Revision Notes of Geography for Class 9 Chapter 4 Climate

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). While weather refers to the state of the atmosphere over an area at any point of time. Their elements are temperature, atmospheric pressure, wind, humidity and precipitation.

Regional Variations in Climatic conditions of India

Temperature

  • In Summer, the mercury occasionally touches 50°C in some parts of the Rajasthan desert whereas it may be around 20°C in Pahalgam in Jammu and Kashmir.
  • In general, coastal areas experience less contrasts in temperature conditions.
  • Seasonal contrasts are more in the interior of the country.

Precipitation

  • It is mostly in the form of snowfall in the upper parts of Himalayas whereas it rains over the rest of the country.
  • The annual precipitation varies from over 400 cm in Meghalaya to less than 10 cm in Ladakh and western Rajasthan.
  • Most parts of the country receive rainfall from June to September.
  • But some parts like the Tamil Nadu Coast gets a large portion of its rain during October and November.
  • There is decrease in rainfall generally from East to West in the Northern Plains.

Climatic Controls

  • There are six major controls of the climate – latitude, altitude, pressure and wind system, distance from the sea (continentally), ocean currents and relief features.
  • Due to curvature of the Earth, the amount of solar energy received varies according to latitude and altitude which generally decreases the air pressure from Equator to 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.
  • Pressure and wind system depends on altitude and latitude of the area which influences the temperature and rainfall pattern.
  • As the distance from the sea increases, its moderating influence decreases and the people experience extreme weather conditions which is known as continentally (very hot during summers and very cold during winters).
  • Ocean currents along with onshore winds affect the climate of the coastal areas.
  • Relief also affects the climate of a place. For example – 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.

Factors affecting India’s Climate

Latitude

  • The Tropic of Cancer passes through the middle of the country from the Rann of Kuchchh in the West to Mizoram in the East.
  • India’s climate has characteristics of tropical as well as subtropical climates.

Altitude

  • India has high Himalayan ranges (average height of about 6,000 metres) in North, which protect India from cold winds of central Asia. It is because of these mountains that this subcontinent experiences comparatively milder winters as compared to central Asia.

Pressure and Winds

  • The climate and associated weather conditions in India are governed by following atmospheric conditions:
    • Pressure and surface winds.
    • Upper air circulation.
    • Western cyclonic disturbances and tropical cyclones.
  • India lies in the region of north easterly winds.

North Easterly Winds

  • These winds originate from the subtropical high-pressure belt of the northern hemisphere.
  • They originate and blow over land as they blow southwards, get deflected to the right due to the Coriolis force, and move towards the equatorial low-pressure area.

Unique Pressure and Wind Conditions over India

  • During Winter: There is a high-pressure area North of the Himalayas and cold dry winds blow from this region to the low-pressure areas over the oceans to the South.
  • During Summer: A low-pressure area develops over interior Asia and North-Western India which causes a complete reversal of the direction of winds during Summer.
  • Southwest Monsoon winds: Air moves from the high-pressure area over the southern Indian Ocean, in a south-easterly direction, crosses the equator, and turns right towards the low-pressure areas over the Indian subcontinent. These winds blow over the warm oceans, gather moisture and bring widespread rainfall over the mainland of India.
  • Upper air circulation: It is dominated by a westerly flow. An important component of this flow is the jet
    stream.

Jet Stream

  • These are a narrow belt of high altitude (above 12,000 m) westerly winds in the troposphere.
  • Their speed varies from about 110 km/h in summer to about 184 km/h in winter.
  • A number of separate jet streams have been identified. The most constant are the mid-latitude and the sub-tropical jet streams.
  • These are located approximately over 27°-30° north latitude, therefore, they are known as subtropical westerly jet streams. Over India, these jet streams blow south of the Himalayas, all through the year except in summer. The Western cyclonic disturbances experienced in the north and north-western parts of the country brought in by these westerly winds.
  • In summer, the subtropical westerly jet stream moves north of the Himalayas with the apparent movement of the Sun.
  • An easterly jet stream, called the sub-tropical easterly jet stream blows over peninsular India, approximately over 14°N during the summer months.

Climate of India

  • The Climate of India is described as the ‘Monsoon’ type which refers to the seasonal reversal in the wind direction during a year.
  • In Asia, this type of climate is found mainly in the south and the southeast.

The Indian Monsoon

  • This phenomenon was first noticed by the sailors who came to India in historic times and were benefited from the reversal of the wind system as they came by sailing ships at the mercy of winds.
  • It was named ‘monsoon’ by the Arabs traders.
  • It is experienced in the tropical area roughly between 20°N and 20°S.
  • Its duration is between 100- 120 days from early June to mid-September.

Factors affecting the Mechanism of Monsoon

  • Differential heating and cooling of land and water creates low pressure on land in India and relatively high pressure on seas.
  • The shift of the position of Inter Tropical Convergence Zone (ITCZ) in Summer, over the Ganga plain (this is the equatorial trough normally positioned about 5°N of the Equator. It is also known as the Monsoon trough during the Monsoon season).
  • The presence of the high-pressure area, east of Madagascar, approximately at 20°S over the Indian Ocean. Its intensity and position affect the Indian Monsoon.
  • During Summer, the Tibetan plateau gets intensely heated which results in strong vertical air currents and the formation of low pressure over the plateau at about 9 km above sea level.
  • The movement of the westerly jet stream to the North of the Himalayas and the presence of the tropical easterly jet stream over the Indian peninsula during Summer.

Other conditions

  • The monsoon is also affected by the pressure conditions over the southern oceans.
  • Normally when the tropical eastern south Pacific Ocean experiences high pressure, the tropical eastern Indian Ocean experiences low pressure. But in certain years, there is a reversal in the pressure conditions and the eastern Pacific has lower pressure in comparison to the eastern Indian Ocean. This is known as the Southern Oscillation or SO.
  • The difference in pressure over Tahiti (Pacific Ocean) and Darwin in northern Australia (Indian Ocean) is computed to predict the intensity of the monsoons.

El Nino Southern Oscillations (ENSO)

  • It relates to the SO in which a warm ocean current that flows past the Peruvian Coast, in place of the cold Peruvian current, every 2 to 5 years.
  • The changes in pressure conditions are connected to the El Nino.
  • The phenomenon is known as ENSO (El Nino Southern Oscillations).

Mechanism of Monsoon

Onset of Monsoon

  • The Monsoon, unlike the trades, are not steady winds but are pulsating in nature which is affected by different atmospheric conditions on its way over the warm tropical seas.
  • The duration of the monsoon is between 100- 120 days from early June to mid-September.
  • Burst of Monsoon: Around the time of its arrival, the normal rainfall increases suddenly and continues constantly for several days.
  • The islands receive the very first monsoon showers, progressively from South to North.
  • It arrives at the southern tip of the Indian peninsula and proceeds into two – the Arabian Sea branch and the Bay of Bengal branch. Both branches merge over the north-western part of the Ganga plains.
  • Delhi generally receives the monsoon showers from the Bay of Bengal branch.
  • Different regions or states welcome monsoon on different dates. By mid-July, the monsoon reaches Himachal Pradesh and the rest of India.

Withdrawal of Monsoon

  • It is a more gradual process which begins in North-Western part of India by early September and withdraws completely from the northern half of the Peninsula by mid-October.
  • The withdrawal from the southern half of the peninsula is fairly rapid. By early December, the monsoon generally withdraws from rest of India.

The Seasons

  • The monsoon type of climate is characterised by a distinct seasonal pattern. The changes are particularly noticeable in the interior parts of the country.
  • The coastal areas do not experience much variation in temperature though there is variation in rainfall pattern.
  • There are four main seasons in India – the cold weather season, the hot weather season, the advancing monsoon and the retreating monsoon with some regional variations.

The Cold Weather Season (Winter)

  • Period: It begins from mid-November in northern India and stays till February. December and January are the coldest months in the northern part of India.
  • The temperature decreases from South to the North and days are warm, and nights are cold.
  • Frost is common in the North and the higher slopes of the Himalayas experience snowfall.
  • During this season, the Northeast trade winds prevail over India. They blow from land to sea and hence, for most part of the country, it is a dry season.
  • Some amount of rainfall occurs on the Tamil Nadu coast as here these winds blow from sea to land.
  • The weather is normally marked by clear sky, low temperatures and low humidity and feeble, variable winds.
  • There is the inflow of low-pressure systems called cyclonic disturbances from the west and the northwest which originate over the Mediterranean Sea and western Asia and move into India with the westerly flow.
  • The total amount of winter rainfall is locally known as ‘mahawat’ which is of immense importance for the cultivation of ‘rabi’ crops.
  • The Peninsular region does not have a well-defined cold season due to the moderating influence of the sea.

The Hot Weather Season (Summer)

  • Due to the apparent northward movement of the Sun, the global heat belt shifts northwards and from March to May, it is hot weather season in India.
  • The summer months experience rising temperature and falling air pressure in the northern part of India.
  • Temperatures remain lower in Peninsular India due to the moderating influence of the oceans.
  • A striking feature of the hot weather season is the ‘loo’ which are strong, gusty, hot, dry winds blowing
    during the day over the north and north-western India.
  • Dust storms are very common during the month of May in northern India. Towards the close of the summer season, pre-monsoon showers are common.
  • This is also the season for localised thunderstorms, associated with violent winds, torrential downpours, often accompanied by hail.

Different names of Thunderstorms

  • In West Bengal, dust storms are known as the ‘Kaal Baisakhi’.
  • ‘Mango showers’: These are Pre-monsoon showers common in Kerala and Karnataka which help in the early ripening of mangoes.

Advancing Monsoon (The Rainy Season)

South-West Monsoon

  • By early June, the low-pressure condition over the northern plains intensifies which attracts the trade winds of the Southern hemisphere.
  • These south-east trade winds originate over the warm sub-tropical areas of the southern oceans and cross the Equator and blow in a south-westerly direction and enter in India. They bring abundant moisture to the sub-continent as these winds blow over warm oceans.
  • The monsoon winds cover the country in about a month except the extreme north-west.
  • The maximum rainfall of this season is received in the north-eastern part of India. Mawsynram in the southern ranges of the Khasi Hills receives the highest average rainfall in the world.
  • Rainfall in the Ganga valley decreases from the East to the West.
  • Break in Monsoon: These are wet and dry spells of monsoon and are related to movement of the monsoon trough. When the axis of the monsoon trough lies over the plains, rainfall is good in these parts whereas when the axis shifts closer to the Himalayas, there are longer dry spells in the plains and widespread rain occurs in the mountainous catchment areas of the Himalayan Rivers.
  • The Monsoon rains are also affected by the frequency and intensity of tropical depressions. These depressions form at the head of Bay of Bengal. The depressions follow the axis of the “monsoon trough of low pressure”.

Retreating/Post Monsoons (The Transition Season)

  • It is a period of transition from hot rainy season to dry winter conditions in the months of October November.
  • During October-November, with the apparent movement of the Sun towards the South, the monsoon trough or the low-pressure trough over the northern plains becomes weaker.
  • The retreat of the monsoon is marked by clear skies and rise in temperature.
  • October Heat: Owing to the conditions of high temperature and humidity, the weather becomes rather oppressive during the day. This is commonly known as ‘October heat’.
  • The low-pressure conditions, over north-western India, get transferred to the Bay of Bengal by early November. This shift is associated with the occurrence of cyclonic depressions, which originate over the Andaman Sea. These cyclones generally cross the eastern coasts of India cause heavy and widespread rain.
  • The thickly populated deltas of the Godavari, the Krishna and the Kaveri are frequently struck by cyclones, which cause great damage to life and property.
  • The bulk of the rainfall of the Coromandel Coast is derived from depressions and cyclones.

Distribution of Rainfall

  • Western coast and north-eastern India receive over about 400 cm of rainfall annually whereas Rajasthan and some parts of Gujarat has less than 60 cm.
  • Rainfall is equally low in the interior of the Deccan plateau, and east of the Sahyadris.
  • Variability of rainfall is high in the regions of low rainfall such as parts of Rajasthan, Gujarat and the leeward side of the Western Ghats.

Interesting points

  • The word monsoon is derived from the Arabic word ‘mausim’ which literally means season.
  • Coriolis Force: It is an apparent force caused by the Earth’s rotation and is responsible for deflecting winds towards the right in the northern hemisphere and towards the left in the southern hemisphere. This is also known as ‘Ferrell’s Law’.
  • Western Cyclonic Disturbances
    • These are weather phenomena of the winter months brought in by the westerly flow from the Mediterranean region.
    • They usually influence the weather of the north and north-western regions of India.
    • Tropical cyclones occur during the monsoon, as well as, in October – November, and are part of the easterly flow and affect the coastal regions of the country.
  • El Nino
    • This is a name given to the periodic development of a warm ocean current along the coast of Peru as a temporary replacement of the cold Peruvian current.
    • The presence of the El Nino leads to an increase in sea-surface temperatures and weakening of the trade winds in the region.
  • Inter Tropical Convergence Zone (ITCZ)
    • It is a broad trough of low pressure in equatorial latitudes where the northeast and the southeast trade winds converge.
    • It lies more or less parallel to the Equator but moves North or South with the apparent movement of the Sun.
    • Mawsynram, the wettest place on the Earth is reputed for its stalagmite and stalactite caves.