Climate Class 9 Notes NCERT and MCQs
This chapter discusses the Indian climate, factors affecting the Indian Climate, Seasons in India, Distribution of Rainfall in India with detailed emphasis on Indian monsoon.
Climate is the sum total of weather events and variations over a large area for a longer duration of time (more than 30 years). Weather is the atmosphere over an area at a particular point of time.
Factors affecting climate of a region
- Latitude – Due to the curvature of the earth, the amount of solar energy received varies according to latitude. Hence, air temperature generally decreases from the equator towards the poles.
- Altitude – As one goes from the surface of the earth to higher altitudes, the atmosphere becomes less dense and temperature decreases.
- Pressure and wind – 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.
- Distance from sea – As the distance from the sea increases, its moderating influence decreases and extreme weather conditions are experienced. This condition is known as continentality (i.e., very hot during summers and very cold during winters).
- Ocean currents – These 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.
- Relief – High mountains act as barriers to 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 relatively dry.
Climate of India
- India has monsoon type of climate in general but there are variations in climate, regionally.
- In Asia, this type of climate is found mainly in the south and the southeast.
- The word monsoon is derived from the Arabic word ‘mausim’ which literally means season. ‘Monsoon’ refers to the seasonal reversal in the wind direction during a year.
Factors affecting Climate of India
- The Tropic of Cancer passes through the middle of the country from the Rann of Kuchchh in the west to Mizoram in the east.
- Almost half of the country, lying south of the Tropic of Cancer, belongs to the tropical area.
- All the remaining area, north of the Tropic, lies in the sub-tropics. Therefore, India’s climate has characteristics of tropical as well as subtropical climates.
- India has mountains to the north, which have an average height of about 6,000 m. India also has a vast coastal area where the maximum elevation is about 30 m.
- The Himalayas prevent the cold winds from Central Asia from entering the subcontinent. It is because of these mountains that this subcontinent experiences comparatively milder winters as compared to central Asia.
- Pressure and Winds
- India lies in the region of north easterly winds which originate from the subtropical high-pressure belt of the northern hemisphere.
- They blow southwards, get deflected to the right due to the Coriolis force, and move towards the equatorial low-pressure area.
- During winter, there is a high-pressure area north of the Himalayas. Cold dry winds blow from this region to the low-pressure areas over the oceans to the south.
- In summer, a low-pressure area develops over interior Asia, as well as, over northwestern India. This causes a complete reversal of the direction of winds during summer.
- 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 are known as the Southwest Monsoon winds. These winds blow over the warm oceans, gather moisture and bring widespread rainfall over mainland of India.
- The upper air circulation in this region is dominated by a westerly flow. An important component of this flow is the jet stream.
- These jet streams 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 are brought in by this westerly flow.
- 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.
The Indian Monsoon
- The Arabs, who had also come to India as traders named this seasonal reversal of the wind system ‘monsoon’.
- The monsoons are experienced in the tropical area roughly between 20°N and 20°S.
- The differential heating and cooling of land and water create low pressure on the landmass of India while the seas around experience comparatively high pressure.
- 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. The intensity and position of this high-pressure area affect the Indian Monsoon.
- The Tibetan plateau gets intensely heated during summer, 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.
- Changes in the pressure conditions over the southern oceans also affect the monsoons.
- 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 periodic change in pressure conditions is known as the Southern Oscillation or SO.
- The difference in pressure over Tahiti (Pacific Ocean, 18°S/149°W) and Darwin in northern Australia (Indian Ocean, 12°30’S/131°E) is computed to predict the intensity of the monsoons.
- If the pressure differences were negative, it would mean below average and late monsoons.
- A feature connected with the SO is the El Nino phenomenon in which a warm ocean current 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 El Nino. Hence, the phenomenon is referred to as ENSO (El Nino Southern Oscillations).
Onset of Monsoon and Withdrawal
- The duration of the monsoon is between 100-120 days from early June to mid-September.
- Around the time of its arrival, the normal rainfall increases suddenly and continues constantly for several days. This is known as the ‘burst’ of the monsoon.
- The monsoon arrives at the southern tip of the Indian peninsula generally by the first week of June. Subsequently, it proceeds into two – the Arabian Sea branch and the Bay of Bengal branch.
- The Arabian Sea branch reaches Mumbai about ten days later on approximately the 10th of June.
- The Bay of Bengal branch also advances rapidly and arrives in Assam in the first week of June.
- The lofty mountains cause the monsoon winds to deflect towards the west over the Ganga plains.
- By mid-June the Arabian Sea branch of the monsoon arrives over Saurashtra-Kuchchh and the central part of the country.
- The Arabian Sea and the Bay of Bengal branches of the monsoon merge over the northwestern part of the Ganga plains.
- Delhi generally receives the monsoon showers from the Bay of Bengal branch by the end of June (tentative date is 29th of June).
- By the first week of July, western Uttar Pradesh, Punjab, Haryana and eastern Rajasthan experience the monsoon. By mid-July, the monsoon reaches Himachal Pradesh and the rest of the country.
- The withdrawal of the monsoon begins in northwestern states of India by early September. By mid-October, it withdraws completely from the northern half of the peninsula.
- The withdrawal from the southern half of the peninsula is fairly rapid. By early December, the monsoon has withdrawn from the rest of the country.
- The islands receive the very first monsoon showers, progressively from south to north, from the last week of April to the first week of May.
- The withdrawal, takes place progressively from north to south from the first week of December to the first week of January.
The Seasons in India
The monsoon type of climate is characterized by a distinct seasonal pattern. The weather conditions vary from season to season. The changes are more visible in the interiors of India while the coastal regions do not experience much variation in temperature though there is variation in rainfall pattern. Four main seasons can be identified in India.
The Cold Weather Season (Winter)
- The cold weather season begins from mid-November in northern India and stays till February.
- The temperature decreases from south to the north.
- 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 the country. 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 from these winds as, here they blow from sea to land.
- In the northern part of the country, a weak high-pressure region develops, with light winds moving outwards from this area.
- There is generally clear sky, low temperatures and low humidity and feeble, variable winds.
- A characteristic feature of this season is the inflow of cyclonic disturbances from the west and the northwest.
- These low-pressure systems, originate over the Mediterranean Sea and western Asia and move into India, along with the westerly flow. They cause winter rains over the plains and snowfall in the mountains.
- Although the total amount of winter rainfall locally known as ‘mahawat’ is small, they are very importance for the cultivation of ‘rabi’ crops.
The Hot Weather Season (Summer)
- Due to the apparent northward movement of the sun, the global heat belt shifts northwards.
- The hot weather season in India is from March to May.
- The summers experience rising temperature and low pressure in the northern part.
- Towards the end of May, an elongated low-pressure area develops in the region extending from the Thar Desert in the northwest to Patna and Chotanagpur plateau in the east and southeast. Circulation of air begins to set in around this trough.
- A characteristic feature of this season is the ‘loo’- strong, gusty, hot, dry winds blowing during the day over north and northwestern India. Sometimes they even continue until late in the evening. Direct exposure to these winds may even prove to be fatal.
- Dust storms bring temporary relief as they lower the temperature and may bring light rain and cool breeze. This is also the season for localised thunderstorms, associated with violent winds, torrential downpours, often accompanied by hail.
- In West Bengal, these storms are known as the ‘Kaal Baisakhi’.
- Towards the close of the summer season, pre-monsoon showers are common especially, in Kerala and Karnataka often referred to as ‘mango showers’.
Advancing Monsoon (The Rainy Season)
- By early June, the low-pressure condition over the northern plains intensifies. It attracts the trade winds of the southern hemisphere.
- These south-east trade winds originate over the warm subtropical areas of the southern oceans.
- They cross the equator and blow in a south-westerly direction entering the Indian peninsula as the south-west monsoon.
- These winds are strong and blow at an average velocity of 30 km per hour. With the exception of the extreme north-west, the monsoon winds cover the country in about a month.
- Early in the season, the windward side of the Western Ghats receives very heavy rainfall, more than 250 cm.
- The Deccan Plateau and parts of Madhya Pradesh also receive some amount of rain in spite of lying in the rain shadow area. The maximum rainfall of this season is received in the north-eastern part of the country.
- 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 due to which Rajasthan and parts of Gujarat get scanty rainfall.
- The monsoon rains take place only for a few days at a time and are interspersed with rainless intervals. These ‘breaks’ in monsoon are related to the movement of the monsoon trough.
- The trough and its axis keep on moving northward or southward, which determines the spatial distribution of rainfall.
- When the axis of the monsoon trough lies over the plains, rainfall is good in these parts.
- On the other hand, whenever the axis shifts closer to the Himalayas, there are longer dry spells in the plains, and heavy rain occurs in the catchment areas of the Himalayan rivers.
- These heavy rains bring in their wake, devastating floods causing damage to life and property in the plains.
- The frequency and intensity of tropical depressions also determine the amount and duration of monsoon rains.
- These depressions form at the head of the Bay of Bengal and cross over to the mainland. The depressions follow the axis of the “monsoon trough of low pressure”.
Retreating/Post Monsoons (The Transition Season)
- 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.
- This is gradually replaced by a high-pressure system. The south-west monsoon winds weaken and start withdrawing gradually.
- By the beginning of October, the monsoon withdraws from the Northern Plains.
- The months of October-November form a period of transition from hot rainy season to dry winter conditions.
- The retreat of the monsoon is marked by clear skies and a rise in temperature.
- Owing to the conditions of high temperature and humidity, the weather becomes rather oppressive during the day.
- This is commonly known as ‘October heat’. In the second half of October, the temperature begins to fall rapidly in northern India.
- 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 and cause heavy rain.
- These tropical cyclones are often very destructive mainly for coastal areas of Andhra Pradesh, Tamil Nadu, Odisha and West Bengal. The bulk of the rainfall of the Coromandel Coast is derived from depressions and cyclones.
Need to Know Facts
- Coriolis force:
- An apparent force caused by the earth’s rotation.
- The Coriolis force 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 ‘Ferrel’s Law’.
- Western Cyclonic Disturbances:
- The western cyclonic disturbances 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.
- These disturbances affect the coastal regions of the country.
- 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 stream.
- Inter Tropical Convergence Zone:
- The Inter Tropical Convergence Zone (ITCZ,) is a broad trough of low pressure in equatorial latitudes.
- This is where the northeast and the southeast trade winds converge.
- This convergence zone lies more or less parallel to the equator but moves north or south with the apparent movement of the sun.
- 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.
- ‘El Nino’ is a Spanish word meaning ‘the child’, and refers to the baby Christ, as this current start flowing during Christmas.
- The presence of the El Nino leads to an increase in sea-surface temperatures and weakening of the trade winds in the region.
- Mawsynram, the wettest place on the earth is also reputed for its stalagmite and stalactite caves.
MCQs based on NCERT Class 9 Geography Chapter 4: Climate
1. Which one of the following places receives the highest rainfall in the world?
Mawsynram in Meghalaya receives the highest rainfall in the world.
2. The wind blowing in the northern plains in summers is known as:
(a) Kaal Baisakhi
(b) Trade Winds
(d) None of the above
The wind blowing in the northern plains in summers is known as Loo. Loo is a hot and dry wind that blows over the area of the Indo-Gangetic Plain during May-June.
3. Which one of the following causes rainfall during winters in north-western part of India?
(a) Cyclonic Depression
(b) Western Disturbances
(c) Retreating Monsoon
(d) Southwest Monsoon
Western Disturbances cause rainfall during winters in north-western part of India.
4. Monsoon arrives in India approximately in-
(a) Early May
(b) Early June
(c) Early July
(d) Early August
Monsoon arrives in India in early June and remains till mid-September.
5. Which one of the following characterizes the cold weather season in India?
(a) Warm days and warm nights
(b) Warm days and cold nights
(c) Cool days and cold nights
(d) Cold days and warm nights
Cold weather season in India starts from mid-November in northern India and remains till February. During this season, days are warm and nights are cold.