Gist of NCERT
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. Youmay have observed that 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. During the summer season the desert area of Rajasthan witnesses 50º temperature whereas Pahalgamsector of Jammu and Kashmir has 20ºC temperature. During winter nights Dras sector of Jammu and Kashmirwitnesses– 45ºC temperature where as Thiruvananth puram has 20ºC. Rainfall also varies in terms of quantity and distribution in the regions of Himalaya rainfall is in the from of snowy balls where as in the rest of part of India it is a general rain. Again annual rainfall varies from 400ºC in the Meghalya to 10ºc in Ladakh and West Rajasthan. In the coastal area the variation of rainfall is less. Whereas in the inner part of country the seasonal variation is more. Accordingly the Indians show their unity in diversity in terms of food, clothing, housing and culture.
Factors determining the climate of India
India’s climate is controlled by a number of factors which can be broadly divided into two groups- (a) factors related to location and relief, and (b) factors related to air pressure and winds.
(a) Factors related to Location and Relief Latitude:
You know that the Tropic of Cancer passes through the central part of India in east- west direction. This, northern part of the India lies in 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 high daily and annual range of temperature.
The Himalayan Mountains: The towering mountain chain provides an invincible shield to protect the subcontinent from the cold northern winds. 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 India Ocean on three sides in the south and girdled by a high and continuous mountainwall 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 decreaseswith 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 temperature of January in Agra is 16ºC whereas it is only 4ºC 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 to its leeward situation along theWestern Ghats.
(b) 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 favourable 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 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 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 ofwesterly 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. On 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 25ºN in February at 200-300 mb level. It is believed that this southern branch of the jet stream exercise an important influence on the winter weather in India.
Western Cyclonic Disturbance and Tropical Cyclones: The western cyclone 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.
Inter Tropical Convergence Zone (ITCZ) The Inter Tropical Convergence Zone (ITCZ) is a low pressure zone located at the equator where trade winds converge, and so, it is a zone where air tends to ascend. In July, the ITCZ is located around 20ºN latitudes (over the Gangetic plain), sometimes called themonsoon trough. This monsoon trough encourages the development of thermal low over north and northwest India. Due to the shift of ITCZ, the trade winds of the southern hemisphere cross the equator between 40ºE and 60ºE longitudes and start blowing from southwest to northeast due to the Coriolis force. It becomes southwest monsoon. In winter, the ITCZ moves southward, and so the reversal of winds from northeast to south and southwest, takes place. They are called northeast monsoons.
Mechanism of Weather in the 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 themiddle of July, the low pressure belt nearer the surface (termed as Inter Tropical Convergence Zone (ITCZ) shifts northwards, roughly parallel to the Himalayas between 20ºN and 25ºN. 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 asmentioned 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. In August, it is confined to 15ºN latitude, and in September up to 22ºN latitudes. The easterlies normally do not extend to the north of 30ºN latitude in the upper atmosphere.
Easterly Jet Stream and Tropical Cyclones: The easterly jet streamsteers 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.
The Nature of Indian Monsoon
Monsoon is a familiar, though a little known climatic phenomenon. Despite the observations spread over centuries, the monsoon continues to puzzle the scientists. 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. A real breakthrough has come recently when it was studied at the global rather than at regional level.
Systematic studies of the causes of rainfall in the South Asian region help to understand the causes and salient features of the monsoon, particularly some of its important aspects, such as:
- The onset of the monsoon.
- Rain-bearing systems (e.g. tropical cyclones) and the relationship between their frequency and distribution ofmonsoon rainfall.
- Break in the monsoon.
Onset of the Monsoon
Towards the end of the nineteenth century, 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 of 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 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 Equator. These winds cross the Equator between 40ºE and 60ºE longitudes.
The shift in the position of the ITCZ is also related to the phenomenon of the withdrawal of the westerly jet streamfromits position over the north Indian plain, south of the Himalayas. The easterly jet stream sets in along 15ºN 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.
Rain-bearing Systems and Rainfall Distribution
There seem to be two rain-bearing systems in India. First originate in the Bay of Bengal causing rainfall over the plains of north India. Second is the Arabian Sea current of the southwestmonsoon 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 fromthe Bay of Bengal varies fromyear 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.
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. EINino 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 10ºC. This results in:
- The distortion of equatorial atmospheric circulation;
- Irregularities in the evaporation of sea water;
- 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 even 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, it 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 associated with dayswhen 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. The meteorologists recognize the following four seasons:
- The cold weather season
- The hot weather season
- The southwest monsoon season
- The retreating monsoon season.
Some Famous Local Storms of Hot Weather Season
Mango Shower: Towards the end of summer. There are pre-monsoon showers which are a common phenomena in Kerala and coastal areas of Karnataka. Locally, they are known as mango showers since they help in the early ripening of mangoes.
Blossom Shower: With this shower, coffee flowers blossom in Kerala and nearby areas.
Nor Westers: These are dreaded evening thunderstorms in Bengal and Assam. Their notorious nature can be understood from the local nomenclature of ‘Kalbaisakhi’, a calamity of the month of Baisakh. These showers are useful for tea, Jute and rice cultivation. In Assam, these storms are known as “Bordoiseela”.
Loo: Hot, dry and oppressing winds blowing in the Northern plains from Punjab to Bihar with higher intensity between Delhi and Patna.
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 21ºC 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 influenced 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.
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 equator. For example, the mean maximum temperature for January at Thiruvanantapuram is as high as 31ºC, and for June, it is 29.5ºC. Temperatures at the hills of Western Ghats remain comparatively low. Pressure andWinds: 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 1019 mb and 1013 mb pass through northwest India and far south, respectively.
As a result, winds start blowing from 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-Brahamputra 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 Medirranean Sea and travelling eastwards across West Asia, Iran, Afghanistan and Pakistan before the 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.
Role of Westerly Jet Stream
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 meager, 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 fromwest to east in the plains and fromnorth 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 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, pick up moisture and causes torrential rainfall over the Tamil Nadu coast, southern Andhra Pradesh, southeast Karnataka and southeast Kerala.
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 30º-32ºC. In March, the highest day temperature of about 38ºC occurs in the Deccan Plateau while in April, temperature ranging between 38ºC and 43ºC are found in Gujarat andMadhya Pradesh. InMay, the heat belt moves further north, and in the north-western part of India, temperatures around 48ºC 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 26ºC and 32ºC. Due to altitude, the temperatures in the hills ofWesternGhats remain below 25ºC. 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 fromthe coast to the interior. The mean daily minimum temperature during the summer months also remains quite high and rarely goes below 26ºC.
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 centred at 25ºN 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 ofWest 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’, blowin the afternoon, and very often, they continue to well intomidnight. Dust storms in the evening are very common duringMay 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.