WIND

WIND :-

Wind is the air in motion. It is a horizontal movement of air. When the air is at rest compared to surrounding objects, we are scarcely aware of its presence, but when it is in motion we readily recognise it.

The chief cause of wind is the unequal heating of the atmosphere by the sun resulting in differences in the pressure between places. Winds always blow from areas of higher pressure toward areas of lower pressure. The greater the difference in pressure between two places, the faster the air will move.

A wind is named according to the direction from which it blows. Thus, a wind from the south blowing towards the north is called a south wind.

The Earth spins on its axis, which affects the direction of the wind. In the Northern hemisphere winds are swung to the right, and in the Southern to the left. This is called "the Coriolis Effect". This is one of the factors affecting the wind direction.

TYPES OF WINDS :-

For an academic study winds can be classified into Planetary Winds (or the Permanent Winds of the Earth) and Local and other Periodic Winds.

Planetary Winds : The Planetary Winds are the general circulation of winds throughout the lower atmosphere of the Earth. Such a circulation of air would be set up on any planet like the Earth, which has an atmosphere envelope and rotates on its axis and which has no uniform land surface.

Some of the important Planetary winds are:

(a) The Doldrums (or the Belt of Equatorial Calms): It is the low pressure belt round the equator, caused by the great heat making the air hot and therefore light. It s a region of calms and very light winds. Here, the North East and the South East Trade Winds coverage on and meet each other. The major movement of air in this region is upward. Although there are no regular winds, violent squalls and thunderstorms, with heavy rains occur frequently. The maximum rainfall is received at equinoxes and minimum at the solstices.

(b) The Trade Winds: The name Trade Winds is derived from the nautical expression "to blow tread" meaning to blow along a regular "tread" or path. These winds seem to tread out a path in the seas for sailing vessels by their steadiness and regularity and so they came to be known as "trade winds".

At the equator there is great heat causing the air over that region to expand and rise up in the atmosphere. This creates a low pressure area. But, as we saw earlier, there are Sub-tropical high pressure belts between 30° to 40° North and South from where winds blow towards the low pressure area around the equator. These winds are the Trade Winds. They are regular both in strength and direction.

These winds blow between approximately 30° north and 30° south and their direction is north-east in the Northern Hemisphere and south-east in the Southern Hemisphere. For this reason, whatever moisture they bring is deposited on the eastern parts of the continents while the western parts have very little rain. The great hot deserts like the Mexican, Kalahari and the Atacama are, therefore, found on the western margins of the continents. There is generally fair weather in the belt of the Trade Winds except where the winds blow from the ocean to a mountainous coast. In the Indian Ocean and among the islands of the south-west Pacific, they are reversed in summer by the Monsoons.

(c) The Prevailing Westerlies (or simply "Westerlies" because they blowout of the west): They blow outside the Tropics, in the Temperate Zone, on the pole ward side of the Trade Winds, and between 30° and 60° north and south. The weather in their area is characterized by a constant procession of cyclones (or depressions) and anticyclones. These winds are also deflected due to the rotation of the earth (to the right in the Northern Hemisphere and left in the Southern Hemisphere). They are not so constant in strength and direction as the Trade Winds. However, in the Southern Hemisphere they are more constant because there are no large land masses to interrupt them. In places they become so strong between latitude 40°- and 65° south, that the sailors use the word "Roaring Forties" the Furious Fifties and the Shrieking Sixties to describe them.

The Westerlies bring rain. They first strike the western coasts of the continents which are therefore rainy, for example, the west coast of Europe and Southern Chile. Similarly, the Roaring Forties bring heavy rain to the western mountainous coasts of Tasmania and New Zealand.

(d) The Polar Winds: Towards the poles and over the ice-covered lands like Greenland and the Antarctica, the air that flows at higher levels is cooled and consequently sinks towards the earth. This cold air forms an area of high pressure (the Polar High) from which the air moves out. These winds are also deflected to the west in both Hemispheres to form the. Polar Easterlies. The amount of deflection due to the earth's rotation is very great. The winds starting form the north pole and from the south pole toward the equator are deflected as much as 90° from their original courses until they blow eastward.

It must be noted that the pattern of the wind systems is greatly affected by certain factors, for example, the different parts of the earth's surface move at different speeds. This has effects on the speed and direction of the winds. Secondly, the earth's surface is composed both of land and sea. This results in the phenomena of land and sea breezes which affect the climate of the places near the seacoasts. For this same reason there are Monsoons developed over the large land masses of India, southern China and the countries of south-east Asia. There are also larger expanses of water in the Southern Hemisphere than in the Northern Hemisphere so that winds are more constant in the Southern Hemisphere. Finally, the position of the Sun in its yearly migration north and south of equator also affects the wind system. The whole wind system swings about 7° north or south with the Sun. It is only when the Sun is shining over the equator that there is equal distribution of heat on either side of it.

LOCAL AND REGIONAL WINDS :-

Land and Water differences: Land gets heated quickly during the day but also loses its heat quickly after sunset, whereas water takes much longer time to get warm, but also longer to get cold. The specific heat of water is very high compared with that of land. Two to five times as much heat is required to raise the temperature of water one degree as for an equal volume of dry earth. Water is also a bad conductor of heat; its transparency and mobility allow the rays of the Sun to penetrate deeper into it, and the heat is spread over a greater volume, hence the same amount of heat produces a greater rise in temperature on land areas, but only moderate over water areas.

Sea Breeze: During the day, the greater heating of the land causes the air to ascend, causing a low pressure area over land and the cool heavy air from the sea moves in to take its place. This is sea breeze. The sea breeze is most noticeable and most regular when temperature changes are most regular, that is, when the pressure gradient is slight and the sky is clear. The strength of the sea breeze also depends on the topography of the coast and the regions. In Temperate regions it is about 15 to 20 km per hour and in the Tropics it may reach 25 to 30 km. Sea breeze is usually cool and fresh. It moderates the weather of the coastal belt on hot summer afternoons.

Land Breeze: During the night the land cools quickly so that it is colder' than the sea. A low pressure area is caused over the sea and the cooler heavier air from the land begins to flow towards the sea.

The land breeze set in by midnight or a few hours later. Like the sea breeze the land breeze is also
influenced by the relief of the land near the coast but it is less developed than the sea breeze.

The general effect of the contrast in heating of land and water areas is to produce cooler winters and
warmer summers in the centers of continents than along coasts.

Monsoon :-

A monsoon is a seasonal prevailing wind that lasts for several months. The term was first used in English in India, Bangladesh, Pakistan, and neighboring countries to refer to the big' seasonal winds blowing from the Indian Ocean and Arabian Sea in the southwest bringing heavy rainfall to the region. In hydrology, monsoon rainfall is considered to be that which occurs in any region that receives the majority of its rain during a particular season. This allows other regions of the world such as North
America, South America, Sub-Saharan Africa, Australia and East Asia to qualify as monsoon regions In terms of total precipitation and total area covered, the monsoons affecting the Indian subcontinent dwarf the North American monsoon. The South Asian monsoon affects a larger number of people due to the high density of population in this part of the world.

"Most summer monsoons have a dominant westerly component and a strong tendency to ascend and produce copious amounts of rain (because of the condensation of water vapor in the rising air). The intensity and duration, however, are not uniform from year to year. Winter monsoons, by contrast, have a dominant easterly component and a strong tendency to diverge, subside, and cause drought."

Process :-

Monsoons are caused by the larger amplitude. of the seasonal cycle of land temperature compared to that of nearby oceans This differential warming happens because heat in the ocean is mixed vertically through a "mixed layer" that may be fifty meters deep, through the action of wind and' buoyancy-generated turbulence, whereas the land surface conducts heat slowly, with the seasonal signal penetrating perhaps a meter or so. Additionally, the specific heat capacity of liquid water is significantly higher than that of most materials that make up land. Together, these factors mean that the heat capacity of the layer participating in the seasonal cycle is much larger over the oceans than over land, with the consequence that the air over the land warms faster and reaches a higher temperature than the air over the ocean. The hot air over the land tends to rise, creating an area of low pressure. This creates a steady wind blowing toward the land; bringing the moist near-surface air over the oceans with it. Similar rainfall is caused by the moist ocean air being lifted upwards by mountains, surface heating, convergence at the surface; divergence aloft, or from storm-produced outflows at the surface. However the lifting occurs, the air cools due expansion in lower pressure, which in turn produces condensation.

In winter, the land cools off quickly, but the ocean keeps the heat longer. The hot air over the ocean rises, creating a low pressure area and a breeze from land to ocean while a large area of drying high pressure is formed over the land, increased by wintertime cooling. Monsoons are similar to sea and land breezes, a term usually referring to the localized, diurnal (daily) cycle of circulation near coastlines everywhere, but they are much larger in scale, stronger and seasonal.

Monsoon Systems :-

As monsoons have become better understood, the term monsoon has been broadened to include almost all of the phenomena associated with the annual weather cycle within the tropical and subtropical land regions of the earth.

Even more broadly, it is now understood that in the geological past, monsoon systems must have always accompanied the formation of super continents such as Pangaea, with their extreme continental climates.

Northeast Monsoon (Southern Asia and Australasia) :-

In Southern Asia, the northeastern monsoons take place from December to early March. The temperature over central Asia is less than 25°C as it is the northern hemisphere winter, therefore creating a zone of high pressure there. The jet stream in this region splits into the southern subtropical jet and the polar jet. The subtropical flow directs northeasterly winds to blow. across southern Asia, creating dry air streams which produce clear skies over India. Meanwhile, a low pressure system develops over South-East Asia and Australasia and winds are directed toward Australia known as a monsoon trough.

Northern Indian Ocean Monsoon :-

Onset dates and prevailing wind currents of the southwest summer monsoon.

South-West Slimmer Monsoon :-

The southwestern summer monsoons occur from June through September. The Great Indian Desert (Thar Desert) and adjoining areas of the northern and central Indian subcontinent heats up considerably during the hot summers. This causes a low pressure area over the northern and central Indian subcontinent. To fill this void, the moisture-laden winds from the Indian Ocean rush in to the subcontinent. These winds, rich in moisture, are drawn towards the Himalayas, creating winds blowing storm clouds towards the subcontinent. However the Himalayas act like a high wall and do not allow the winds to pass into Central Asia, forcing them to rise. With the gain in altitude of the clouds, the temperature drops and precipitation occurs. Some areas of the subcontinent receive up to 10,000 mm of rain.

The southwest monsoon is generally expected to begin around the start of June and dies down by the end of September. The moisture-laden winds on reaching the southernmost point of the Indian peninsula, due to its topology, become divided into two parts:

- Arabian Sea Branch of the SW Monsoon
- Bay of Bengal Branch of the SW Monsoon

The Arabian Sea Branch of the SW Monsoon first hits the Western Ghats of the coastal state of Kerala, India and hence Kerala is the first state in India to receive rain from the South- West Monsoon. This branch of the monsoon moves northwards along the Western Ghats giving rain to the coastal areas west of the Western Ghats. It is to be noted that the eastern parts of the Western Ghats do not receive much rain from this monsoon as the wind does not cross the Western Ghats.

The Bay of Bengal Branch of SW Monsoon flows over the Bay of Bengal heading towards North-Eastern India and Bengal, picking up more moisture from the Bay of Bengal. Its hits the Eastern Himalaya and provides a huge amount of rain to the regions of North-East India, Bangladesh and West Bengal. Mawsynram, situated on the southern slopes of the Eastern Himalaya in Shillong, India is one of the wettest places on Earth. After striking the Eastern Himalaya it turns towards the West, travels over the Indo-Gangetic Plain, at a rate of roughly 1-2 weeks per state' pouring rain all along its way.

The monsoon accounts for 80 percent of the rainfall in the country. Indian agriculture (which accounts for 25 percent of the GDP and employs 70 percent of the population) is heavily dependent on the rains, especially crops like cotton, rice oil seeds and coarse grains. A delay of a few days in the arrival of the monsoon can, and does, badly affect the economy, as evidenced in the numerous droughts in India in the 90s.

June 1 is regarded as the date of onset of the monsoon in India, which is the average date on which the monsoon strikes Kerala over the years for which scientific data is available with the Indian Meteorological Department.

North-East Monsoon (Retreating Monsoon) :-

Around September, with the sun fast retreating south, the northern land mass of the Indian subcontinent begins to cool off rapidly. With this air pressure begins to build over northern India. The Indian Ocean and its surrounding atmosphere still holds its heat. This causes the cold wind to sweep down from the Himalayas and Indo-Gangetic Plain towards the vast spans of the Indian Ocean south of the Deccan peninsular. This is known as the North-East Monsoon or Retreating Monsoon.

While traveling towards the Indian Ocean, the dry cold wind picks up some moisture from the Bay of Bengal and pours it over peninsular India. Cities like Chennai, which get less rain from the South-West Monsoon, receives rain from the Retreating Monsoon. About 50% - 60% of the rain received by the state of Tamil Nadu is from the North-East Monsoon.

It is worth noting that North-East Monsoon (or the Retreating Monsoon) is not able to bring as much rain as the South- West Monsoon.

North American Monsoon :-

The North American Monsoon (NAM) occurs from late June or early July into September, originating over Mexico and spreading into the southwest United States by mid-July. It affects Mexico along the Sierra Madre Occidental as well as Arizona, New Mexico, Nevada, Utah, Colorado, West Texas, and California. It pushes as far west as the Peninsular Ranges and Transverse Ranges of southern California, but rarely reaches the coastal strip (a wall of desert thunderstorms only a half-hour's drive away is a common summer sight from the sunny skies along the coast during the monsoon). The
North American monsoon is known to many as the Summer, Southwest, Mexican or Arizona monsoon. It is also sometimes called the Desert Monsoon as a large part of the affect.

African Monsoon :-

The monsoon of western sub-Saharan Africa is the result of the seasonal shifts of the Intertropical Convergence Zone arid the great seasonal temperature differences between the Sahara and the equatorial Atlantic Ocean. It migrates northward form the equatorial Atlantic in February, reaches western Africa on June 22, then moves back to the south by October'[15]. The dry, northeasterly trade winds, and their more extreme form, the harmattan, are interrupted by the northern shift in the ITCZ and resultant southerly, rain-bearing winds during the summer. The semiarid Sahel and Sudan depend upon this pattern for most of their precipitationed area is desert.

South American Monsoon :-

Much of Brazil experiences seasonal wind patterns that bring a summer maximum to precipitation. Rio de Janeiro is infamous for flooding as a result of monsoon rains.

OTHER LOCAL WINDS :-

The Sirocco is the name given to the southerly wind experienced in North Africa, Sicily and Southern Italy. It originates in the Sahara Desert and reaches the sea as a very hot dry wind. Where it descends from a mountain range, as for example, on the Algerian coast, its heat and dryness are increased. The Sirocco withers vegetation and often causes much damage to crops, especially if it blows while the vines and olives are in blossom. In Egypt this type of wind is known: as the khamsin.

The Mistral is the name given.to the strong, northerly or north-westerly wind experienced on the shores of the north-west Mediterranean. It is most prevalent during the winter. The wind is strong and may sometimes have a speed of over 100 km per hour. It is also very cold and harmful to plant life.

The Foehn is the name given to the hot dry wind which blows down the leeward slope of a mountain.

Foehn winds often blow with great violence and cause much discomfort. In spring they cause snow to disappear very quickly and thus make pasture available for animals sooner than would otherwise be the case. Similar winds blowing eastwards across the Prairies of North America from the Rockies are known as the Chinook winds.

Loo is a hot wind which blows usually in the afternoon in the plains of northern India during May and June. Its temperature may range between 45°C and 50°C which is hot enough to cause sunstroke's.

Norwesters are violent thunderstorms which occur on the passage of a strong wind that approaches from the west or north-west, hence the name "Nor westers". They occur in the Bengal and Assam region during the hot season (April to June) before the onset of the South- West Monsoon.

CYCLONES AND ANTICYCLONES CYCLONES :

A Cyclone is a small low pressure system with winds blowing anti-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The pressure falls rapidly in the center with strong winds spiraling around it. Cyclones are roughly circular or oval in form, hence the isobars on weather maps are also shown circular or oval.

Cyclones originate as a wave along a prong separating two masses of air differing in temperature, densities and directions. They bring rain because in a cyclone the warm moist air is made to rise over a mass of cold heavier air. With the approach of a cyclone the sky becomes dull and overclouded and these is heavy rain accompanied by lightning and thunder.

A cyclone is always on the move and follows in the direction of the regular wind system in the particular area. Thus in the region of the Westerlies, most cyclones move forward eastwards, while in the trade wind regions the movement is in a westerly direction.

Cyclones and Depressions: Tropical cyclones are somewhat different from the cyclone (or Depressions as they are called) of the Temperate regions. The Depressions in the Temperate regions are much larger, sometimes being as much a 3000 km across and are generally.oval in shape. They are not violent and they travel in fairly well defined paths at average speeds of 35 to 50 km an hour.

Tropical cyclones originate mostly over the oceans in the tropical or sub-tropical regions. The winds are more violent and cause severe damage to life and property. They occur frequently in the Gulf of Mexico, the Carribean Sea, the western Pacific, of the east and west coast of India, east of Southern Africa and north of Australia. They are known by different names in different parts of the world: hurricanes in the West Indies, typhoons in the China Seas and cyclones in the Indian Ocean. Similar storms of northern Australia are locally known as willy willies. Another type of cyclone is the tornado which often blows in the Mississippi basin in the U.S.A. The winds in them may attain the velocity of over 300 km per hour. Where it touches the ground it causes unbelievable destruction.

Anticyclones: An Anticyclone is an area of high atmospheric pressure which goes on diminishing outward from the center. The winds are usually light and blow clockwise in the Northern Hemisphere (anti-clockwise in the Southern Hemisphere). They are also roughly circular OT oval in shape.

The anticyclones do not move in any definite direction. They also move very slowly and sometimes may drift about and gradually disperse or remain stationary for several days. As the air is warmed when descending, it has a tenancy to gather moisture rather than deposit it. There is, therefore, less cloudiness and very little rain in an anticyclone. Jet Streams: They are powerful currents of air that move along west to east courses at heights between 9,000 metres and 15,000 metres. They attain speeds of 650 km per hour. They are created by air temperature differences where weather fronts meet. Scientists say that their behavior has a great effect on weather patterns throughout the world.

Clouds : Cloud is a mass of minute water droplets or tiny crystals of ice formed by the condensation of the water vapour in free air at considerable elevations. As the clouds are formed at some height over the surface of the earth, they take various shapes. According to their height, expanse, density and transparency or· opaqueness clouds are grouped under four types: (i) cirrus; (ii) cumulus; (iii) stratus; (iv) nimbus.

Cirrus : Cirrus clouds are formed at high 'altitudes (8,000-12,000 m). They are thin and detached clouds having a feathery appearance. They are always white in colour.

Cumulus : Cumulus clouds look like cotton wool. They are generally formed at a height of 4,000-7000 m. they exist in patches and can be seen scattered here and there. They have a flat base.

Stratus: As their name implies, these are layered clouds covering large portions of the sky. These clouds are generally formed either due to loss of heat or the mixing of air masses with different temperatures.

Nimbus: Nimbus clouds are black or dark gray. They form at middle levels or very near to the surface of the earth. These are extremely dense and opaque to the rays of the sun. sometimes, the clouds are so low that they seem to touch the ground. Nimbus. clouds are shapeless masses of thick vapour.

How to measure Wind?

The two most important things about the wind are its speed and direction in which it is blowing. We use a weather vane or a windsock (a kind of long cloth tube through which the wind is funnelled) to see wind direction. It is expressed in compass points. Wind speed is measured by the Beaufort Scale, windsocks or by special scientific instruments called anemometers. The unit of measurement is kilometers per hour (km/h) or knots.

The Beaufort Scale The Beaufort Scale was invented in 1805 by Admiral Beaufort to estimate wind speed through observations of objects. The original scale was for use at sea but it has been adapted for use on land.

The Beaufort Scale :-

Force Strength weather symbol km/h Effect
0 Calm - 0-1 Smoke rises vertically
1 Light air - 1-5 Smoke drifts slowly
2 Light breeze - 6-11 Wind felt on face; leaves rustle
3 Gentle breeze - 12-19 Twigs move; light flag unfurls
4 Moderate breeze - 20-29 Dust and paper blown about; small branches move
5 Fresh breeze - 30-39 Wavelets on inland water; small trees move
6 Strong breeze - 40-50 Large branches sway; umbrellas turn inside out
7 Near gale - 51-61 Whole trees sway; difficult to walk against wind
8 Gale - 62-74 Twigs break off trees; walking very hard
9 Strong gale - 75-87 Chimney pots, roof tiles and branches blown down
10 Storm - 88-101 Widespread damage to buildings
11 Violent Storm - 102-117 Widespread damage to buildings
12 Hurricane - Over 119 Devastation