CHAPTER NO.7 WINDS

CHAPTER NO.7 WINDS

Variation in air pressure at different places gives birth to winds. Atmosphere

is a mixture of gases therefore it is highly mobile. This mobility originates out of difference in air pressure of various places, in simple words, known as winds. Winds move from high pressure to low pressure regions. This cycle of winds moves on the whole Earth which divide and distribute the temperature and humidity from Equator to Poles. Winds always try to cover the disimilanties in pressure of air.

According to Coriolis Effect which is known as Ferrel’s law also, independently moving flows in Northern Hemisphere tend to turn to their nght and in Southern Hemisphere towards their left because of rotation of Earth. This effect is seen in winds also.

Winds are divided into three types on the basis of term and tenure their of :—

1. Planetary or Perennial Winds

2. Seasonal Winds

3. Local Winds.

1. Planetary Winds : These winds move in one direction throughout the year. They

move from high pressure areas of major pressure belts to low pressure areas. These winds are also known as stable winds. Eastern or Equatorial winds, Tradewinds,Western winds and Polarwinds are examples of these winds.

(a) Eastren or Equatorial Winds : Region between 5°N latitude to 5°S latitude is

known as Equatorial low pressure belt. This belt receives vertical rays of sun because of which temperture is very high and moreover the temperature of air present near the surface of Earth increases and air starts moving upward. These are known as equatorial winds or Doldrums.

In this belt mostly the winds move vertically and daily in the afternoon they bning heavy thunder showers, this region is also known as Inter-tropical convergence zone (IICZ)because trade winds of both hemispheres join here.

(b) Trade winds : Region of trade winds extends from Equatorial low pressure belt to 30° North and South lattitude. In this region winds move from sub tropical high pressure areas to low pressure areas of equatorial low pressure belt.

Earth rotates an its axis from west to east and in northen hemisphere these winds deflect towards right under the coriolis effect and according to the rule/law of Farrel because of which these are known as North-Eastern trade winds. On the other hand in southern hemisphere these winds deflect towards left therefore these are known as

south western trade winds.

According to the myth the name ‘Trade’ winds has been derived from the word

‘Track’ of German language which means a defi nite/particular path. Trade winds blow continously in one direction with adefimte speed. In ancient times sailors sail from one place to another with the help of these winds. These winds cross the oceans and bring rainfall in the eastern parts of continents while reaching upto western parts these

become dry because of which mostly the deserts are found on the western parts.

latitudes in both the hemispheres. This region matches to the vertical wind direction of equatorial region winds while in direction being exactly opposite toit. Hot air rises up in the equatonal regions but in this region air begins to descend toward the surface

which results in cool and dry weather. This region is also known as ‘Horse latitudes’because according to a myth, in ancient times when sailors were dependent on winds for the movement of ships, they face problem in movement due to the reduction of speed of wind in this region. They used to drop their horses in the sea so that the weight

of ship could be reduced and they could save it from sinking.

(d) Westerlies : Westerlies are the prevailing winds in the middle latitudes (i.e.between 35 and 65 degree) in both hemispheres like Trade winds, these winds also change their direction due to Coriollis Effect. In northern hemisphere these blow from south west and in southern hemisphere these blow from north west. These winds bring rainfall in the western parts of continents.

Variations are found in speed and direction of these winds due to the irregular division of water and land in northern hemisphere. On the other hand most of southern hemisphere is covered by water in these latitudes because of which these winds are more strong and regular in this sphere. These winds also have some local names given by sailors like. Roaring Forties for 40° latitude, "Furious Fifties" (50 to 60 degrees south) and Shirking sixties for 60° latitude.

e) Polar winds : Polar winds blow from high pressure polar regions to low pressure sub polar regions. Polar regions have low temperature and these are covered by ice.These winds are dry and donot bring rainfall because these are cold and do not attain moisture. Polar regions are extremely cold and no one is living there because of which

we donot have much information about these regions. But still we can say that as westerlies these winds are more regular in southern hemisphere as compare to northern hemisphere.

Shifting of wind belts : Circulation of wind on the earth’s relief 1s quite different from the above mentioned conditions. We all know that earth rotates on its axis and it also

revolves around sun and moreover it is inclined at 23 1/2°. Because of this the position of sun on Earth change continously in a year. Sun rays fall vertically on Equator as well as on Tropic of Cancer and Tropic of Capricon.

Commonly we say that sun rays fall vertically on equator but if we study world air pressure maps for the month of January and July, we will come to know that air pressure belts change their position during these months.

On June 21 sun rays fall vertically on Tropic of Cancer. During these days Northern hemisphere has summer season and Southern hemisphere has winter season.Equatorial high temperature and low pressure regions shift towards north. Similarly other air pressure regions in northern hemisphere shit toward north and regions in southern hemisphere shift towards Equator.

Average situations are found twice in a year when sun rays fall directly on the Equator i.e. March 21 and September 23. This region has high temperature and low air pressure. Air pressure belts also shift on December 22 but their direction is opposite to the direction of June 21.

Due to the change in the situation of Sun and Earth, airpressure and wind belts shift from their definite position because of which various effect have been observed in white world, some of which are as follows :

i. Region which extends from 5° to 10° latitude on the both sides of equator remians hot and humid due to low air pressure in summer and in winters it remains dry by the effect of trade winds.

ii. The western parts of the continents which lie between 30° to 45° lattitudes in both the hemispheres remain dry in summers due to effect of north eastern and south eastern Trade winds. When air pressure belts shift towards Equator in Northern hemisphere,Westerlies bring rainfall in the western parts of the continents.

ii. Change in positioning of sun brings some difference in weather and seasons

particularly in the regious falling between 60° and 70° latitudes in both the hemispheres.In Northen hemisphere where these higher latitudes find effect of Westerlies, snow melts and vegetation starts growing. Similarly, cold effect 1n winter brings dry weather with Polar winds.Generally Western winds bring rain while Polar winds bring dry spell with them.

iv. Regions of monsoon winds have been created in India and world due to the shifting of air pressure belts and wind belts. Moreover due to this effect these winds blow completely opposite to each other in summer and winter.

2.Seasonal or Temporary winds : Seasonal winds are effected by the change in temperature and air pressure according to the change in season Monsoon winds are the finest example of these winds.

Origin of Indian Monsoon

The Term ‘Monsoon’ has been derived from the Arabic word ‘mausim’ — meaning

‘Season’. Monsoon is a wind system in which there is compleat reversal of prevailing direction of winds after every six months, 1.e. from Summer to Winter and Vice Versa.Concepts about the origin of Monsoon Monsoon is acomplex meteorological phenomenon. Some of the important concepts about the origin of monsoon have been discussed as under :

The Thermal Concept or Classical theory was propounded of Halley in 1686.

According to this concept, Monsoons are the extended land breeze and sea breeze on a large sale, produed by the differential heating of continents and ocean bodies. During the summers in northern hemisphere, When the sun rays are vertical over tropic of cancer, the huge landmas of Asia heats quickly and develops a strong low pressure centre near lake Baikal (Siberia) and Peshwara (Pakistan). Moreover the poleward

shift of the Inter tropical Convergence Zone (ITCZ) to a position over Southern Asia reinforces the thermally induced low pressure centre in comparison to this, the pressure over the oceans towards the Indian and Pacific oceans is relatively high. Under these conditions, a sea to land pressure gradient develops. Consequently, the surface air flow is from the high pressure over the ocean towards the low pressure areas over the heated land. Contrary to this during the winter season in the northern hemisphere these develops a high pressure over near Baikal (Siberia) and Peshwar and Indian and Pacific oceans are comparatively warm haivng a low pressure. The winds blow from Land to Sea.

Dynamic Concept

The Dynamic Concept about the origin of Monsoon was put forward by Flohn in 1951. In his opinion, The Monsoon is the result of Seasonal Migration of Planetary Winds and pressure belts (fig.) The Inter-Tropical Convergence Zone At the time of Summer Solstice (21 June), when the sun rays are vertical over the tropic of cancer the North ITCZ is extended upto 30°N latitude, covering South and South-East Asia and thus equitorial westerlies are established over these Areas. The equatorial westerlies are established over these areas (fig.) The equatorial westerlies become South-West or Summer Monsoon. The NITCZ is associated with numerous atmospheric strams (Cyclones).

Recent Concepts

Tibetan Plateau and the circum Polar whirl. In 1973. The Monsoon Expedition

(Monex) was organized under the Joint Venture of the former Soviet. Moon and India.On the basis of investigation the meteorologists arrived at the conclusion that the Tibet Plateau plays a vital role in initiating the monsoon circulation over the Indian Subcontinent. Tibet Plateau 1s a high table land with dimensions 2000 x (600— 1000)square Kilometer it has an average height of 4 Kilometer. Tibet Plateau affects the atmosphere in two ways (1) as a physical barrier, and (11) as a high level heat source.At the begining of Juen the Sub tropical Jet Stream disappears completely over northrn

India. (fig.). At this time the Jet Stream shifts to thenorth of Himalayas and Tibet and takes up a position at about 40°N. The plateau of Tibet accentuates the northward displacement of Jet Stream. Thus the abrupt onset of Summer Monsoon at the beginning of June is prompted by the hydro-dynamic effect of the Himalayas and not by the thermally induced low pressure centre over north West India in the middle of

October the Plateau Proves to be the most important factor in causing the advance of the Jet Stream South of Himalayas or bifurcate it into twoparts.

EL Nino and Monsoon

EL Nino meaning “Child Christ’ is a warm ocean current appearing along the Peru (South America) coast generally in December. It replaces the cold Peru or Humboldt current flowing along the Perm coast in normal years. Under Normal conditions, the Peru is Cold Water Current, While over the Western Pacific (Indonesia and Eatern Australia) the ocean current is warm and deep. (Fig.). The appearance of EL-Nino ‘reversal’ the condition’ these develops warm conditions over the eastern Pacific (Peru Coast) and Cold conditions in Western Pacific. Whenever this usually warm ocean current (EL-Nino) is produced near the Peruvian C oast in South America, the amount of Precipitation in the coastal regions is usually high while the Australian and Indonesian coasts record drought conditions in brief, the occurrence of EL-Nino results into weak Monsoon causing droughts floods and failure of crops.

The Indian ocean has been warming at a rate faster than ever before (1.2°C during the Past Century). It is also the largest consistent contributor to the global ocean warming treds. The Western Indian Ocean, traditionally thought to have cooler sea surface temperatures than the central and eastern Idnian Ocean, is surprisingly showing an even

stronger summer warming trend over the whole of the 20" Century then the central and eastern Indian Ocean.

EI-Ninois abnormal warming of sea surface of south east Pacific ocean. during El-Nino event, Plankton does not flourish luxuriantly. Krill fish disappears. Penguins in Antarctica climate observe baby depression. Wild purple flower are found everywhere

on Atacama sands. Thermocline either go deeper or changes its ends. Tropical diseses increases manyfold. Landslides intrupt the system of life over predmont plains of Andes or the western coast of South America.

South east Pacific ocean always remain a zone of confrontation between counter equitonal current and Humboldt cold ocean current, they tend to overpower each other. The constant upwelling of the cold water of Humboldt current generates cold sea surface and high air pressure. If west wind drift does not push sufficent cold water toit, counter

equitorial current establishs its warm water over the larger areas. and raises sea surface temperature and induces El Nino conditions.

A recent study focussed on the causes of its warming and found that it was mainly due to EL-Nino events. Which are getting longer and more frequent during recent decades,Possibly due to changing climate. These EL Nino events eaken the summer westerly winds over the Indian Ocean. Winds have the effect of cooling the sea surface. Strong winds cause evaporation and loss of latent heat from the ocean leading to cooling.When the winds are weakened the opposite happens - The ocean warms.

The study published recently in the ‘Journal of Climate’ was undertaken by Dr. Roxy Mathew Koll of the Indian institute of Tropical Meteorology, Pune and has found that the ocean atmospheric Phenomenon-The EL-Nino, and the influence on the walker circulation were responsible for periodic weakening of Monsoon Westerlies and led to abnormally high summer sea surface temperature in Western Indian Ocean. The study has found that “The Frequency and magnitude of El-Ninos have also increased

in the recent decades. Possibly due to global warming. This means a piling up of heat on the Indian Ocean.

Mostly this effect has been observed in Asian countries like India, Malayasia, China,Korea, Japan and Taiwan. Moreover, we can also see this effect in USA, Northern Australia and Western Africa.

These winds originate due to the difference between the temperature of land and water regions. Moreover, shifting of air pressure belts due to change in the position of sunand

Earth also act as a major reason for the origin of these winds.

Research about these winds is going on at world level with advanced scientific

techniques so that we can understand the nature of these winds accurately. Study about the higher atmospheric activities is also going on, specially about the role of Jet stream.

La Nina:

The sea surfce temperature of south east pacific ocean subsides by 3*to 5*Celsius. it leads to stronger Walker circulation. Robust, south east trade winds carry more moisture and all the places will be wetter than normal conditions on the eastern margins of the continents. Western margins will exibit excessive drought conditions.

Pre advent of Monsoon, excessive rainfall over south east and Eastern Africa are frequently associated with La-Nina. It occures when counter equitorial current is weak and west wind drift pushes more water in Humboldt current. protracted La Nina conditions takes place on occassions. Mid 1998 to early 2001, the sea surface temperature of south east Pacific ocean remains excessively below normal. Last L

Nina occures in 2010-11. itis expected, 2017 is having an extra ordinary La-Nina

year.

3. Local winds : Local winds originate due to the difference between local relief and temperature and their effect is also local. These winds are limited upto particular region and local people give name of these winds. Name of these winds describe the affect on that particular region.

These winds can be further classified into two parts :

(i) Hot winds

(11) Cold winds

(i) Hot Winds : Hot winds originate by advection of hot air from warm source region and its movement toward adjacent region and moreover temperature of wind rises when it descends in mountainous region.

Foehn : Fohn or Foehn is a type of dry, warm, down slope wind that occurs in the

northern slopes of Alps mountain for its heal and dryness is its dry adicbetic rate which increases temperature even upto 10°C per kolometer while decending from mountain tops. In spring season these winds increase the temperature because of which the ice melts and conducive enviornment is created for the cultivation of wheat and growth of

fodder for animals. Increase is temperature and decrease in humidity brings soothing effect in weather.

Chinook : These are the hot and dry winds which descend toward Praire plains from Rocky mountains of North America, particularly in Colorado, Wyoning, British Colmbia and Montana states in spring season locally known as ‘Chinook’. ‘Chinook’ means ‘snow eater’ for locals. Like Foehn winds these are also very helpful for local people. These winds create conducive environment for the cultivation of crops, fodder

for animals and also give relief from cold. Canadian Punjabis have given them a new name, “Shoonkan’ .

Santa Ana : These are hot and dry down slope winds like Foehn and Chinook, they

blow in southern parts of California (USA) from Santa Ana mountaineous regions to coastal plains. But these are harmful for orchards and other vegetations in plains and trees start dying because of dryness.

Loo : These are hot and dry winds which blow in parts of northern Indiai.e. Punjab,Harayana, UP and Bihar during the month of May and June. These are very harmful for human beings and every year large number of people die because of these winds.

Khamsin : These are hot and dry winds which blow in Egypt (Africa) from April to June and they carry dust particles with them. At times the blow off big dewellings.

Sirocco : Hot, dry and dusty winds which blow towards north from Sahara desert are knwon as ‘Sirocco’ winds. These winds cross the Medditranian sea and enter in Italy and Spain. These winds attain moisture while crossing the sea and bring rain laiden with dust. This rainfall is harmful for crops.

Harmattan : Harmattan winds are hot and dry and they blow from Sahara desert to

Gulf of Guinea and carry dust particles with them. These winds decrease the humidity on the western coasts because of which local people have given the name ‘doctors’to these winds.

(ii) Cold winds :

Mistral : Cold and dry winds blowing in spain and France during winter season are known as Mistral winds. These winds blow from Mediterranean plateaus to valleys of Rhone. These winds decrease the temperature abruptly which affects the crops and human life.

Berg : Cold and dry winds decending from the southern slopes of Alps mountain range are knwon as ‘Berg’. These winds affect the eastern coasts of * Adriatic Sea’ (Italy).

Blizzard : Cold and dry snowy winds blowing in snow /ice covered polar regions are known as Blizzard. As dust storms these winds also decrease the visibility. These winds blow in USA, Canada, Siberia and Antartica.

CYCLONES

Cyclone is a system of low pressure in which the barometric graident is steep. Ina cyclone winds circulate, blowing inward in an anti-clock-wise in the NorthernHemisphere and in aclockwise direction in the Soutern Hemisphere.

The Cyclones are mainly of two types : (1) The Temperate Cyclones also known as mid-latitude or extra tropical cyclone and (11) the tropical cyclons known by different names in different countries of the world, like Hurricane in U.S.A. and Typhoons in China.

(1) Temperature Cyclones (Mid latitude or Extra tropic Cyclones or Depressions)

The low pressure system of Temperate latitude is known as temperate cyclone,

recently replaced by the term ‘depressions’. In general, a depress is a region where the atmospheric pressure is low relative to that of its surroundings. The main characteristics of a temperate cyclone are as under :

1. The isobars of a temperate cyclone are more or less oval or elongated in shape.

2. The isobars of a lowest pressure is nearest to the centre of depression.

3. In size (Diameter) it may be 150-3000 km (100-2000 Miles).

4, It may be practically stationary or moving at about800 to 1100 km (600 — 700

Miles) per day.

5. In the Northern Hemisphere, the winds of a depressoin (Cyclone) circulate round the centre in an anticlockwise direction, while in the Southern Hemisphere, the wind blow in aclockwise direction.

6. The temperate cyclone originates where warm tropical air meets cold polar air, the former ascending over the latter with the formation of frontal surfaces.

7. The depressions are characteristics with unsettled and variable weather.

8. Their general direction of movement is from west to eaast in themid-latitudes (westerlies) but specific paths are often curved and sometime erratic.

9. The Average speed of the temperate cyclone is about 30 to 50 km per hour or 800 to 1100 km per day. Speed is however, greater in Winters than in the summers.

10. The rainfall is light to moderate which occurs in the form of light showers. Fog and poor visibility are common in the precipitation areas.

11.A few hours after the front has passed, clean weather (Anticyclone) prevails.Before the arrival of a temperate cyclone the high wispy cirrus clouds often in the form of mare’s tail appear first over the Western Horizon As the front appraches, the clouds

lower and thicken Progressively to Cirrostratus, altostratus and nimbostratus. The temperate cyclones rarely appear alone and they move in form of a family. Most frequently, three or four such cyclone forms a series, and this is called acyclone family.

Geographical Distribution of Temperate Cyclones :The Major Areas and tracks of temperate cyclones have been shown in the map. In General they occur between 0° to 60° latitudes in both the Hemispheres However, in the Southern Hemisphere, the belt of maximum frequency of Cyclones is almost

cotinuous around the world. The Great strom in U.K. 1987 “Great Strom’ and Wilma in United States in 2005 were disastrous.

TROPICAL CYCLONES

Tropical Cyclones is a system of low pressure occuring in tropical latitude extend from the Tropic of Cancer 2342°N to the Tropic of Capricorn 232° south, encompassing the equitonal belt of Doldrums the between 10°N and 10° South. They originate near the Western Flanks of the oceans, where warm tropical currents supply an abundance of Water Vapours. Tropical Cyclone is a powerful manifestation of earth’s energy and

moisture systems. The following weather conditions are necessary for the origin of a tropical cyclone :

1. There should be continuous supply of abundant warm and moist air.

2. The sea temperature in lower latitude should be around 27°C.

3. They develops in a Inter Tropical Convergence Zone (ITCZ).

4. Existence of weak tropical disturbance is also required. The Pre-existing milk tropical disturbance, under favourable conditions, intensifies and develops into a Violent tropical cyclones.

5. There should be anticyclonic circulation at the height of 9 KM to 15 KM above the surface disturbance. The upper air anticyclonic circulation sucks the air from the ocean surface above, thus the upward movement of air is accelerated and low Pressure Centre at the surface is intensified.

The central part of the Cyclone are known by different names in different parts of the world. For example, in China Sea they are called as Typhoons (From the Arabic Word *Toofan’. Baguios’ in Philippines, *Taifu’ in Japan, in the Carribean Sea and the Gulf of Maxico as Hurricanes, in the north West Australia as * Willy- Willies’ and in the Bay of Bengal, Arabian Sea, Madagascar and Africa, as simply cyclones.

Approximately 80 tropical cyclones occur annually worldwide. The Direction of winds in the tropical cyclones in anti clockwise in the Northern Hemisphere and Clockwise in the Southern Hemisphere. The main characteristics of tropical cyclones are as under :

1. In a tropical cyclone, the isobars are generally circular, and close to each others resulting into steep Pressure gradient.

2. The Whole Cyclone has a diameter of about 150 to 300 Km.

3. They originate in the tropical regionon the Western Margins of oceans, where the sea temperature exceed 27°C.

4. Most of them originates in the equitorial belt of calms — the doldrums, when this belt is at its farthest limit from the equator.

5. They are most frequent in late summers and autumn (August to October).

6. Violent Wind of Hurricane Speed (119 Km/h) Circulate around the centre of ‘eye’of the strom — The eye of the Cyclone covers a restricted area, being often about 15 to 30 per in diameter in the eye of the Cyclone, the atmospheric pressure is excessively low, the air is sometimes calm, and the sky clear. Passage of the eye may take about

half an hour, after which the strom strikes with renewed ferocity, but with winds in the opposite direction.

7. A tropical cyclone is accompanied by towering cumulonimbus clouds, torrential

rainfall, violent winds, thunder and lightning.

8. The Majority of Cyclones decay when they come outer land or when they recurve northward over oceans.

The Tropical Cyclone cause immense domage to the coastal areas as they destroy buildings, roads, railways, bridges and shipyards. Moreover, trees are uprooted, crops damaed and Platations ruined. Thousand of lives have been lost in Bangladesh, India,

Myanmar, Malaysia, Maxico, U.S.A. etc. because of Such Cyclone in the year 2014

Cyclone ‘Hudhud’ ‘Nan, Nilofer and in 2015 *Ashobaa’ and Komen have caused

widespread damage in India (Map Given) In 2013 Mahasen Philin, Helen and Lehar

Cyclonic storm ‘Roanu' in the Bay of Bengal in may 2016 Andaman Islands and

remaining parts of North Andaman Sea were important.

TORNADOES

Tornado is a rapidly rotating column of air developed around a very intense low

pressure centre. It is associated with dark funnel shaped cloud and with extremely violent winds (More than 400 Km/hour). The precise mechanism about the origin and development of a tornade is not fully understood but the following atmospheric conditions appear to be necessary for Tornado development:

(1) A layer of warm moist air at low latitude.

(2) A layer of dry air at higher altitude with an intense cold front.

(3) Solar heating of the ground.

Tornados are generally regarded as having the greatest wind intensity of all the meterological hazards. The updraught of Tornado is associated with acold front squall line and cumulommbus clouds. The tornado appears as a dark funnel cloud hanging from the base of a dense cumulonimbus clouds. Tornadoes occurin many parts of the world but are most strongly associated with and most common in the united states —about 1000 are recorded there each year.

Devastation from a tornado is often complete with in the narrow limit ofits path. The precise mechanism about the origin and development of a tornados not fully understood but the following atmospheric conditions appear to be necessary for Tornado development :

Anticyclone (The High)

The Concept of anticyclone was put forward by Sir francis Galton in 1861. A centre of high pressure is known as anticyclone. In other words, it is a region in which the atmospheric pressure is high compared with that of adjacent areas, and which shows at least one closed isobar. Generally, there is a series of concentric closed isobars,

approximately circular or oval in shape, the highest pressure being at the centre.

Diameter of anticyclone range from a few hundred to a few thousand Kilometers.

The anticyclones of sub tropics are associated with the sub tropics highs. They are more stationary. In the Northern Hemisphere the genral and circulation is clockwise

round the anticyclone and in the Southern Hemisphere, anticlockwise. The associated weather is settled and stable, generally warm, sunny and dry in Summer and Cold,frosty and clear (or foggy) in Winter. In the basis of their structure then may be classified into (1) The sub-tropical highs,

(2) The Polar Continental Highs

(3) High with in the Cyclone Series

(4) The Polar Outbreak Highs blowing in an anti-clockwise spiral, but accompanied by violent down draughts.