Types of Winds
The winds at the surface may be classified into four main types, viz. (1) the planetary winds, (2) the monsoon winds, (3) cyclones and anticyclones (4) local winds.
1. Planetary winds:
The general distribution of winds throughout the lower atmosphere is known as planetary winds. Confined within some latitudinal belts, these winds blow rather regularly throughout the year and are basically controlled by the latitudinal pressure belts. The main planetary winds are (i) the North-east and the South-east Trade winds (ii) the Temperate Westerlies and (iii) the Polar Easterlies, which blow from the polar high pressure area to the temperate low pre sure area.
Trade winds: Trade winds blow in a belt lying between 5°N-30°N in the northern hemisphere and 5°S-30°S in the southern hemisphere. From the equator ward side of the Sub-tropical highs in the Northern hemisphere air flows towards the Equatorial low but it is deflected right according to Ferrle’s law and as a result instead of blowing as northerly wind, it bends westward to become North-east trade winds. In the Southern hemisphere winds originating from the Sub-tropical high pressure and blowing towards the Equatorial low pressure are similarly deflected westward to become the prevailing South-east trades. Trade winds are noted for their consistency, both in force and direction in many areas especially over open seas and derive their name from the nautical expression ‘to blow trade’ meaning ‘to blow along a regular track’. Zones of sub-tropical highs in latitudes about 30°-35°N and 30°-35°S are areas of descending air and are characterize by calms light variable winds, comparatively dry air and quiet, stable weather conditions. These zones of latitudes are called Horse latitude.
The Westerlies: The Westerly winds blow across latitudes 35°-60° of both hemispheres. The air streams flowing pole wards from the Sub-tropical high pressure areas deflects eastward in the Northern he sphere to form South-westerlies. Similar winds in the Southern hemisphere are known as North-westerlies. Unlike the trade winds, the westerlies are very variable in force and direction especially in the Northern hemisphere. In the Southern hemisphere, on the other hand, the Westerlies blow with great strength and regularity throughout the year over the almost uninterrupted expanse of ocean and have given the name Roaring forties to the region specially between latitudes 40°S and 50°S. Sometimes the name is applied to the winds themselves as they give a roaring sound on account of high speed.
The Polar easterlies: The Polar easterlies blow from the Polar high pressure area to the Temperature low pressure area. On their equator ward journey they are deflected westward to become North easterlies in the Northern hemisphere and South easterlies in the Southern hemisphere.
2. Monsoon winds:
The type of wind system in which there is a complete or almost complete reversal of prevailing direction from season to season is known as the monsoon winds. The largest and best developed monsoonal area in the world is the SE Asia including India. During the summer (April to September) the interior of the continents is intensely heated. This creates a low pressure into which winds are drawn from over the cooler surrounding oceans. In India South-west monsoon wind blows in summer. As the air from over the oceans is very moist, it results in heavy rainfall. During the winter (October to March) the continental interior becomes much cooler than the surrounding oceans; the wind direction is thus reversed, blowing from the continental high pressure to the low pressure over the oceans. This creates the NE monsoon over India, which is generally a cool and dry wind.
3. Cyclone and Anticyclone:
A cyclone is a region of low atmospheric pressure and associated storm system, of which there are two types. The first, characteristics of temperate latitudes, is now usually referred to as a depression. The second in general covers a smaller area and typical of the tropics, is usually called a Tropical cyclone. The Tropical cyclones are generally violent with wind speeds as high as 120-280 km/hr.
Anticyclone is a region of atmospheric high pressure and associated with light winds. Both cyclones and anticyclones are manifested by circular oval or wedge shaped arrangement of isobar with lowest pressure at the centre in case of cyclones and highest pressure at the centre in case of anticyclones. The law governing here known as Buys Ballot’s law, states that in the northern hemisphere winds move in an anticlockwise direction around the centre of low pressure and clockwise around centers of high pressure; in the southern hemisphere the reverse is true. Both cyclones and anticyclones move as systems, cyclones moving much faster than the anticyclones. As a result the wind directions at a place shift with the passage of a cyclone or an anticyclone.
4. Local winds:
Winds, caused by local factors and confined to a limited area compared to planetary winds, are called local winds. Some well known examples of local winds are given below:
Land breeze: It is a common local wind that affects only coastal area. During the night the land becomes very much cooler than the sea as land is quickly chilled than the sea. The air adjacent to the surface is also chilled with the result that there is a marked high pressure over land. Thus the cooler, heavier, denser air over the land flows towards the sea and land breeze occurs.
See breeze: Unlike land breeze it blows the opposite way from the sea to the land. During the day land becomes quickly heated compared to sea with the result there is a marked low pressure over the land. Thus air is drawn into the land from the comparatively high pressure area of the adjacent seas or oceans. The southerly sea breeze in summer is highly welcome in Kolkata.
Fohn: Fohn is a warm dry wind that blows down the valleys of the north facing slopes of the Alps and is most common in spring and autumn. It occurs when a depression to the north of the Alps draws air from the south over the mountains.
Chinook: Chinook is a warm dry southwesterly wind, similar to the Fohn in origin and character. It need be pointed out here that the Chinook or the Fohn is relatively much warmer than the air into which it is invading but in the absolute sense it may be even cold at times below freezing point of water.
Tornado: Tornado also sometimes known as twistu in the U.S.A. is a violently rotating storm in which winds whirl around a small area of extremely low pressure.
Some other important local winds: Harmattan is a very dry and dust laden West African wind that blows south-east from the Sahara to the coast especially between Octobers to February. Its incursion into the coast of Gulf of Guinea gives relief to stifling humidity. The sirocco, a hot dry south wind blowing from the Sahara to the Mediterranean, is most unpleasant. In Egypt this wind is called khamsin. The mistral and bora are cold north winter winds. Mistral is experienced in southern France, especially in the Rhone delta and bora blows down the mountains to the east coast of Adriatic Sea and North Italy. Norwester is a squall occurring during hot season (April-June) in North India and may bring rain or hail. The Loos are hot and dry summer westerly winds that sweep the Upper and Middle Ganga Plains.