Wind can be described as the movement of air from the high-pressure area to the low-pressure area. There are many types of wind such as permanent, seasonal and local winds. A wind is named after the direction from which it blows, e.g. the wind blowing from the west is called westerly.
Forces Controlling Winds
- Pressure Gradient and Winds
- The Coriolis Effect and Wind
Types of Winds
The wind direction changes with the season and in still others the winds are so variable that no pattern is discernible. Despite these difference, the winds are generalized under three categories.
- (a) Planetary winds or permanent winds
- (b) Periodic winds or Seasonal winds
- (c) Local winds
(a)Planetary Winds
Planetary or permanent winds blow from high pressure belts to low pressure belts in the same direction throughout the year. They blow over vast area of continents and oceans. They are easterly and westerlies and polar easterlies.
i) The Easterlies
- The winds that blow from sub-tropical high pressure areas towards equatorial low pressure areas called trade or easterly winds.
- The word trade has been derived from the German word ‘trade’ which means track.
- To blow trade means ‘to blow steadily and constantly in the same direction’.
- Because of the Coriolis effect the northern trade winds move away from the subtropical high in north-east direction.
- In southern hemisphere the trade winds diverge out of the sub-tropical high towards the equatorial low from the southeast direction
- As the trade winds tend to blow mainly from the east, they are also known as the Tropical easterlies. (see fig.)
(ii) The Westerlies
- The winds that move poleward from the sub-tropical high pressure in the northern hemisphere are deflected to the right and thus blow from the south west.
- These in the southern hemisphere are deflected to the left and blow from the north-west. Thus, these winds are called westerlies (see fig.)
(iii) Polar Easterlies
- Polar easterlies blow from polar regions towards sub-polar low pressure regions.
- Their direction in the northern hemisphere is from north-east to southwest and from south-east to north-west in the southern hemisphere.
(b) Periodic Winds
- The direction of these winds changes with the change of seasons.
- Monsoon winds are the most important periodic winds.
- Monsoon Winds:
- The word ‘Monsoon’ has been derived from the Arabic word ‘Mausim’ meaning season.
- The winds that reverse their direction with the change of seasons are called monsoon winds.
- During summer the monsoon winds blow from sea towards land and during winter from land towards seas.
- The Asiatic monsoon is the result of interaction of both planetary wind system and regional factors, both at the surface and in the upper troposphere.
- India, Pakistan, Bangladesh, Myanmar(Burma), Sri Lanka, the Arabian Sea, the Bay of Bengal, South-east Asia, North Australia, China and Japan are important regions where monsoon winds are prevalent.
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