Climate refers to the sum total of weather conditions and variations over a large area for a long period of time (more than thirty years).
Weather refers to the state of the atmosphere over an area at any point of time. The elements of weather and climate are the same, i.e. temperature, atmospheric pressure, wind, humidity and precipitation. The weather conditions fluctuate very often even within a day. But there is some common pattern over a few weeks or months, i.e. days are cool or hot, windy or calm, cloudy or bright, and wet or dry. On the basis of the generalized monthly atmospheric conditions, the year is divided into seasons such as winter, summer or rainy seasons
India is separated from the rest of Asia by the wall of Himalayan mountain ranges. It restricts the cold and dry winds of Central Asia to the North of Himalayas. These mountain ranges also act as an effective physical barrier for rain bearing southwest monsoon winds and force them to shed their moisture within the country. Thus, it acts as an effective climatic divide between the Indian subcontinent and central Asia.
There are six major controls of the climate of any place. They are: latitude, altitude, pressure and wind system, distance from the sea (continentality), ocean currents and relief features.
Due to the curvature of the earth, the amount of solar energy received varies according to latitude. As a result, air temperature decreases from the equator towards the poles.
As one goes from the surface of the earth to higher altitudes, the atmosphere becomes less dense and temperature decreases. The hills are therefore cooler during summers.
The pressure and wind system of any area depend on the latitude and altitude of the place. Thus, it influences the temperature and rainfall pattern.
The sea exerts a moderating influence on climate: As the distance from the sea increases, its moderating influence decreases and the people experience extreme weather conditions. This condition is known as continentality (i.e. very hot during summers and very cold during winters).
Ocean currents along with onshore winds affect the climate of the coastal areas, for example, any coastal area with warm or cold currents flowing past it, will be warmed or cooled if the winds are onshore.
Finally, relief too plays a major role in determining the climate of a place. High mountains act as barriers for cold or hot winds; they may also cause precipitation if they are high enough and lie in the path of rain-bearing winds. The leeward side of mountains remains dry.
4.1. Climate of India
The world is divided into a number of climatic regions. The climate of India is described as the ‘monsoon’ type. This type of climate is found mainly in the south and Southeast Asia.
The monsoon regime emphasizes the unity of India with the rest of the southeast Asian region. This view of broad unity of the monsoon type of climate should not, however, lead one to ignore its regional variations which differentiate the weather and climate of different regions of India. For example, the climate of Kerala and Tamil Nadu in the south are so different from that of Uttar Pradesh and Bihar in the north, and yet all of these have a monsoon type of climate.
The climate of India has many regional variations expressed in the pattern of winds, temperature and rainfall, rhythm of seasons and the degree of wetness or dryness. These regional diversities may be described as sub-types of monsoon climate.
Let us take a closer look at these regional variations in temperature, winds and rainfall. While in the summer the mercury occasionally touches 55°C in the western Rajasthan, it drops down to as low as minus 45°C in winter around Leh. Churu in Rajasthan may record a temperature of 50°C or more on a June day while the mercury hardly touches 19°C in Tawang (Arunachal Pradesh) on the same day. On a December night, temperature in Drass (Jammu and Kashmir) may drop down to minus 45°C while Thiruvananthapuram or Chennai on the same night records 20°C or 22°C. These examples confirm that there are seasonal variations in temperature from place to place and from region to region in India. Not only this, if we take only a single place and record the temperature for just one day, variations are no less striking. In Kerala and in the Andaman Islands, the difference between day and night temperatures may be hardly seven or eight degree Celsius. But in the Thar desert, if the day temperature is around 50°C, at night, it may drop down considerably up to 15°-20°C.
Now, let us see the regional variations in precipitation. While snowfall occurs in the Himalayas, it only rains over the rest of the country. Similarly, variations are noticeable not only in the type of precipitation but also in its amount. While Cherrapunji and Mawsynram in the Khasi Hills of Meghalaya receive rainfall over 1,080 cm in a year, Jaisalmer in Rajasthan rarely gets more than 9 cm of rainfall during the same period. Tura situated in the Garo Hills of Meghalaya may receive an amount of rainfall in a single day which is equal to 10 years of rainfall at Jaisalmer. While the annual precipitation is less than 10 cm in the northwest Himalayas and the western deserts, it exceeds 400 cm in Meghalaya.
The Ganga delta and the coastal plains of Orissa are hit by strong rain-bearing storms almost every third or fifth day in July and August while the Coromandel coast, a thousand km to the south, goes generally dry during these months. Most parts of the country get rainfall during June-September, but in the coastal areas of Tamil Nadu, it rains in the beginning of the winter season. In spite of these differences and variations, the climate of India is monsoonal in rhythm and character.
4.2. Factors Influence the Climate of India
India’s climate is controlled by a number of factors which can be broadly divided into two groups:
factors related to location and relief,
and factors related to air pressure and winds.
4.2.1. Factors related to Location and Relief
Latitude: The Tropic of Cancer passes through the central part of India in east-west direction. Thus, the northern part of India lies in a sub-tropical and temperate zone and the part lying south of the Tropic of Cancer falls in the tropical zone. The tropical zone being nearer to the equator, experiences high temperatures throughout the year with small daily and annual range. Area north of the Tropic of Cancer being away from the equator, experiences extreme climate with a high daily and annual range of temperature.
The Himalayan Mountains: The lofty Himalayas in the north along with its extensions act as an effective climatic divide. The towering mountain chain provides an invincible shield to protect the subcontinent from the cold northern winds. These cold and chilly winds originate near the Arctic circle and blow across central and eastern Asia. The Himalayas also trap the monsoon winds, forcing them to shed their moisture within the subcontinent.
Distribution of Land and Water: India is flanked by the Indian Ocean on three sides in the south and girdled by a high and continuous mountain-wall in the north. As compared to the landmass, water heats up or cools down slowly. This differential heating of land and sea creates different air pressure zones in different seasons in and around the Indian subcontinent. Difference in air pressure causes reversal in the direction of monsoon winds.
Distance from the Sea: With a long coastline, large coastal areas have an equable climate. Areas in the interior of India are far away from the moderating influence of the sea. Such areas have extremes of climate. That is why, the people of Mumbai and the Konkan coast have hardly any idea of extremes of temperature and the seasonal rhythm of weather. On the other hand, the seasonal contrasts in weather at places in the interior of the country such as Delhi, Kanpur and Amritsar affect the entire sphere of life.
Altitude: Temperature decreases with height. Due to thin air, places in the mountains are cooler than places on the plains. For example, Agra and Darjeeling are located on the same latitude, but the temperature of January in Agra is 16°C whereas it is only 4°C in Darjeeling.
Relief: The physiography or relief of India also affects the temperature, air pressure, direction and speed of wind and the amount and distribution of rainfall. The windward sides of Western Ghats and Assam receive high rainfall during June-September whereas the southern plateau remains dry due to its leeward situation along the Western Ghats.
4.2.2. Factors Related to Air Pressure and Wind
To understand the differences in local climates of India, we need to understand the mechanism of the following three factors:
Distribution of air pressure and winds on the surface of the earth.
Upper air circulation caused by factors controlling global weather and the inflow of different air masses and jet streams.
Inflow of western cyclones generally known as disturbances during the winter season and tropical depressions during the south-west monsoon period into India, creating weather conditions favorable to rainfall.
The mechanism of these three factors can be understood with reference to winter and summer seasons of the year separately.
Mechanism of weather in the Winter Season
Surface Pressure and Winds: In winter months, the weather conditions over India are generally influenced by the distribution of pressure in Central and Western Asia. A high pressure centre in the region lying to the north of the Himalayas develops during winter. This centre of high pressure gives rise to the flow of air at the low level from the north towards the Indian subcontinent, south of the mountain range. The surface winds blowing out of the high pressure centre over Central Asia reach India in the form of a dry continental air mass. These continental winds come in contact with trade winds over northwestern India. The position of this contact zone is not, however, stable. Occasionally, it may shift its position as far east as the middle Ganga valley with the result that the whole of northwestern and northern India up to the middle Ganga valley comes under the influence of dry northwestern winds.
Jet Stream and Upper Air Circulation: The pattern of air circulation discussed above is witnessed only at the lower level of the atmosphere near the surface of the earth. Higher up in the lower troposphere, about three km above the surface of the earth, a different pattern of air circulation is observed. The variations in the atmospheric pressure closer to the surface of the earth have no role to play in the making of upper air circulation. All of Western and Central Asia remains under the influence of westerly winds along the altitude of 9-13 km from west to east. These winds blow across the Asian continent at latitudes north of the Himalayas roughly parallel to the Tibetan highlands. 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 bifurcated. 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 has its mean position at 25°N in February at 200-300 mb level. It is believed that this southern branch of the jet stream exercises an important influence on the winter weather in India.
Western Cyclonic Disturbance and Tropical Cyclones: The western cyclonic disturbances which enter the Indian subcontinent from the west and the northwest during the winter months, originate over the Mediterranean Sea and are brought into India by the westerly jet stream. An increase in the prevailing night temperature generally indicates an advance in the arrival of these cyclones’ disturbances.
Tropical cyclones originate 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 destructive due to high wind velocity and torrential rain that accompanies it.
Mechanism of weather in Summer Season
Surface Pressure and Winds: As the summer sets in and the sun shifts northwards, the wind circulation over the subcontinent undergoes a complete reversal at both, the lower as well as the upper levels. By the middle of July, the low-pressure belt nearer the surface [termed as Inter Tropical Convergence Zone (ITCZ)] shifts northwards, roughly parallel to the Himalayas between 20° N and 25° N. By this time, the westerly jet stream withdraws from the Indian region. In fact, meteorologists have found an interrelationship between the northward shift of the equatorial trough (ITCZ) and the withdrawal of the westerly jet stream from over the North Indian Plain. It is generally believed that there is a cause and effect relationship between the two. The ITCZ being a zone of low pressure, attracts inflow of winds from different directions. The maritime tropical air mass (mT) from the southern hemisphere, after crossing the equator, rushes to the low-pressure area in the general southwesterly direction. It is this moist air current which is popularly known as the southwest monsoon.
Jet Streams and Upper Air Circulation: The pattern of pressure and winds as mentioned above is formed only at the level of the troposphere. An easterly jet stream flows over the southern part of the Peninsula in June, and has a maximum speed of 90 km per hour (Figure 4.3). In August, it is confined to 15oN latitude, and in September up to 22o N latitudes. The easterlies normally do not extend to the north of 30o N latitude in the upper atmosphere.
Easterly Jet Stream and Tropical Cyclones: The easterly jet stream steers the tropical depressions into India. These depressions play a significant role in the distribution of monsoon rainfall over the Indian subcontinent. The tracks of these depressions are the areas of highest rainfall in India. The frequency at which these depressions visit India, their direction and intensity, all go a long way in determining the rainfall pattern during the southwest monsoon period.
4.3. Factors Affecting Indian Monsoon
Tibet Plateau (TP)
It is the most mountainous in the world, with a mean height that exceeds 4000 m above sea level. The mechanical effect of the plateau has determinative roles upon the formation of a regional climate system in Asia.
The thermal effect of the plateau may profoundly affect the atmospheric circulation as well because the sensible and latent heat fluxes from TP reach a higher altitude than its surroundings. The atmosphere above TP is heated more strongly than the surrounding atmosphere at the same level.
The roles of the Tibetan Plateau (TP) upon the transition of precipitation in the South Asian summer monsoon are investigated using a simplified regional climate model. Before the onset of the south Asian monsoon, descending flow in the mid-troposphere, which can be considered as a suppressor against precipitation, prevails over northern India.
The descending motion gradually weakens and retreats from this region before July, consistent with the northwestward migration of the monsoon rainfall. To examine a hypothesis that the dynamical and thermal effects of TP cause the mid-tropospheric subsidence and its seasonal variation, a series of numerical experiments are conducted using a simplified regional climate model. The mechanical effect of the TP generates robust descending flow over northern India during winter and spring when the zonal westerly flow is relatively strong, but the effect becomes weaker after April as the westerly flow tends to be weaker.
The thermal effect of the TP, contrastingly, enhances the descending flow over north India in the pre-monsoonal season. The descending flow enhanced by the thermal effect of the TP has a seasonal cycle because the global-scale upper-level westerly changes the energy propagation of the thermal forcing response.
The subsidence formed by the mechanical and thermal effects of the TP disappears over northern India after the subtropical westerly shifts north of the plateau, the seasonal change of which is in good agreement with that in the reanalysis data. The retreat of the descending flow can be regarded as the withdrawal of the pre-monsoon season and the commencement of the South Asian monsoon. After that, the deep convection, indicating the onset of the Indian summer monsoon, is able to develop over north India in relation to the ocean- atmosphere and land-atmosphere interaction processes. Northwest India is known to be the latest region of summer monsoon onset in south Asia. Thus, the thermal and mechanical forcing of the TP has a great impact on the transition of the Indian monsoon rainfall by changing the mid-troposphere circulation.
Recently it has been found that there is a positive correlation between Tibetan snow cover and Indian summer monsoon rainfall (IMR). The seasonal monsoon wind shift and weather associated with the heating and cooling of the Tibetan plateau is the strongest such monsoon on earth.
Jet Stream Theory
This theory tries to explain the establishment of both the NE and SW Monsoons as well their unique features like bursting and variability. The jet streams are a system of upper-air westerlies. It gives rise to slowly moving upper-air waves, with 250 knots winds in some air streams.
Over India, a subtropical westerly jet develops in the winter season which is replaced by the tropical easterly jet in the summer season. The high temperature over the Tibetan Plateau, as well as over Central Asia in general, during the summer is believed to be the critical factor leading to the formation of the tropical easterly jet over India in summer. The mechanism affecting monsoon is that the westerly jet causes high pressure over northern parts of the subcontinent during the winter. This results in the north to south flow of the winds in the form of the NE Monsoon. With the northwards shift of the vertical sun, this jet shifts northwards too. The intense heat over the Tibetan Plateau, coupled with associated terrain features of high altitude of the plateau, etc. generate the tropical easterly jet over Central India. This jet creates a low-pressure zone over the northern Indian plains influencing the wind flow towards these plains, assisting the establishment of the SW Monsoon.
Tropical easterly jet
Due to low pressure over Tibetan plateau, high pressure exists in the upper troposphere. Due to high pressure, a tropical easterly jet stream blows from Tibetan plateau to Mascarene high (low pressure at troposphere) near Madagascar. This helps in sudden onset of monsoon.
Subtropical westerly jet stream
Subtropical westerly jet stream flows entirely south of the Himalayas over north India. (during winter). They form depressions and high pressure over northern India.
Somali current and Somali jet stream (Phinlander jet)
During June-July-August due to creation of high pressure at Mascarene high basin, air diverges due to which it goes towards the horn of Africa. Here it is divided into two parts – one goes towards Africa and another goes towards Indian subcontinent.
It is a low-level jet stream (appear only in summer)
Its occurrence predicts a good monsoon in India.
Somali jet Stream is a low-level jet stream which flows from Somalia to Indian mainland
Shifting of ITCZ to Tibetan Plateau
Intertropical Convergence Zone (ITCZ) is a zone of low-pressure system which lies at the equator normally. But during summer in the Northern hemisphere, due to intense heating of Tibetan Plateau, a low-pressure system is developed and ITCZ shifts to Tibet Plateau. This causes a thermally induced condition to attract southwest monsoon towards Indian sub-continent.
Indian Ocean dipole
Two poles are formed as:
Mascarene high basin & Tibetan plateau
Mascarene high basin & Western pacific pool
When strong low pressure is created over western pacific pool then more winds will flow towards Mascarene high basin and more will be the flow of winds towards low pressure Tibetan plateau. (Sign of good monsoon) this dipole like situation in Indian ocean is called IOD (Indian Ocean Dipole)
EI-Nino – Due to impact of EI-Nino, western pacific pool becomes high pressure (due to reversal of walker cell) and low-pressure system develops at Peruvian coast dipole system is destroyed and hence no wind blow from western pacific pool towards Mascarene high and hence Indian monsoon is impacted.
Due to unusual heating of Peruvian current at western margin the L.P system does not develop in east Australian coast which causes break in dipole. Instead, a new system was emerged by H.P in the west Pacific Ocean.
4.4. The Mechanism of Indian Monsoon
Many attempts have been made to discover the exact nature and causation of monsoon, but so far, no single theory has been able to explain the monsoon fully. To understand the mechanism of monsoon in India we have to understand the following three aspects:
The onset of the monsoon.
Rain-bearing systems (e.g. tropical cyclones) and the relationship between their frequency and distribution of monsoon rainfall.
Break in the monsoon
4.4.1. The Onset of Monsoon
It was believed that the differential heating of land and sea during the summer months is the mechanism which sets the stage for the monsoon winds to drift towards the subcontinent. 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 trade deflected towards the Indian subcontinent after crossing the Equator. These winds cross the Equator between 40°E and 60°E longitudes.
The shift in the position of the ITCZ is also related to the phenomenon of the withdrawal of the westerly jet stream from its position over the north Indian plain, south of the Himalayas. The easterly jet stream sets in along 15°N latitude only after the western jet stream has withdrawn itself from the region. This easterly jet stream is held responsible for the burst of the monsoon in India.
Entry of Monsoon into India: The southwest monsoon sets in over the Kerala coast by 1st June and moves swiftly to reach Mumbai and Kolkata between 10th and 13th June. By mid-July, southwest monsoon engulfs the entire subcontinent.
4.4.2. Rain-bearing Systems and Rainfall Distribution
There seem to be two rain-bearing systems in India. First originated in the Bay of Bengal causing rainfall over the plains of north India. Second is the Arabian Sea current of the southwest monsoon which brings rain to the west coast of India. Much of the rainfall along the Western Ghats is orographic as the moist air is obstructed and forced to rise along the Ghats.
The intensity of rainfall over the west coast of India is, however, related to two factors:
The offshore meteorological conditions.
The position of the equatorial jet stream along the eastern coast of Africa.
The frequency of the tropical depressions originating from the Bay of Bengal varies from year to year. Their paths over India are mainly determined by the position of ITCZ which is generally termed as the monsoon trough. 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. The rain which comes in spells, displays a declining trend from west to east over the west coast, and from the southeast towards the northwest over the North Indian Plain and the northern part of the Peninsula.
4.4.3. Break in 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 a 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 associated with days when winds blow parallel to the coast.
4.5. Seasons in India
The climatic conditions of India can best be described in terms of an annual cycle of seasons. The meteorologists recognised the following four seasons:
the cold weather seasons
the hot weather seasons
the southwest monsoon seasons
the retreating monsoon seasons.
4.5.1. The Cold Weather Season
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. The mean daily temperature remains below 21°C over most parts of northern India. The night temperature may be quite low, sometimes going below freezing point in Punjab and Rajasthan. 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 a 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 influence of the sea and the proximity to the equator. For example, the mean maximum temperature for January at Thiruvananthapuram is as high as 31°C, and for June, it is 29.5°C. Temperatures at the hills of Western Ghats remain comparatively low.
Pressure and Winds: By the end of December (22nd December), the sun shines vertically over the tropic of Capricorn in the southern hemisphere. The weather in this season is characterized by feeble high-pressure conditions over the northern plain. In south India, the air pressure is slightly lower. The isobars of 1019mb and 1013mb pass through northwest India and far south respectively.
As a result, winds start blowing from the northwestern high-pressure zone to the low air pressure zone over the Indian Ocean in the south. Due to low pressure gradient, the light winds with a low velocity of about 3-5 km per hour begin to blow outwards. By and large, the topography of the region influences the wind direction. They are westerly or northwesterly down the Ganga Valley. They become northerly in the Ganga-Brahmaputra delta. Free from the influence of topography, they are clearly northeasterly over the Bay of Bengal.
During the winters, the weather in India is pleasant. The pleasant weather conditions, however, at intervals, get disturbed by shallow cyclonic depressions originating over the east Mediterranean Sea and travelling eastwards across West Asia, Iran, Afghanistan and Pakistan before they reach the northwestern parts of India. On their way, the moisture content gets augmented from the Caspian Sea in the north and the Persian Gulf in the south.
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 western 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. The precipitation goes on decreasing from west to east in the plains and from north to south in the mountains. The average winter rainfall in Delhi is around 53 mm. In Punjab and Bihar, rainfall remains between 25 mm and 18 mm respectively.
Central parts of India and northern parts of the southern Peninsula also get winter rainfall occasionally.
Arunachal Pradesh and Assam in the northeastern parts of India also have rains between 25 mm and 50 mm during these winter months.
During October and November, northeast monsoon while crossing over the Bay of Bengal, picks up moisture and causes torrential rainfall over the Tamil Nadu coast, southern Andhra Pradesh, southeast Karnataka and southeast Kerala.
4.5.2. The Hot weather Season
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 most parts of India, temperatures recorded are between 30°-32°C. In March, the highest day temperature of about 38°C occurs in the Deccan Plateau while in April, temperatures ranging between 38°C and 43°C are found in Gujarat and Madhya Pradesh. In May, the heat belt moves further north, and in the north-western part of India, temperatures around 48°C are not uncommon.
The hot weather season in south India is mild and not so intense as found in north India. The Peninsular situation of south India with moderating effect of the oceans keeps the temperatures lower than that prevailing in north India. So, temperatures remain between 26°C and 32°C. Due to altitude, the temperatures in the hills of Western Ghats remain below 25°C. 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. The mean daily minimum temperature during the summer months also remains quite high and rarely goes below 26°C.
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 position centered at 25°N in July. Roughly, this elongated low-pressure monsoon trough extends over the Thar desert in the north-west to Patna and Chotanagpur plateau in the east-southeast. The location of the ITCZ attracts a surface circulation of the winds which are southwesterly on the west coast as well as along the coast of West Bengal and Bangladesh. They are easterly or southeasterly over north Bengal and Bihar. It has been discussed earlier that these currents of southwesterly monsoon are in reality ‘displaced’ equatorial westerlies. The influx of these winds by mid-June brings about a change in the weather towards the rainy season.
In the heart of the ITCZ in the northwest, the dry and hot winds known as ‘Loo’, blow in the afternoon, and very often, they continue well into midnight. Dust storms in the evening are very common during May in Punjab, Haryana, Eastern Rajasthan and Uttar Pradesh. These temporary storms bring a welcome respite from the oppressing heat since they bring with them light rains and a pleasant cool breeze. Occasionally, the moisture-laden winds are attracted towards the periphery of the trough. A sudden contact between dry and moist air masses gives rise to local storms of great intensity. These local storms are associated with violent winds, torrential rains and even hailstorms.
4.5.3. The Southwest Monsoon Season or Advancing Monsoon Season
As a result of rapid increase of temperature in May over the northwestern plains, the low-pressure conditions over there get further intensified. By early June, they are powerful enough to attract the trade winds of the Southern Hemisphere coming from the Indian Ocean. These southeast trade winds cross the equator and enter the Bay of Bengal and the Arabian Sea, only to be caught up in the air circulation over India. 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 lightning, is often termed as the “break” or “burst” of the monsoons. The monsoon may burst in the first week of June in the coastal areas of Kerala, Karnataka, Goa and Maharashtra while in the interior parts of the country, it may be delayed to the first week of July. The day temperature registers a decline of 5°C to 8°C between mid-June and mid-July. As these winds approach the land, their southwesterly direction is modified by the relief and thermal low pressure over northwest India. The monsoon approaches the landmass in two branches:
The Arabian Sea branches
The Bay of Bengal branch.
Monsoon Winds of the Arabian Sea
The monsoon winds originating over the Arabian Sea further split into three branches.
It’s 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 Coastal 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 Aravallis, causing only a scanty rainfall. In Punjab and Haryana, it too joins the Bay of Bengal branch. These two branches, reinforced by each other, cause rains in the western Himalayas,
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. It’s one branch moves westward along the Ganga plains reaching as far as the Punjab plains. The other branch moves up the Brahmaputra 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 monsoon.
It lies in the rain shadow area of the Arabian Sea branch of the south-west monsoon.
Characteristics of Monsoonal Rainfall
Rainfall received from the southwest monsoons is seasonal in character, which occurs between June and September.
Monsoonal rainfall is largely governed by relief or topography. For instance, the windward side of the Western Ghats registered a rainfall of over 250 cm. Again, the heavy rainfall in the northeastern states can be attributed to their hill ranges and the Eastern Himalayas.
The monsoon rainfall has a declining trend with increasing distance from the sea. Kolkata receives 119 cm during the southwest monsoon period, Patna 105 cm, Allahabad 76 cm and Delhi 56 cm.
The monsoon rains occur in wet spells of a few days’ duration at a time. The wet spells are interspersed with rainless intervals known as ‘breaks. These breaks in rainfall are related to the cyclonic depressions mainly formed at the head of the Bay of Bengal, and their crossing into the mainland. Besides the frequency 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 runoff and soil erosion.
Monsoons play a pivotal role in the agrarian economy of India because over three-fourths of the total rain in the country is received during the southwest 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 earlier than usual, causing great damage to standing crops and making the sowing of winter crops difficult.
4.5.4. The retreating Monsoon Season
The months of October and November are known for retreating monsoons. By the end of September, the southwest monsoon becomes weak as the low-pressure trough of the Ganga plain starts moving southward in response to the southward march of the sun. The monsoon retreats from the western Rajasthan by the first week of September. It withdraws from Rajasthan, Gujarat, Western Ganga plain and the Central Highlands by the end of the month. By the beginning of October, the low pressure covers northern parts of the Bay of Bengal and by early November, it moves over Karnataka and Tamil Nadu. By the middle of December, the centre of low pressure is completely removed from the Peninsula.
The retreating southwest monsoon season is marked by clear skies and rise in temperature. The land is still moist. Owing to the conditions of high temperature and humidity, the weather becomes rather oppressive. This is commonly known as the ‘October heat’. In the second half of October, the mercury begins to fall rapidly, particularly in northern India. 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, October and November are the rainiest months of the year.
The widespread rain in this season is associated 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 Coromandel coast is derived from these depressions and cyclones. Such cyclonic storms are less frequent in the Arabian Sea.
4.6. Distribution of Rainfall in India
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 than 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 Orissa, 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 Deccan Plateau receive rainfall between 50-100 cm.
Areas of Inadequate Rainfall: Parts of the Peninsula, especially in Andhra Pradesh, Karnataka and Maharashtra, Ladakh and most of western Rajasthan receive rainfall below 50 cm. Snowfall is restricted to the Himalayan region.
4.7. Climatic Classification of India
The two most significant factors in defining different climates are temperature and precipitation. A place’s location on a continent, its topography, and its elevation may also have an impact on the climate. Although India has tropical monsoon climate as a whole, there are large regional variations in important climatic elements such as rainfall and temperature. Because of these variations India can be divided into various climatic regions.
4.7.1. Koppen’s Classification of Climatic Regions
Wladimir Koppen gave a classification of climate based upon annual and monthly means of temperature and precipitation. It accepts the native vegetation as the best expression of the totality of a climate, so that many of the climatic boundaries are based upon vegetation.
Koppen has expressed the view that the effectiveness of precipitation in vegetation growth depends not only upon the amount of precipitation, but also upon the intensity of evaporation and transpiration. Much of the water obtained from precipitation is lost from the soil and plants by evaporation and transpiration and is not available for vegetation growth. Thus, a certain amount of rain falling in hot and dry climates may not be as useful to vegetation as the same amount of rain falling in a cool and humid climate. Koppen has suggested five major types of climate which correspond with five principal vegetation groups. Each climatic type is represented by a capital letter explained below:
A: Tropical rainy climate with no cool season. Temperature of the coolest month above 18°C.
B: Dry climate in which there is an excess of evaporation over precipitation.
C: Middle-Latitude rainy climate with mild winters. Average temperature of coldest month below 18°C but above -3°C. Average temperature of warmest month over 10°C.
D: Middle-latitude rainy climate with severe winters. Average temperature of coldest month below -3°C and that of warmest month above 10°C.
E: Polar climate with no warm season. Average temperature of the warmest month below 10°C.
The above mentioned major climatic types are further subdivided depending upon the seasonal distribution of rainfall or degree of dryness or cold. They are designated by small letters a, c, f, h, m, g, s and w each having a specific meaning as per details given below:
a: hot summer, average temperature of the warmest month over 22°C.
c: cool summer, average temperature of the warmest month under 22°C.
f: no dry season.
w: dry season in winter.
s: dry season in summer.
g: Ganges type of annual march of temperature; hottest month comes before the solstice and the summer rainy season.
h (heiss): average annual temperature under 18°C.
m (monsoon): short dry season.
The capital letters S and W are employed to designate the two subdivisions of dry climate: semiarid or Steppe (S) and arid or desert (W). Capital letters T and F are similarly used to designate the two subdivisions of polar climate: tundra (T) and icecap (F).
Koppen divided India into nine climatic regions making use of the above scheme (Fig.)
Amw (Monsoon type with short dry winter season). This climate is found in the western coastal region, south of Mumbai. This area receives over 300 cm of annual rainfall in summer from the south-west monsoons.
As (Monsoon type with dry season in high sun period). This is the region in which rainfall occurs in winter and summer is dry. Coromandel Coast experiences this type of climate. Coastal Tamil Nadu and adjoining areas of Andhra Pradesh are included in it. The amount of rainfall mostly in winter is 75-100 cm and is received from the retreating monsoons.
Aw (Tropical Savanna type). This climate is found in most parts of the peninsular plateau barring Coromandel and Malabar coastal strips. The northern boundary of this climatic region roughly coincides with the Tropic of Cancer. The average annual rainfall is about 75 cm which is received in the summer season from the south west monsoons. Winter season remains dry.
BShw (Semi-arid Steppe type). Some rain shadow areas of Western Ghats, large parts of Rajasthan and contiguous areas of Haryana and Gujarat have this type of climate. Rainfall varies from 12 to 25 cm and most of it occurs in summer. Winter is completely dry. Some arid steppe vegetation is found here.
BWhw (Hot desert type).? Most of western Rajasthan has a hot desert type of climate where the amount of annual rainfall is less than 12 cm. Temperatures are very high in summer. Natural vegetation is almost absent.
Cwg (Monsoon type with dry winters). This type of climate is found in most parts of the Ganga Plain, eastern Rajasthan, Assam and in Malwa Plateau. The summer temperature rises to 40°C which falls to 27 °C in winter. Most rainfall occurs in summer and winter is dry.
Dfc (Cold, Humid winters type with shorter summer). Some of the north-eastern states such as Sikkim, Arunachal Pradesh and parts of Assam have this type of climate. Winters are cold, humid and of longer duration. The winter temperatures are about 10°C. Summers are short but humid.
Et (Tundra Type). This climate is found in the mountain areas of Uttarakhand. The average temperature varies from 0 to 10°C. There is fall in temperature with altitude.
E (Polar Type). The higher areas of Jammu & Kashmir and Himachal Pradesh experience polar climate in which the temperature of the warmest month varies from 0° to 10°C. These areas are covered with snow for most part of the year.
4.8. Impact of Monsoon
Monsoon is that axis around which revolves the entire agricultural cycle of India. It is because about 64 per cent people of India depend on agriculture for their livelihood and agriculture itself is based on southwest monsoon.
Except Himalayas all the parts of the country have temperatures above the threshold level to grow the crops or plants throughout the year.
Regional variations in monsoon climate help in growing various types of crops.
Variability of rainfall brings droughts or floods every year in some parts of the country.
Agricultural prosperity of India depends very much on timely and adequately distributed rainfall. If it fails, agriculture is adversely affected particularly in those regions where means of irrigation are not developed.
Sudden monsoon bursts create problems of soil erosion over large areas in India.
Winter rainfall by temperate cyclones in north India is highly beneficial for rabi crops.
Regional climatic variation in India is reflected in the vast variety of food, clothes and house types.