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THE CLIMATE OF INDIA

THE CLIMATE OF INDIA

INTRODUCTION

  • Climate of a country includes the study of temperature, rainfall, atmospheric pressure as well as the direction and velocity of winds over a long period of time. Weather is variation in above elements over a short period of time, say over a day or a week.
  • These elements of climate are largely influenced by latitudinal extent, physiographic (relief) features and areal distribution of land and water.

FACTORS INFLUENCING CLIMATE OF INDIA | THE CLIMATE OF INDIA 

Temperature and rainfall are the main factors which determine the weather of a region. These are in turn determined by following factors:

  • Location:
  • Location to the north and south of the Tropic of Cancer — location around tropics is chiefly responsible for hot weather conditions.
  • Encircling by water bodies comprising Arabian Sea and Bay of Bengal —Oceanic effect imparts the maritime climate near the coasts and moderates the effect of high temperatures.
  • Deep hinterlands on the hand experience continental climate. Continental climate includes high extremities of maximum and minimum temperature and low rainfall due to absence of moisture in large quantity.
  • The 30-35° latitudes experience anti-cyclonic circulation as a result of which the north-west India experiences dry, stable winds and less annual rainfall.
  • Topography:
  • The tall and lofty Himalayan ranges create climatic divide between Indian subcontinent and rest of the Asia. Himalayas prevent the cold winds of Central Asia from entering the Indian subcontinent otherwise the North India would have been colder than it remains today. Similarly, Himalayas obstruct the northward moving monsoons at their foothills and give much needed rainfall to lands otherwise located in anti-cyclonic zone.
  • Topographic variations have a deep impact particularly on rainfall. Rainfall pattern changes on the windward side and leeward side of mountain ranges such as Western Ghats, Meghalaya hills.
  • Altitude: Temperature decreases with height. Due to thin air, places in the mountains are cooler than places on the plains. That is why despite being at the same latitude; Shimla is cooler than Patiala in Punjab.
  • Atmospheric phenomena:
  • Monsoon wind is the most dominating factor of Indian climate. The complete reversal of monsoon winds brings about a sudden change in the Seasons and gives it a rhythmic character. The South-west Monsoon winds bring moisture along with them and yield around 80% of annual rainfall.
  • Upper air circulation of westerly jet stream and easterly jet stream significantly influence rainfall conditions in all the seasons.
  • Tropical cyclones originating in Arabian Sea and Bay of Bengal as well as Western disturbances from Mediterranean Sea affect the rainfall.
  • El Nino current is responsible for widespread droughts and floods in tropical regions of the world including India.
  • La Nina, another global weather phenomenon is generally the harbinger of heavy monsoons in India.
  • Southern Oscillation — when the winter air pressure is high over the Pacific Ocean and low over the Indian Ocean, the southwest monsoons in India tend to be stronger. In reverse case, monsoons are most likely to be weak er.

CLIMATE OF INDIA | THE CLIMATE OF INDIA 

  • It is difficult to classify India’s climate generally because of variations in climate due to its sub-continental size and topographic variations. But India is generally described as tropical monsoon country.
  • It is tropical because Tropic of Cancer passes through the middle of country affecting the temperature distribution in the country except the Himalayas. It is also Monsoonal because all the seasons are dominated by monsoon winds.
  • On, the other hand, there are contrasting differences in the climate of India. India possesses almost all types of climates found in the world. There are huge diversities in Indian climate ranging from equatorial (Andamans) to arid (Jaisalmer) in terms of precipitation, from polar (Siachen) to hot desert type (Barmer) for temperature, from monsoonal (Meghalaya) to tropical savanna type (peninsular plateau) to continental semi-arid steppe (Punjab). But in spite of these differences the climate of India has got monsoonal imprint in character and rhythm.

MONSOON

  • The word ‘monsoon’ is derived from Arabic word `mausin’ or `mausam’ or Malayan `Monsin’. Monsoon implies seasonal reversal in the direction of prevailing winds. Monsoons are large scale seasonal wind system flowing over wider geographical area in which there is dominant wind flowing in one direction and it changes its direction completely in another season. The 180° change of direction is a keynote feature of monsoon.
  • The winds flow from Sea to Land (onshore) during summer and land to sea (offshore) during winter. In other words, monsoon is a double system of seasonal winds that is the sum of summer and winter winds.
  • The seasonal change in the direction of wind is a principle feature of the Indian climate. With greater continentality and lesser oceanic influences, the northwestern part of India becomes the centre of high pressure in winter and low pressure in summer.
  • The cold and dry winds blowing from this part of the country in winter are known as north east monsoons and the warm moist winds in summer blowing in from the oceans are known as south west monsoons. The phenomenon of monsoon itself develops from combination of multiple phenomena, as described next.
  1. Inter Tropical Convergence Zone (ITCZ)
  • ITCZ is a low pressure zone located near the equator where trade winds from both the hemispheres converge. Due to convergence of winds it becomes a zone of ascending air, depressions and cyclones, maximum clouds and heavy rainfall. The location of ITCZ shifts north and south of the equator with the corresponding position of the Sun across the equator during summer solstice and winter solstice.
  • During winter in northern hemisphere, ITCZ is present near or south of equator. So at this time the winds on Indian subcontinent are north-easterly trade winds blowing from northeast to south and southwest direction.

Later during summer solstice (northward migration of Sun in summer as seen from the earth) the ITCZ shifts to around 20° to 25° N latitude over ind6-Gangetic plain. ITCZ over India is generally termed as the monsoon trough. The ITCZ becomes significant for two reasons:

  • With the northward shift of ITCZ, the trade winds from southern hemisphere cross the equator. When the trade winds from southern hemisphere cross the equator and enter northern hemisphere, the winds change their direction due to Coriolis force and blow across Indian subcontinent from southwest to northeast direction. Thus they became southwest monsoon. Again in next winter, ITCZ moves southward after winter solstice and winds on Indian subcontinent revert back to northeast monsoons.
  • As ITCZ are associated with depressions and cyclones, they yield cyclonic rainfall. The paths of these tropical depressions over India are mainly determined by the position of ITCZ which is generally termed as the monsoon trough. This elongated low pressure monsoon trough extends over the Thar Desert in the north-west to Patna and Chhota Nagpur plateau in the east-southeast. As the axis of the monsoon trough oscillates, there are fluctuations in the track and direction of these depressions, and the intensity and the amount of rainfall vary from year to year.
  • Jet Streams
  • Jet stream is a band of very strong and rapidly moving circumpolar air circulation (150-300 km/hr) found in the upper troposphere. The westerly jet stream, most common one, blows from west to east at the height of 12 km while the easterly jet stream blows from east to west at the height of 13 km.
  • The westerly jet stream is produced due to thermally induced high pressure zone at the poles (Extreme cold temperature produces high pressure, while low temperature produces low pressure zone). Jet streams withdraw from an area when the temperature over there increases. In case of Indian subcontinent, the position of jet stream is different during winters and during summers.
  • Jet stream in winter
  • In winter, the upper air westerly jet streams are bifurcated into two branches due to physical obstruction of the Himalayas and Tibetan Plateau. One branch is located South of Himalayas and the other north of Tibetan Plateau. A ‘high’ pressure system is formed south of the jet stream over Afghanistan and north-west Pakistan from which air tends to subside over India. These anti-cyclonic conditions lead to atmospheric stability and dry conditions over northern India. It also causes the flow of north-east winter monsoons from north to south. The ‘western disturbances’ from Mediterranean Sea are also moved in to northern India under the force of west flowing Jet Stream.
  • Jet stream in summer
  • During April – May, low pressure areas are developed on the ground surface in northwest India and Pakistan due to intense heating of ground surface. But so long as the position of upper air jet streams is maintained above the surface low pressure zone (to the south of Himalayas), the dynamic anti-cyclonic conditions persist over north-west India, Pakistan and Afghanistan. The winds descending from upper air high pressure system obstruct the ascent of winds from the surface low pressure areas with the result the weather remains warm and dry. This is why the months of April and May are warm and dry in spite of high temperature and evaporation.
  • By the middle of June, the upper air westerly jet stream (southern branch of winter jet stream) is withdrawn from over India. This happens because of weakening of polar surface high pressure and upper air circumpolar whirl due to increased temperature for prolonged period in the northern hemisphere. This withdrawal of southern branch of jet stream is sudden. The sudden withdrawal of jet stream produces an upper tropospheric low pressure zone over northwest India and Pakistan. Consequently the upper air low pressure zone (dynamic depression) sucks the already warm, air (thermal depression) on ground surface. This mechanism causes vigorous inflow of the South West trade winds into India causing sudden ‘burst’ of south -west monsoon.
  • Meanwhile the ITCZ, which was in southern hemisphere during northern winter, (and southern summer) moves northward across the equator with northward movement of Sun as well as it is pushed by the southern circumpolar whirl (Jet stream in southern hemisphere). As a result the originally southeasterly trade winds cross the equator and take south-westerly direction due to Coriolis force and rush towards India. Coriolis force is the deflective force caused due to the rotatory velocity of the earth. Coriolis force deflects the winds towards right side in northern hemisphere. The ITCZ being low pressure zone carries with it cyclonic waves and vortices from Indian Ocean which produce wet weather while their occlusion produces dry weather which continues till new vortices are formed.
  • Role of Tibetan Plateau and Easterly Jet stream
  • The Tibetan Plateau affects the atmosphere in two ways: (a) as a physical barrier and (b) as a high level heat source. In winter, the excessive cooling of Tibetan plateau causes advancement of jet stream south of the Himalayas, thus bifurcating the jet stream in two parts. In summer (June), relatively more heated plateau fastens up the northward displacement of westerly jet stream. This contributes to the ‘burst’ of monsoon.
  • Also the summer time heating of the Tibetan plateau makes it a high level heat source which produces thermal anticyclone over this region in the mid-troposphere. This anti-cyclogenesis weakens the western subtropical jet stream. But it also produces tropical Easterly Jet stream over the longitudes east of India which then moves westward across India and the Arabian Sea to the eastern Africa.
  • It descends over southern Indian Ocean and intensifies its high pressure cell so as to push the south-west monsoon winds. in fact, the higher the intensity of Tropical Easterly jet (due to higher heating of Tibet), stronger would be the Easterly Jet, greater would be the potency of high pressure cell over Indian Ocean and stronger would be the south west monsoon.
  • Role of Oceanic Bodies (El Nino, Southern Oscillation, Walker Circulation and Somali Ocean Current)
  • El Nino meaning ‘Child Christ’ is a warm ocean current appearing along the Peru coast in December. It replaces the Peru or Humboldt Cold Ocean current flowing over this region in normal years. Under normal conditions the water layer over the eastern Pacific Ocean (near Peru and Ecuador) is cool and Shallow, while over the western Pacific (Indonesia and Western Australia) it is warm and deep. Such conditions are helpful for strong southwest monsoons. The appearance of El Nino reverses the situation creating warm condition over eastern Pacific and cold in Western Pacific.
  • Also it is seen that whenever the surface pressure is high over eastern pacific, the pressures over the Indian Ocean tend be low and vice versa. This see-saw pattern of meteorological changes is called ‘Southern Oscillation’. As pressures are inversely related to rainfall, this suggests that when low pressure prevails over the Indian Ocean (positive S01), the chances are that coming monsoon rains will be good.
  • During El Nino anomaly, the above conditions are reversed -the surface pressure over eastern Pacific becomes low, pressure over western Pacific and Indian Ocean becomes high, the Southern Oscillation Index becomes negative and this combination called ENSO event reduces the monsoon rainfall in India. But in the end, it also must be noted that one to one correspondence between ENSO event and drought in India is not yet conclusively established.
  • Apart from above factors, a strong Somali Ocean Current too is associated with good monsoon.

SUMMARY OF MONSOON DEVELOPMENT | THE CLIMATE OF INDIA 

  • In winter, a high pressure zone develops over northwest India due to cold temperature. At the same time northeast monsoon winds are blowing over rest of India from north to south. During summer, a low pressure zone develops by end of June over northwest India due to persistent high temperature. The southern branch of jet stream weakens and is finally withdrawn from the southern slopes of Himalayas by mid-June, leading to the formation of dynamic depression over the surface thermal low.
  • The Inter Tropical Convergence Zone moves northward and occupies a position over 25° N by mid-June and thus allows the equatorial westerlies and southeast trade winds to gush in over the subcontinent. The cyclonic vortices developed in the ITC zone help build up cyclonic rainfall in the country.
  • Obstruction of monsoon winds by the mountains causes orographic rainfall as well. The tropical easterly jet stream originating due to thermal heating of Tibetan plateau intensifies Indian Ocean high pressure cell which further strengthens the monsoons. The south-east trade winds are also pushed by the Antarctic circumpolar whirl to develop as southwest monsoon.

WEATHER SEASONS | THE CLIMATE OF INDIA 

While southwest monsoon and northeast monsoon are two distinct seasons in India, the year can be conveniently divided into following four seasons based on Indian Meteorological Department’s classification:

  • The Cold Weather Season — December to February
  • The Hot Weather Season — March to May
  • The Southwest Monsoon Season — June to September
  • The Retreating Monsoon Seasons (Post Monsoon Season) — October and November.

COLD WEATHER SEASON | THE CLIMATE OF INDIA 

  • The Sun in these days shines over Tropic of Capricorn in southern hemisphere. Bright weather prevails over most of the peninsular plateau. But the coastal Tamil Nadu may experience rainy weather from the northeast monsoon. There is hardly any seasonal change in the distribution pattern of the temperature in coastal areas because of moderating influence of the sea and the proximity to equator.
  • In north India, the north-easterly trade winds are re-established. The north-west India located away from marine influences, experiences low temperature and feeble atmospheric pressure conditions. In the south India and Indian Ocean, the pressure is relatively lower due to warm temperatures. As a result, cold and dry winds blow from north to south towards low’ pressure area.
  • In Indo-Gangetic Plains, the season is marked by light westerly winds, low humidity, fog, low temperature and large day time variations of temperature. The urban areas like Delhi, Agra etc. are particularly characterized by dense smog. When the temperature goes below 6° C from the normal, it is called Cold Wave. The frequency of severe cold waves decreases towards East and South. Light north-easterly winds blow down the Brahmaputra and Cachar Valleys and northerly and northwesterly over rest of eastern India. The night temperature in Indo-Gangetic plains sometimes goes below the freezing point producing ground frost conditions. There are three main reasons for the excessive cold in north India during this season:
  • States like Punjab, Haryana and Rajasthan being far away from the moderating influence of sea experience continental climate.
  • The snowfall in the nearby Himalayan ranges creates cold wave situation; and
  • 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.
  • Winter monsoons in the form of north-east trade winds do not cause much rainfall. It is because firstly, since they move from land to the sea, 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. During cold weather season number of cyclonic depressions known as Western Disturbances are developed in the Mediterranean Sea.
  • These depressions move eastward, towards India, at the height of 3000 m above sea level, under the influence of westerly jet stream. On their way, the moisture content gets augmented from the Caspian Sea in the north and the Persian Gulf in the south. They yield some rainfall. It is of great economic significance for rabi crops of wheat and gram as they provide much needed moisture to these crops. Also there is snowfall over the hills of Jammu and Kashmir, Himachal Pradesh and Uttarakhand.
  • This snow provides water to the Himalayan Rivers during summer season. The frequency of disturbances is 2 to 4 every month and goes on decreasing in summer. They can travel till Bengal in the east. The precipitation goes on decreasing from west to east in the plains and from north to south in the mountains. The south-eastern peninsula too gets occasional rainfall in association with cyclonic storms and depressions carried over north-east monsoon from Bay of Bengal.

HOT WEATHER SEASON | THE CLIMATE OF INDIA 

  • April, May and June are the months of summer in north India. There is continuous and rapid rise of temperature and fall in pressure from March to May. While northeast India and hilly areas have cool weather, peninsular India has relatively lower temperature than northwestern India. The northern and Central parts of India experience heat waves in this season. According to Indian Convention, a temperature of 6° to 7° C above normal is termed ‘moderate’ and 8° C or more as ‘severe’ heat wave.
  • In most parts of India, temperatures recorded are above 35°. In March, the highest day temperature of about 38°C occurs in the Deccan Plateau while in April temperature ranging between 38°C and 43°C are found in Central India. In May, the heat belt moves further north. During late May and June the north-western India as well as central India sometimes experience temperatures around 48°C.
  • The atmospheric pressure is low all over the country due to high temperature. In May and June, because of steep pressure gradient, the hot, strong and dust laden winds, known as ‘loo’ start to blow in north India. They reach maximum intensity in afternoon. Sometimes, strong dust storms (aandhi) which are actually thunder storms are developed. These are followed by torrential rains and sometimes hall causing great destruction.
  • These temporary storms bring a welcome respite from the oppressing heat since they bring with them rains and a pleasant cool breeze. These thunderstorms occur in afternoon and evenings and they appear to strike from north-west direction for which they are termed as nor westers. They are known as Kalbaisakhi in West Bengal and as Bordoichilla in Assam. The rainfall is of great significance for tee crop, rice end jute, In the peninsula thunderstorm rain occurs chiefly in April and May and is known as Mango Showers. Kerala gets some rainfall in May on account of temporary invasion of the south-most monsoon. In northwest peninsula this is a season of dry winds with little or no rains.

SOUTHWEST MONSOON SEASON/WET SEASON | THE CLIMATE OF INDIA 

  • As a result of continuously rising temperatures, low pressure conditions develop over northern India. On the other hand, Indian Ocean is relatively under high pressure conditions. As seen earlier, with the withdrawal of southern branch of jet stream, the south-east trade winds suddenly gush in, The monsoon approaches the landmass in two branches: a) Arabian Sea branch and, b) Bay of Bengal branch: Bay of Bengal stream strikes Andaman and Nicobar by about 20th May while the Arabian Sea Current of South-West monsoon reaches kerala by 1st June. The rein begins suddenly accompanied by thunders and lightening in the afternoon of a scorching day. This is called monsoon ‘burst’. The further advance of monsoon is shown in following diagram.
  • While the’ windward side of Western Ghats receives around 200-250 cm rainfall, the leeward or rain shadow side receives only about 50-100 cm rainfall resulting in
    frequent drought situations. THE CLIMATE OF INDIA
  • The sub-branch of Arabian Sea current going to Gujarat Western Rajasthan fails to yield much rainfall because of absence of mountain barrier, parallel position of Aravallis to monsoon winds and shunting effect of hot and dry air from Baluchistan. The northwest India generally receives much less rainfall as moisture content of monsoon winds reduces substantially by the time they reach Rajasthan, Haryana and Punjab.
  • After being diverted northwards by Arakan Yoma mountains the Bay of Bengal branch splits into two. One branch moves westward along the Ganga plains under the channeling effect of the Himalayas and the thermal low in northwest India.
  • The other branch moves up the Brahmaputra valley in the north and the northeast, causing widespread rains. A small branch of the Bay of Bengal current is obstructed by Garo and Khasi hills leading to heavy rainfall as in Cherrapunji (1087cm) and Mawsynram (1141cm).
  • The weather is also affected by 20-25 cyclonic depressions in a season which enter the country through Bay of Bengal and Arabian Sea. These yield heavy rainfall and cause lot of destruction.
  • The active period of monsoon are separated by gap of 1 to 2 weeks when there is no rainfall at all and cloudiness decreases. This gap is called ‘monsoon break’. The breaks are believed to be due to weakening of Tibetan high and so easterly jet stream. As a result the rain-bearing depressions and storms from Bay of Bengal are not pushed along the monsoon trough    or the   ITCZ over this region, On the other hand, during break period the monsoon trough shifts northwards to Himalayan foothills and yields orographic rainfall. So when most part of the country is rainless, the Himalayan Rivers are flooded due to heavy rainfall. Over the west coast the dry spells are associated with days when winds blow parallel to the coast.

SEASON OF RETREATING MONSOON | THE CLIMATE OF INDIA 

  • The southwest monsoon begins to retreat from northern India by 2nd or 3rd week of
  • September with Southward migration of the Sun and consequent weakening of    low pressure area over north western area. However unlike           the sudden ‘burst’, the retreat is gradual. The return of jet stream is accompanied by restoration of north east trade winds. In October, sky is clear, temperature is around 30-35° and soil and air still moist. As a result, hot and humid weather is observed during day while nights are cool. This is famously known as ‘October heat’.
  • The eastern coast of peninsula receives some of severest tropical cyclones in this period. Tamil Nadu and Andhra coast receive their major rainfall from retreating south west and incoming north-east monsoons. The weather all over the country, otherwise, is fine and clear.
  • A notable feature during these months is the occurrence of moderate to severe cyclones in October and November in the Bay of Bengal and Arabian Sea. On an average, 5 to 6 tropical cyclones form every year, of which two or three could be severe. The months of October-November and May-June are known to produce cyclones of severe intensity (bi-modal peak). Tropical cyclones developing during the monsoon months (July to September) are generally not so intense. This is because of development of strong vertical wind shear in the troposphere which inhibits cyclone development.
  • Frequency of cyclones in the Bay of Bengal is more than the Arabian Sea (4:1 ratio). This is because remnants of typhoons over Northwest Pacific move across South China Sea to Indian Seas as well as Bay of Bengal being relatively warmer than Arabian Sea. Andhra Pradesh, Odisha, Tamil Nadu and West Bengal and Pondicherry on the East Coast and Gujarat on the West Coast are more vulnerable to cyclone disasters. The disaster potential is high due to the accompanying destructive wind, torrential rainfall, floods and storm surges. 40% of the total population lives within 100 km of coastline. The positive side of these depressions and cyclones is that a bulk of the rainfall of the Coromandel Coast is derived from them.

DISTRIBUTION OF RAINFALL| THE CLIMATE OF INDIA 

  • There is a great regional and temporal variation in the distribution of rainfall. Over 80% of annual rain is received in four rainy months of June to September and even in these there are active spells and break periods. During cold weather season, country gets some rainfall and snowfall from Western Disturbances. The southeastern peninsula gets about 25 cm rainfall from retreating south-west monsoon and north-eastern monsoon. During hot weather season, thunderstorms cause rainfall. Temporary invasion of southwest monsoon causes some rains in Kerala during same time. THE CLIMATE OF INDIA
  • The average annual rainfall in India is 125 cm but it has great spatial variations. Proximity to the sea and orographic features play great roles in regional variation of rainfall. Very high rainfall occurs (> 200 cm) occurs along the West Coast, Western Ghats, the Islands, north eastern hills and sub Himalayan regions. The high rainfall is mostly orogenic, except on the islands where it is cyclonic. The areas of high rainfall (100-200 cm) are Brahmaputra Valley, Himachal Pradesh, Uttarakhand, Chhattisgarh, northern Gangetic Plains, eastern Maharashtra and Madhya Pradesh. Western Rajasthan, Kachchh and Ladakh receive < 50 cm rainfall. Rest of the regions gets 50­100 cm rainfall.
  • These include rainshadow regions of Western Ghats as well as north-west India. Two general trends are observed in rainfall distribution. First, in peninsular India, except Tamil Nadu, rainfall decreases eastward and second, in north India it decreases northward and north-westward.

VARIABILITY OF RAINFALL | THE CLIMATE OF INDIA 

  • The characteristic feature of the monsoon rainfall is its variability. The actual rainfall of a place in a year may deviate from its mean rainfall by 10 to 50%. The variability of annual rainfall may be expressed in terms of coefficient of variability.
  •                           CV = Standard Deviation x 100
  • Mean The coefficient  of variation indicates the amount of fluctuation in rainfall over a long period of time from the mean values. It generally ranges between 15% and 35% for India as a whole.
  • Indian summer monsoon exhibits large spatial variability. Regions of high rainfall such as West Coast and north eastern regions are associated with lowest variability of 10-20% and the regions of lowest rainfall (north west India — Kachchh, Rajasthan desert, Haryana, Punjab and Ladakh) have highest variability of more than 40%. The variability is also high for rainshadow regions of the peninsula as about 25 to 35% as seen from the Diagram.
  • Thus rainfall variability is inversely proportional to total amount of rainfall and reliability of rainfall is inversely proportional to coefficient of variability. Thus coefficient of variability helps in long range prediction of rainfall. The variability has significant effect on agricultural productivity. The areas of high variability have low rainfall, chronic deficiency of water, frequent droughts and crop failures.

PECULIARITIES AND SIGNIFICANCE OF INDIAN MONSOON

  • The extremes of temperature, rainfall and humidity of Indian climate are particularly distinct. The climate is primarily dominated by south west monsoon. But it is highly variable, both regionally and temporally, and quite unreliable thus causing agricultural and economic loss. Also it is concentrated in four months of the year.
  • In fact it is torrential in nature. Because of monsoon breaks, there are just 40-45 actual rainy days. This results in floods and soil erosion. Although it should also be noted that one part or the other of the country receives rainfall in each month of the year due to south west monsoon, north east monsoon, western disturbances, local thunderstorms etc.

WEATHER | THE CLIMATE OF INDIA 

  • FORECASTING
  • Weather forecasting is the application of science and technology to predict the state of the atmosphere for a given location. In India weather forecasting is done by various organisations like Indian Meteorological Department, Indian Institute Tropical Meteorology. National Centre for Medium Range Weather Forecasting, Indian National Centre for Ocean Information Services, National Remote Sensing Centre, etc. under the umbrella of Ministry of Earth Sciences, India also has international collaboration with meteorological organisations of countries like US, UK, France, Korea, Japan, etc. as well as organisations like World Meteorological Organisation.
  • The weather forecasting involves observation of land-ocean-atmosphere interaction, observation of Indian Ocean, climate monitoring and prediction Hydrological cycle, etc. The Government of India has launched Integrated Weather Forecasting and Communication System in 2010. Under this system, data from various observatories, sensors, Doppler radars and automatic weather stations is collected and run in computer models for forecasting the weather. It aims to provide location specific weather predictions in a customized manner. For India, INSAT 3A and Kalpana-1 provide the satellite data for weather now cast and forecast.
  • Among all elements, the prediction of monsoon rainfall is most important as the life in India is often said to be revolving around monsoon. The variability of Indian Monsoon directly affects the weather sensitive activities like agriculture, water resources, transportation, health, power, disaster management and the very livelihood of everyone. Depending on time scale, the monsoon prediction can be classified into following categories:
  • Short Range (up to 3 days): It involves prediction of weather upto 3 days. Forecasting of location specific heavy rainfall event during southwest monsoon season are the main challenging areas of monsoon forecasting in this time scale. It is done by IMD. THE CLIMATE OF INDIA
  • Medium Range: Medium Range weather forecast is defined as weather forecast from 4 days up to 7 days in tropical regions. It gives better idea about monsoon, its advancement, withdrawal, persistence and cessation of dry spells, etc. It is done by National Centre for Medium Range Forecasting.
  • Extended Range or Infra Seasonal: It involves weather forecast beyond 7 days up to a month. It includes prediction of ‘break’ periods and ‘active’ periods of monsoon which are essential for agricultural planning of sowing, giving fertilizers, harvesting and water management. Extended range weather prediction is done by IITM.
  • Long Range or Seasonal Forecasting: Long range forecasting is done for a season by IMD. It involves two stage forecast according to which the first stage forecast for the seasonal rainfall over the country as a whole is issued in April Later in June an update forecast for seasonal rainfall including that for four broadly homogenous rainfall regions of India is issued. IMD also generates operational forecasts for rainfall during second half of monsoon season.

MONSOON MISSION

  • The accurate prediction of monsoon involves many challenges of in depth understanding the mechanism of monsoon, better observation facilities, acquisition supercomputing facilities (2.5 -3 pets flop is needed) and equipments, use of dynamic model instead of present statistical model for weather prediction, etc. To overcome these challenges, the Earth System Science Organisation (ESSO), Ministry of Earth Sciences, Government of India launched Monsoon Mission in 2012. It’s objective is to improve monsoon prediction over the country on all time and space scales using dynamical modelling framework. It will be implemented during next five years (2012-2017).
  • The mission will be undertaken through two submissions on different time scales — (i) Extended range to seasonal time scale to be coordinated by Indian Institute of Tropical Meteorology, Pune; and (ii) short to medium range scale to be coordinated by the National Centre for Medium Range Weather Forecasting. The Indian National Centre for Ocean Information Services (INCOIS) will provide ocean observations. IMD has the responsibility of operationalizing the programme.

CLIMATIC REGIONS OF INDIA | THE CLIMATE OF INDIA 

Indian climate varies from one part to another. Some parts get a lot of rain while some remain arid. The coastal areas experience a very low annual range of temperature while northwestern part of the North Plain experiences a much higher range of temperature. India can be divided into climatic regions on the basis of a number of climatic classification schemes. One of the most popular classifications is provided by Trewartha. On the basis of this scheme India can be divided into following climatic regions‑

  • Tropical Rainforest Climate – It includes western coastal region and parts of northeast India. Rainfall is above 200 cm and temperatures may go below 18.2° and may rise up to 29° Mawsynram lies in this region. High temperatures accompanied with high humidity are favorable for dense forests.
  • Tropical Savanna climate – It covers most of peninsular region except semi-arid zone to the east of the Sahyadris where rain shadow effect of this mountain range makes climate still more arid. Rainfall varies from 76 cm to 152 cm and temperatures are above that of tropical rainforest climate.
  • Tropical Semi-Arid Steppe Climate – It includes the region of rain shadow belt extending from central Maharashtra to Tamil Nadu. Rainfall varies from 38.1 to 72.2 cm and temperature varies from 20°C to 28.8°C in December and 32.8°C in May. These areas are highly prone to drought and crop failures. Dry farming and cattle rearing are the ideal activities from the climatic point of view.
  • Tropical and Sub-tropical Steppe Climate – It includes a broad area from Punjab to Kutch between the Thar desert on the west and more humid areas of Gangetic plains in the east and peninsular region to its south and southeast. Rainfall varies from 30.5 cm to 63.5 cm and temperature ranges from 12°C and 35° These regions are characterized by dry farming and animal husbandry. However due to availability of water, intensive farming can also be practiced.
  • Tropical Desert Climate – It prevails in western parts of Rajasthan and a part of Rann of Kuchchh. Rainfall is very scanty and temperatures in summers are very high and hot dry winds called loo blow strongly. The mean temperature of these areas is above 34.5°C and occasionally the day temperature may rise above 50° Temperature decreases in winter and in north it goes as low as 11.6°C in January. Vegetation cover is very scanty and is composed mainly of grasses. THE CLIMATE OF INDIA
  • Humid Sub-Tropical Climate with Dry Winter – This type of climate is experienced in a large area south of Himalayas, east of the areas of Tropical and sub-tropical Steppe and North of Tropical Savanna. The region extends from Rajasthan, east of Aravalli. Rainfall varies from 63.5 cm to 125.4 cm. Amount of rainfall decreases away from the Himalaya and also from east to west. Summer is the rainy season and winters are dry except for some areas receiving a little rainfall from the westerly depressions.
  • Mountain climate – The Himalayan region experiences this type of climate. The mountain climate is characterized by sharp contrasts in temperatures between sunny and the shaded slopes. The diurnal range of temperature is high and amount of rainfall varies a great deal from west to east and also between windward and leeward side. The climate becomes dry and cold in the Trans-Himalayan region where the southwest monsoon fails to reach. Due to this reason, the northwestern part of the Himalayan region is a cold desert. Ladakh region has a typical cold desert type of climate. Temperatures in the high altitude regions as well as in the typical cold desert type of areas fall below the freezing point during winters. Where the southern slopes are protected from cold northerly winds, the climate is somewhat warmer. Such areas also receive a heavier rainfall. Although the climate of entire Himalayan zone is defined as mountain climate, in reality the climatic conditions in this vast mountain region vary a great deal.

AGRO-CLIMATIC ZONES OF INDIA | THE CLIMATE OF INDIA 

  • Another way of looking at the climatic types is by segregating them into Agro­climatic zones. Recently, the agricultural development planning based on agro-climatic zones has been considered more appropriate in order to reduce the regional imbalances and to realize the unexploited potential.
  • The strategy of Agro-climatic planning aims at more scientific utilization of resources. The holistic approach of assessing climate, soil type, topography, water resources and irrigation facility would provide the base for exploiting full potential of the region. Accordingly the country has been divided into 15 agro-climatic zones on the basis of homogeneity in factors like soil type, rainfall, temperature, water resources etc. These areas include‑
  • Western Himalaya region
  • Eastern Himalaya region
  • Lower Gangetic plains region.
  • Medium Gangetic plains region
  • Upper Gangetic plains region
  • Trans Gangetic plains region
  • Eastern plateau and Hills region
  • Western plateau and Hills region
  • Southern plateau and Hills region
  • East Coast plains and Ghats region.
  • West Coast plains and Ghats region.
  • Central plateau and Hills region
  • Gujarat plains and Hills region
  • Western Dry region.
  • The Island region.
  • The strategies for developing land and water resources and suitable cropping patters for each zone have been worked out. Also, the non-crop based activities such as forestry, fisheries, animal husbandry and agro-processing activities have been emphasized for a holistic development of each agro-climatic zone. The agro-climatic approach is important from the viewpoint of providing more even development of agriculture and allied activities in different zones.
  • It helps in achieving higher productivity while ensuring sustainable development of agriculture through adoption of crops, land use and agricultural practices that are suitable to a particular zone in view of its soil type, climate and availability of water resources. THE CLIMATE OF INDIA
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