Warm	Rockies (USA and Canada)
Sirocco	Hot	North Africa
Fohn	Warm	Alps/Europe
Khamsin	Hot	Egypt
Blizzard	Cold	Siberian, Canada, USA