NCERT Summary: Climate - 1 Notes | EduRev

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Introduction 

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.

NCERT Summary: Climate - 1 Notes | EduRev

The elements of weather and climate are the same, i.e. temperature, atmospheric pressure, wind, humidity and precipitation. You may 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 Pahalgam sector of Jammu and Kashmir has 20ºC temperature. During winter nights Dras sector of Jammu and Kashmir witnesses– 45ºC temperature where as Thiruvananthapuram has 20ºC.
  • Rainfall also varies in terms of quantity and distribution in the regions of Himalaya rainfall is in the form 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 Meghalaya 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.NCERT Summary: Climate - 1 Notes | EduRev


Factors determining 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.
  • factors related to air pressure and winds.

Try yourself:Punjab state falls under which climatic region?
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(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 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 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 the Western 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.
    NCERT Summary: Climate - 1 Notes | EduRev
  • 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 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 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.
Try yourself:Which of the following factors is/are that determine the climate of a region?
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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. NCERT Summary: Climate - 1 Notes | EduRev
    A) Idealized winds generated by pressure gradient and Coriolis Force. B) Actual wind patterns owing to land mass distribution.
  • In July, the ITCZ is located around 20ºN latitudes (over the Gangetic plain), sometimes called the monsoon 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 Wind
  • 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 20ºN and 25ºN. By this time, the westerly jet stream withdraws from the Indian region.
    NCERT Summary: Climate - 1 Notes | EduRevIndian Climate: Summer
  • 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.
  • 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 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.
Try yourself:The Inter-Tropical Convergence Zone (ITCZ) is a low-pressure zone located at the
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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 (Example: tropical cyclones) and the relationship between their frequency and distribution of monsoon 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.NCERT Summary: Climate - 1 Notes | EduRev
  • 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 stream from its 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 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:

(i) The offshore meteorological conditions.
(ii) 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.


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.NCERT Summary: Climate - 1 Notes | EduRev
  • The system involves oceanic and atmospheric phenomena with the appearance of warm currents off the coast of peru in the Eastern Pacific and affects weather in many places including India. EI-Nino is merely an extension of the warm equatorial current which gets replaced temporarily by cold Peruvian current or Humboldt current. This current increases the temperature of water on the Peruvian coast by 10ºC. This results in:

    (i) The distortion of equatorial atmospheric circulation;
    (ii) Irregularities in the evaporation of seawater;
    (iii) 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.

Try yourself:What is the effect of El Nino on Indian monsoon?
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