Climate Profile of India

INTRODUCTION #

India is a land of contrast of relief and climate. Geographically India is a peninsular extension of the great Eurasian landmass. Climatologically India comes in tropical, sub tropic and temperate regimes. India is basically a tropical country although its northern part is situated in the temperate belt. In the south, the Indian coasts are washed by the Arabian Sea and the Bay of Bengal branches of the Indian Ocean which give it a typical tropical monsoon climate.

FACTORS DETERMINING THE CLI- MATE OF INDIA #

1. Location and Latitudinal Extent: The Trop- ic of Cancer passes through the central part of India in east-west direction. Thus, north- ern part of India lies in sub-tropical and tem- perate zone and the southern part falls in the tropical zone. The tropical zone being nearer to the equator, experiences high temperatures throughout the year with small daily and an- nual range. Area north to the Tropic of Can- cer being away from the equator, experiences extreme climate with high daily and annual range of temperature.
2. Distance from the Sea: Arabian Sea and Bay of Bengal surround the Indian peninsula and make climatic conditions mild along the coastal areas. Areas in the interior of India are far away from the moderating influence of the sea thus having extremes of climate. That is why the annual range of temperature at Kochi does not exceed 3°C whereas it is as high as 20°C at Delhi.

• The Himalayas: The Himalayan Ranges pro- tect India from the bitterly cold and dry winds of Central Asia during winter. Further, these ranges act as an effective physical barrier for rain-bearing south-west monsoon winds to cross the northern frontiers of India. Thus, the Himalayas act as a climatic divide between the Indian Subcontinent and Central Asia.
• Physiography: Physical map of India is closely related to the climatic conditions of the country. Places located at higher altitude have cool climate even though they are locat- ed in the southern India, i.e., Ooty. Similarly, though Agra and Darjeeling are located on the same latitude, the temperature of Agra in January is 16°C whereas it is only 4°C in Darjeeling. The physiography of India also affects the direction and speed of wind and the amount and distribution of rainfall. The windward sides of the Western Ghats and As- sam receives high rainfall during June-Sep- tember whereas the southern plateau remains dry due to its leeward situation along the Western Ghats. It is due to physiography that the funnel shaped Cherrapunji valley is the wettest place on Earth.
• Monsoon Winds: The most dominating fac- tor of the Indian climate is the ‘Monsoon winds’ as a result of which it is often called the Monsoon Climate. The south-west sum- mer monsoons from the Arabian sea and the Bay of Bengal bring rainfall to the entire country. The north-eastern winter monsoons travel from land to sea and does not cause much rainfall except along the Coromandel

coast after getting moisture from the Bay of Bengal.

• Upper Air Circulations: The changes in the upper air circulation over Indian landmass in- fluence the climate of India to a great extent.
  • Westerly Jet Stream: All of Western and Central Asia remains under the influ- ence of westerly winds along the altitude of 9-13 km from west to east. These are known as Jet Streams. Tibetan highlands act as a barrier in the path of these jet streams. As a result, jet streams get bi- furcated. One of its branches blows to the north of the Tibetan highlands, while the southern branch blows in an eastward direction, south of the Himalayas. It is believed that this southern branch of the jet stream exercises an important influ- ence on the winter weather in India as they bring western disturbances from the Mediterranean region to India.
  • Easterly Jet: Reversal in upper air cir- culation takes place in summer due to the apparent shift of the sun’s vertical rays in the northern hemisphere.

The westerly jet is replaced by the easterly jet stream which owes its origin to the heating of the Tibetan plateau. This leads to the devel- opment of an easterly cold jet stream centered around 15°N latitude and blowing over pen- insular India. This helps in the sudden onset of the southwest monsoons. The easterly jet stream steers the tropical depressions into In- dia. These depressions play a significant role in the distribution of monsoon rainfall over the Indian subcontinent.

• Tropical Cyclones: Tropical cyclones orig- inate over the Bay of Bengal and the Indian ocean. These tropical cyclones have very high wind velocity and heavy rainfall and hit the Tamil Nadu, Andhra Pradesh and Orissa coast. Most of these cyclones are very de- structive due to high wind velocity and tor- rential rain.
• Western Disturbance: The western distur- bances, which enter the Indian subcontinent from the west and the northwest during the

winter months, originate over the Mediterra- nean Sea and are brought into India by the westerly jet stream. They influence the winter weather conditions over most of the Northern plains and Western Himalayan region.

• El-Nino Effect: El-Nino is a narrow warm current which occasionally replaces the cold Peru current. This is responsible for wide spread floods and droughts in the tropical re- gions of the world. It is believed that the se- vere drought of 1987 in India was caused by El-Nino.
• La Nina: The returning of the weather condi- tions to normal after an El-Nino is called La Nina. The presence of La Nina is the harbin- ger of heavy monsoon showers in India.

MONSOON #

Monsoon comes from an Arabic word ‘MAUSAM’ which means season. Monsoon are seasonal winds which reverse their direction of flow with the change of season. They flow from sea to land during the summer and from land to sea dur- ing winter. The theories of monsoons are generally divided into following two broad categories:

1. Classical theory: Halley explained the mon- soon as resulting from thermal contrasts be- tween continents and oceans due to their dif- ferential heating. In summer the sun shines vertically over the tropic of cancer resulting in high temperature and low pressure in cen- tral Asia while the pressure is still sufficient- ly high over Arabian Sea and Bay of Bengal. This induces air flow from sea to land and brings heavy rainfall to India and her neigh- bouring countries. In winter the sun shines vertically over the tropic of Capricorn. The north western part of India grows colder than Arabian Sea and Bay of Bengal and the flow of the monsoon is reversed. It was lacking in the physical ingredient of the effect of rota- tion of the earth.

2.     Modern theory: #

• Role of ITCZ; according of FEOHN monsoon is only the normal seasonal mi- gration of planetary winds following the sun. According to him the existence of

Asian monsoon is not due to contrast be- tween land and sea but mainly due to the annual migration of thermally produced planetary winds and pressure belts un- der continental influence. The southeast trade winds of the southern hemisphere cross the equator and start flowing from southwest to northeast direction under the effect of Coriolis force. These dis- placed trade winds are called south west monsoon and bring monsoon to the re- gion.

• Role of jet streams; M.T. Yin had given this concept stating that the burst of mon- soon depends upon the upper air circu- lation. Two prominent jet streams effect the monsoon winds:
  • The sub tropical westerly jet stream, this jet stream dominates in winter time in the upper troposphere circu- lation of the northern latitudes. It has a global extent between latitudes 25- 32° N and can be located over south Asia at an elevation of about 12 km. The jet stream is split owing to the presence of Himalayan mountain system in its path. The winds tend to descend over northwestern part of India resulting in atmospheric stabil- ity.
  • Equatorial easterly jet stream, this jet is a prominent feature of the up- per air circulation during the Indian monsoon season appearing as a band of strong easterlies extending from south East Asia across the Indian Ocean and Africa to the Atlantic. The western and eastern jet streams flow in the north and south of the Hi- malayas respectively. The eastern jet becomes powerful and stationed and this results in more active south west monsoon.
  • Role of Tibetan plateau, the Tibetan plateau is located more than 4500 km above sea level with a length of 2000 km

and with a width of 600 km in the west and 1000 km in the east. This plateau is considered to be one of the key factors in the development of monsoon. The Tibetan plateau exerts its influence as a mechanical barrier as well as high heat plateau. An anticyclone appears in upper troposphere due to latent heating over the Tibetan plateau. It generates an area of rising air, during its ascent the air spreads outwards and gradually sinks over the equatorial part of the Indian Ocean. It picks up moisture from the Indian Ocean and causes rainfall in India and adjoining countries.

Characteristics of Monsoonal Rainfall #

• Rainfall from the southwest monsoons is seasonal in character, which occurs be- tween June and September.
• Monsoonal rainfall is largely governed by relief or topography. For instance the wind- ward side of the Western Ghats registers a rainfall of over 250 cm. The heavy rainfall in the northeastern states can be attributed to their hill ranges and the Eastern Himala- yas.
• The monsoon rainfall has a declining trend with increasing distance from the sea. Kol- kata receives 119 cm during the southwest monsoon period, Patna 105 cm, Allahabad 76 cm, Delhi 56 cm and Bikaner 24 cm.
• The monsoon rains are characterized by ‘Breaks’. These breaks in rainfall are re- lated to the cyclonic depressions formed at the head of the Bay of Bengal, and their crossing into the mainland. The frequency of such depressions is 2 to 4 per months, from June to September. Besides the fre- quency and intensity of these depressions, the passage followed by them determines the spatial distribution of rainfall.
• The summer rainfall comes in a heavy downpour leading to considerable run off and soil erosion.
• Monsoon is the pivot of the agrarian econ- omy of India because over three fourths

of the total rain in the country is received during the southwest monsoon season.

• Its spatial distribution is quite uneven rang- ing from 12 cm in western Rajasthan to more than 400 cm in Meghalaya.
• The beginning of the rains sometimes is considerably delayed over the whole or a part of the country. The rains sometimes end considerably earlier than usual, causing great damage to standing crops and making the sowing of winter crops difficult. This is why monsoons are extremely unpredictive and uncertain.

RAINFALL DISTRIBUTION #

1.     Areas of very high rainfall (annual rainfall of 200 cm and above) #

These include the west coast from Thiruva- nanthapuram in the South to Mumbai in the North (Avg. annual rainfall 200 – 400 cm). Almost the whole of Assam, Nagaland, Me- ghalaya, Mizoram, Arunachal Pradesh, Sik- kim, parts of Manipur, Tripura and northeast- ern tip of West Bengal also receive 200 cm or more, with isolated pockets receiving over 400 cm. Meghalaya (The abode of clouds) is the wettest part of the country with Mawsyn- ram and Cherrapunji getting 1221 cm and 1102 cm of annual rainfall respectively.

2.     Areas of High rainfall (100-200 cm annual rainfall) #

These include eastern slopes of the Western Ghats, major part of the northern plain, Oris- sa, M.P. Andhra Pradesh, and Tamil Nadu.

3.     Areas of Low rainfall (50 – 100 cm annual rainfall) #

Include large parts of Gujarat, Maharashtra, Western M.P., Andhra Pradesh, Karnataka, eastern Rajasthan, Punjab, Haryana and parts of Uttar Pradesh.

4.     Areas of very Low rainfall (Less than 50 cm of annual rainfall) #

These are desert and semi-deserts areas. They include large parts of Western Rajasthan, Kuchchh, and most of Ladakh region of Jam- mu and Kashmir.

WESTERN DISTURBANCES #

With the southward shift of the polar front in winter, the tracks of middle latitude cyclones pass across the northern portion of the Indian subconti- nent during the period October to June. In the other months the tracks shift far to pole ward and do not usually affect the Indian region. The temperate cyclones originates in western Asia and Mediterra- nean Sea and reach the Indian area in the course of their eastward passage from west India and refer to as western disturbances. These western disturbances causes.

• Snowfall in higher reaches of Himalayas.
  • Rainfall in north west plains which are ben- eficial for rabi crop.
  • The sudden cold wave that decreases the tem- perature of that area and hail also takes place.

LOCAL WINDS #

During summers the atmospheric pressure is low all over the country. In the months of May and June high temperature in North West India builds up steep pressure gradient and under such conditions dust laden and strong winds blows.

• LOO: It is hot dust laden winds that usually starts in the morning and reaches its peak at afternoon. This increases the temperature of the area and causes high humidity.
• AANDHIS: These are basically thunder- storms which move like a solid wall of dust and sand. The winds velocity is high and vis- ibility reduces to few meters only. Such dust storms are common in Rajasthan, Haryana, Delhi, Uttar Pradesh etc. In West Bengal and adjoining areas of Jharkhand, Orissa the di- rection of storm is mainly from the North West and are called NORWESTERS.
• KAL BAISAKHIS: These are violent storms causes heavy damage to standing crops, live- stock and human beings. Maximum occurs in month of March and April.

The Nature of Indian Monsoon

Onset of the Monsoon #

• During April and May when the sun shines vertically over the Tropic of Cancer, the large

landmass in the north of Indian ocean gets intensely heated. This causes the formation of an intense low pressure in the northwestern part of the subcontinent.

• Since the pressure in the Indian Ocean in the south of the landmass is high as water gets heated slowly, the low pressure cell attracts the southeast trades across the Equator.
  • These conditions help in the northward shift in the position of the ITCZ. The southwest monsoon may thus, be seen as a continuation of the southeast trades deflected towards the Indian subcontinent after crossing the Equa- tor. These winds cross the Equator between 40°E and 60°E longitudes.
  • Entry of Monsoon into India : The southwest monsoon sets in over the Kerala coast.

Rain-bearing Systems and Rainfall Distribution #

There seem to be two rain-bearing systems in India.

1. In the Bay of Bengal causing rainfall over the plains of north India.
2. The Arabian Sea current of the south- west monsoon which brings rain to the west coast of India. Much of the rainfall along the West- ern Ghats is orographic as the moist air is ob- structed and forced to rise along the Ghats. The intensity of rainfall over the west coast of India is, however, related to two factors:
   1. The offshore meteorological conditions.
   1. The position of the equatorial jet stream along the eastern coast of Africa.

EI-Nino and the Indian Monsoon #

• EI-Nino is a complex weather system that appears once every three to seven years, bringing drought, floods and other weather extremes to different parts of the world.

The system involves oceanic and atmospheric phenomena with the appearance of warm currents off the coast of Peru in the Eastern Pacific and affects weather in many places including India.

EI-Nino is merely an extension of the warm equatorial current which gets replaced temporarily

by cold Peruvian current or Humbolt current (locate these currents in your atlas). This current increases the temperature of water on the Peruvian coast by 10°C. This results in:

• the distortion of equatorial atmospheric cir- culation;
• irregularities in the evaporation of sea water; and
• reduction in the amount of planktons which

further reduces the number of fish in the sea.

The word EI-Nino means ‘Child Christ’ because this current appears around Christmas in December. December is a summer month in Peru (Southern Hemisphere).

EI-Nino is used in India for forecasting long range monsoon rainfall. In 1990-91, there was a wild EI-Nino event and the onset of southwest monsoon was delayed over most parts of the country ranging from five to twelve days.

Break in the Monsoon #

During the south-west monsoon period after having rains for a few days, if rain fails to occur for one or more weeks, it is known as break in the monsoon. These dry spells are quite common during the rainy season. These breaks in the different regions are due to different reasons:

• In northern India rains are likely to fail if the rain-bearing storms are not very frequent along the monsoon trough or the ITCZ over this region.
• Over the west coast the dry spells are associ- ated with days when winds blow parallel to the coast.

The Rhythm of Seasons #

The climatic conditions of India can best be described in terms of an annual cycle of seasons. Four seasons :

• the cold weather season;
• the hot weather season;
• the southwest monsoon season;
• the retreating monsoon season.

The Cold Weather Season #

1. Temperature: Usually, the cold weather season sets in by mid-November in northern

India. December and January are the coldest months in the northern plain.

There are three main reasons for the exces- sive cold in north India during this season :

• States like Punjab, Haryana and Rajas- than being far away from the moderating influence of sea experience continental climate.
  • The snowfall in the nearby Himalayan ranges creates cold wave situation.
  • Around February, the cold winds coming from the Caspian Sea and Turkmenistan bring cold wave along with frost and fog over the northwestern parts of India.

The Peninsular region of India, however, does not have any well-defined cold weather season. There is hardly any seasonal change in the distribution pattern of the temperature in coastal areas because of moderating influ- ence of the sea and the proximity to equator.

• Pressure and Winds: By the end of Decem- ber (22nd December), the sun shines vertical- ly over the Tropic of Capricorn in the south- ern hemisphere. The weather in this season is characterised by feeble high pressure condi- tions over the northern plain.
  • Rainfall: Winter monsoons do not cause rainfall as they move from land to the sea. It is because firstly, they have little humidity; and secondly, due to anti cyclonic circulation on land, the possibility of rainfall from them reduces. So, most parts of India do not have rainfall in the winter season.

However, there are some exceptions to it

• In northwestern India, some weak temperate cyclones from the Mediterranean sea cause rainfall in Punjab, Haryana, Delhi and west- ern Uttar Pradesh. Although the amount is meagre, it is highly beneficial for rabi crops. The precipitation is in the form of snowfall in the lower Himalayas. It is this snow that sustains the flow of water in the Himalayan rivers during the summer months.
  • During October and November, northeast monsoon while crossing over the Bay of Ben- gal, picks up moisture and causes torrential

rainfall over the Tamil Nadu coast, southern Andhra Pradesh, southeast Karnataka and southeast Kerala.

The Hot Weather Season #

1. Temperature: With the apparent northward movement of the sun towards the Tropic of Cancer in March, temperatures start rising in north India. April, May and June are the months of summer in north India.

In the coastal regions, the north-south extent of isotherms parallel to the coast confirms that temperature does not decrease from north to south rather it increases from the coast to the interior.

• Pressure and Winds: The summer months are a period of excessive heat and falling air pressure in the northern half of the country. Because of the heating of the subcontinent, the ITCZ moves northwards occupying a po- sition centered at 25°N in July. .

The Southwest Monsoon Season #

Increase of temperature in May over the north- western plains, the low pressure conditions over there get further intensified. By early June, they are pow- erful enough to attract the trade winds of Southern Hemisphere coming from the Indian Ocean. These southeast trade winds cross the equator and enter the Bay of Bengal and the Arabian Sea. Passing over the equatorial warm currents, they bring with them moisture in abundance. After crossing the equator, they follow a southwesterly direction. That is why they are known as southwest monsoons.

The rain in the southwest monsoon season begins rather abruptly. One result of the first rain is that it brings down the temperature substantially. This sudden onset of the moisture-laden winds associated with violent thunder and lightening, is often termed as the ‘break’ or ‘burst’ of the monsoons.

The monsoon approaches the landmass in two branches:

• The Arabian Sea branch
  • The Bay of Bengal branch.

Monsoon Winds of the Arabian Sea #

The monsoon winds originating over the Arabian Sea further split into three branches:

• Its one branch is obstructed by the Western Ghats. These winds climb the slopes of the Western Ghats from 900-1200 m. Soon, they become cool, and as a result, the windward side of the Sahyadris and Western Coast- al Plain receive very heavy rainfall ranging between 250 cm and 400 cm. After crossing the Western Ghats, these winds descend and get heated up. This reduces humidity in the winds. As a result, these winds cause little rainfall east of the Western Ghats. This region of low rainfall is known as the rain-shadow area.
• Another branch of the Arabian sea monsoon strikes the coast north of Mumbai. Moving along the Narmada and Tapi river valleys, these winds cause rainfall in extensive areas of central India. The Chotanagpur plateau gets 15 cm rainfall from this part of the branch. Thereafter, they enter the Ganga plains and mingle with the Bay of Bengal branch.
• A third branch of this monsoon wind strikes the Saurashtra Peninsula and the Kachchh. It then passes over west Rajasthan and along the Aravalis, causing only a scanty rainfall.

Monsoon Winds of the Bay of Bengal #

The Bay of Bengal branch strikes the coast of Myanmar and part of southeast Bangladesh. But the Arakan Hills along the coast of Myanmar deflect a big portion of this branch towards the Indian subcontinent. The monsoon, therefore, enters West Bengal and Bangladesh from south and southeast instead of from the south-westerly direction.

From here, this branch splits into two under the influence of the Himalayas and the thermal low is northwest India. Its one branch moves westward along the Ganga plains reaching as far as the Punjab plains. The other branch moves up the Brahmapu- tra valley in the north and the northeast, causing widespread rains. Its sub-branch strikes the Garo and Khasi hills of Meghalaya. Mawsynram, located on the crest of Khasi hills, receives the highest average annual rainfall in the world.

Here it is important to know why the Tamil Nadu coast remains dry during this season. There are two factors responsible for it:

• The Tamil Nadu coast is situated parallel to the Bay of Bengal branch of southwest mon- soon.
• It lies in the rainshadow area of the Arabian Sea branch of the south-west monsoon.

Characteristics of Monsoonal Rainfall #

• Rainfall received from the southwest mon- soons is seasonal in character, which occurs between June and September.
• Monsoonal rainfall is largely governed by relief or topography. For instance the wind- ward side of the Western Ghats register a rainfall of over 250 cm.
• The monsoon rainfall has a declining trend with increasing distance from the sea.
• The monsoon rains occur in wet spells of few days duration at a time. The wet spells are interspersed with rainless interval known as ‘breaks’.
• The summer rainfall comes in a heavy downpour leading to considerable run off and soil erosion.
• Monsoons play a pivotal role in the agrar- ian economy of India because over three- fourths of the total rain in the country is received during the south- west monsoon season.
• Its spatial distribution is also uneven which ranges from 12 cm to more than 250 cm.
• The beginning of the rains sometimes is considerably delayed over the whole or a part of the country.
• The rains sometimes end considerably ear- lier than usual, causing great damage to standing crops and making the sowing of winter crops difficult.

Season of Retreating Monsoon #

• The months of October and November are known for retreating monsoons.
  • By the end of September, the southwest mon- soon becomes weak as the low pressure trough of the Ganga plain starts moving southward in response to the southward march of the sun.

• The retreating southwest monsoon season is marked by clear skies and rise in temperature. The land is still moist. Owing to the condi- tions of high temperature and humidity, the weather becomes rather oppressive. This is commonly known as the ‘October heat’.
  • The weather in the retreating monsoon is dry in north India but it is associated with rain in the eastern part of the Peninsula. Here, Octo- ber and November are the rainiest months of the year.
  • The widespread rain in this season is associ- ated with the passage of cyclonic depressions which originate over the Andaman Sea and manage to cross the eastern coast of the southern Peninsula.
  • These tropical cyclones are very destructive. The thickly populated deltas of the Godavari, Krishna and Kaveri are their preferred targets. Every year cyclones bring disaster here.
  • A few cyclonic storms also strike the coast of West Bengal, Bangladesh and Myanmar. A bulk of the rainfall of the Coromondal coast is derived from these depressions and cyclones.
  • Such cyclonic storms are less frequent in the Arabian Sea.

Distribution of Rainfall #

The average annual rainfall in India is about 125 cm, but it has great spatial variations :

• Areas of High Rainfall: The highest rainfall occurs along the west coast, on the Western Ghats, as well as in the sub-Himalayan areas is the northeast and the hills of Meghalaya. Here the rainfall exceeds 200 cm. In some parts of Khasi and Jaintia hills, the rainfall exceeds 1,000 cm. In the Brahmaputra valley and the adjoining hills, the rainfall is less then 200 cm.

• Areas of Medium Rainfall: Rainfall between 100-200 cm is received in the southern parts of Gujarat, east Tamil Nadu, northeastern Peninsula covering Odisha, Jharkhand, Bihar, eastern Madhya Pradesh, northern Ganga plain along the sub-Himalayas and the Cachar Valley and Manipur.
  • Areas of Low Rainfall: Western Uttar Pradesh, Delhi, Haryana, Punjab, Jammu and Kashmir, eastern Rajasthan, Gujarat and Dec- can Plateau receive rainfall between 50-100 cm.
  • Areas of Inadequate Rainfall: Parts of the Peninsula, especially in Andhra Pradesh, Kar- nataka and Maharashtra, Ladakh and most of western Rajasthan receive rainfall below 50 cm.

Snowfall is restricted to the Himalayan region.

Climatic Regions of India #

Major climatic types of India based on Koep- pen’s scheme: Koeppen based his scheme of Climatic classification on monthly values of temperature and precipitation. He identified five major climatic types, namely:

• Tropical climates, where mean monthly tem- perature throughout the year is over 18°C.
• Dry climates, where precipitation is very low in comparison to temperature, and hence, dry. If dryness is less, it is semi arid (S); if it is more, the climate is arid(W).
• Warm temperate climates, where mean tem- perature of the coldest month is between 18°C and minus 3°C.
• Cool temperate climates, where mean tem- perature of the warmest month is over 10°C, and mean temperature of the coldest month is under minus 3°C.
• Ice climates, where mean temperature of the warmest month is under 10°C.