CBSE Class 9 Social Science Climate Chapter Notes

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CLIMATE

WEATHER AND CLIMATE

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(a) India has Diverse Climatic Conditions:

We can take two important elements-temperature and precipitation, and examine how they vary from place to place and season to season.

Temperature –

(i) In summer, the mercury occasionally touches 50o C in some parts of the Rajasthan desert, whereas it may be around 20C in Pahalgam in Jammu and Kashmir. On a winter night, temperature at Drass in Jammu and Kashmir may be as low minus 45o C. Tiruvananthpuram, on the other hand, may have a temperature of 20o C.

(ii)In certain places there is a wide difference between day and night temperatures. In the Thar Desert the day temperature may rise to 50C, and drop down to near 15o C the same night. On the other hand, there is hardly any difference in day and night temperatures in the Andaman and Nicobar islands or in Kerala. 

Precipitation –

There are variations not only in the form and types of precipitation but also in its amount and the seasonal distribution.

(i)While precipitation is mostly in the form of snowfall in the upper parts of Himalayas, it rains over the rest of the country.

(ii)The annual precipitation varies fro over 400 cm in Meghalaya to less than 10 cm in Ladakh and western Rajasthan.

(iii) Most parts of the country receive rainfall from June to September. But some parts like the Tamil Nadu coast get most of its rain during October and November.Coastal areas experience less contrast in temperature conditions, seasonal contrast are more in the interior of the country.

CLIMATIC CONTROLS

The climate of a place is determined by the interplay of various factors such as location, altitude, distance from the sea, pressure and winds and upper air circulation. 

(i) 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.

(ii) As one goes from the surface of the earth to higher to higher altitudes, the atmosphere becomes less dense and temperature decreases. The hills are therefore cooler during summers.

(iii) The pressure and wind system any area depend on the latitude and altitude of the place. Thus it       influences the temperature and rainfall pattern.

(iv) The sea exerts a moderating influences on climate: As the distance form the sea increases, its moderating influence decreases and the people experience extreme weather conditions. This condition is known as continentality.

(v) Ocean currents along with onshore winds affect the climate of the coastal areas.

(vi) Relief too plays a major role in determining the climate of a place. High mountains act as barriers for cold or not 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.

FACTORS AFFECTING INDIA’S CLIMATE

(a) Latitude:

Indian is situated roughly between 8o N and  37o N latitudes. India is divided in almost two equal parts by the tropic of cancer. The southern half lies in the tropical zone and the western half in the subtropical zone. Therefore, India’s climate has characteristics of tropical as well as subtropical climates.

(b) Altitude:

India has mountains to the north, which have an average height of about 6000 meters. 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.

(c) Pressure and Winds:

India lies in the subtropical high pressure belt, thus, the winds originate from the land and move outwards towards the equatorial low pressure belt. These winds are known as northeast trade winds and are devoid of any moisture. But due to unequal heating of land and water in the summer, a low pressure develops over the interior of land masses. This low pressure attracts the winds from south of the equator. After crossing the equator the southeast trade winds get deflected and are known as southwest monsoons. The climate of India is also affected by jet streams. This is a fast flowing wind blowing in a narrow zone in the upper atmosphere. The jet streams are responsible for sudden outbreak of monsoons in Northern India. a subtropical westerly jet stream bring in the western disturbances in winter. These disturbances cause heavy snowfall on the mountains and light rains on the northwestern part of India.

THE INDIAN MONSOON

The climate of India is strongly influenced by monsoon winds. the Arabs, who had come to India as traders benefited from the reversal of the wind system as they came by sailing ships at the mercy of winds, they named this seasonal reversal of the wind system ‘monsoon’.

The monsoons are experienced in the tropical area roughly between 20o N 20o S. to understand the mechanism of the monsoons, the following facts are important.

(i) The differential heating and cooling of land and water creates low pressure on the landmass of India while the seas around experience comparatively high pressure.

(ii)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 5o N of the equator – also known as the monsoon-trough during the monsoon season).

(iii) The presence of the high-pressure area, east of Madagascar, approximately at 20o S over the Indian Ocean. The intensity and position of this high-pressure area affects the Indian Monsoon.

(iv) The Tibetan plateau gets intensely heated during summer, which results in strong vertical air currents and the formation of high pressure over. The plateau at about 9 km above sea-level.

(v)   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. The difference in pressure over Tahiti (Pacific Ocean, 18o S/149o W) and Darwin in northern Australia (Indian Ocean, 12o 30’S/ 131oE) is computed to predict the intensity of the monsoons. if the pressure differences are negative, it means below average and late monsoons.

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THE MONSET OF THE MONSOON AND WITHDRAWAL

The Monsoon, unlike the trades, are not steady winds but are pulsating in nature, affected by different atmospheric conditions encountered by it, on its way over the warm tropical seas.The duration of the monsoon is between 100-120 days from early June to mid-September. The monsoon arrives at the southern tip of the Indian peninsula generally by the first week of June. Subsequently, it divides in to 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 arrives is Assam in the first week of June. The lofty mountain s 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. 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.

withdrawal or the retreat of the monsoon is a more gradual process. 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 SEASONS

Four main seasons can be identified in India –

(i)    Cold weather season                                            --                                 December to February

(ii)   Hot weather season                                             --                                 March to May

(iii)  Advancing monsoon season                            --                                 June to September

(iv)  Retreating monsoon season                              --                                 October and November

(A) The Cold Weather Season (Winter):

(i) The cold weather season begins from mid-November in India and stays till February.

(ii) December and January are the coldest months in the northem part of India. The temperature decreases as one moves from south to the north.

(iii) Days are warm and the nights are cold. Frost is common in the north and the higher slopes of the Himalayas experience snowfall.

(iv) 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.

(v) In the northern part of the country, a feeble high-pressure region develops, with light winds moving outwards from this area.

(vi) The weather is normally marked by clear sky, low temperatures and low humidity and feeble variable winds.

(vii) 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 the much-needed winter rains over the plains and snowfall in the mountains. Locally known as ‘mahawat’ are of immense importance for the cultivation of ‘rabi’ crops.

(viii) The northeast trade winds cause fair amount of rainfall in Chennai or the Coromandel Cast in winter.

(b) Hot weather season (Summer):

(i) Due to the apparent movement of the sun, the global heat belt shifts northward. as such, from March to May, it is not weather season in India.

(ii) Temperature increases from south to north. In peninsular India, temperatures remain lower

(iii) High temperature between 38o C and 48C in the plains.

(iv) Local dust storms accompanied with light rains.

(v) Hot dry winds, ‘loo’is common in May and June.

(vi) Kerala and Kamataka coast receivers pre-monsoon showers. (Mango showers)

(vii) West Bengal and Assam are affected by northwesterly winds. (Kalbaisakhi).

(c) Advancing Monsoon (The Rainy Season):

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The climate of India is described as of monsoon type. Derived from an Arabic word ‘mausim’, monsoons refer to the seasonal reversal in the wind direction through the year.

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Mechanism of the Monsoon:

The word monsoon denotes a season in which the wind regime is completely reversed. The southeast winds, after crossing the equator in the Indian Ocean, take a southwesterly direction. The dry and hot land bearing trades are thus completely replaced by sea bearing winds full of moisture. This phenomenon of complete reversal of winds is confined to tropical lands lying between 20o N and 20o S. this phenomenon account for 75 to 90 percent of the annual rainfall just from June to September

Characteristics of the Monsoon:

(i) Almost all over the country, the rains occur from June to September.

(ii) 75% to 90% of the total annual rainfall is concentrated over this period.

(iii) There is great variation in the advance and withdrawal dates of the monsoons.

(iv) The monsoons occur in wet spell, interspersed by dry spells.

(v) The amount of rainfall also varies, causing floods and drought conditions.

 “Distribution of rainfall received from the southwest monsoons is governed mainly by the relief of the country.”

(i) The windward side of the Western Ghats receives a rainfall of over 250 cm. On other hand, the leeward side of the Western Ghats receives less than 50cm.

(ii) The heavy rainfall in the northeastern states can be attributed to the hill and mountain ranges.

(iii) Rainfall in the Northern Plains decreases westward.

Breaks in monsoons are related to the frequency and intensity of tropical depressions. They are formed at the head of the Bay of Bengal and cross over the mainland. The depressions follow the axis of the monsoon trough of the low pressure. For various reasons 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 widespread rains in the mountainous catchment area of the Himalayan Rivers.

 “Rainfall decreases from east to west in the Northern Plains while it increases in the Peninsular India”.

Pattern of Rainfall in the Northern Plains:

(i) The Northern Plains get much of their rainfall by Southwest Monsoons which strike the eastern part of the country first and give heavy rainfall there. so eastern parts of the country like Assam, Meghalaya, Bengal etc. get much rainfall.

(ii) Then the monsoons arising from the Bay of Bengal move westwards along the Himalayas. their capacity to cause rain become lesser and lesser as they move westward because they continue to become drier and drier.

Pattern of Rainfall in Peninsular India:

(i) The Peninsular India also gets much rainfall because of another branch of the South-West Monsoons which rises from the Arabian Sea. These monsoon winds first of all strike the Western Ghats and cause mu8ch rain there.

(ii) These winds while reaching the other side of the Western Ghats become dry and cause less rain. As they go on moving to the eastern side they become drier and drier and so the rain goes decreasing from west to east.

Mumbai receives rainfall in summer while Chennai has in Winter :

Mumbai receives more rainfall in summer as it is situated on the Arabian Sea coast, and receives all its rains from the Arabian Sea branch of the southwest monsoons from June to September only. Chennai receives two-third of the rainfall from the retreating southwest monsoons aided by cyclones on October to December.        

Western Rajasthan has desert type of climate:

(i) Western Rajasthan lies in the rain shadow areas of the Aravalli Mountains. (Leeward side).

(ii)  Arabian sea branch of S.W. monsoons blows parallel to the Aravalli range; hence the Aravallis fail to check it.

(ii)  By the time the Bay Bengal branch reaches here it is almost dry, moreover it lies on the leeward side of the Aravallis.

(iii) Monsoon winds become warmer and increase their capacity to hold moisture instead of causing rain.

(D) Retreating Monsoons:

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During October-November, the monsoon trough of low pressure becomes weaker and is gradually replaced by high pressure. The outreach of te monsoon becomes unsustainable and it starts withdrawing gradually. this is known as the retreat of monsoon.

October Heat:

(i) The retreat of the monsoons is marked by clear skies and rise in temperature.

(ii) The land is still moist.

(iii) Owing to the conditions of high temperature and humidity, the weather become oppressive, which is known as October heat in northern India.

Cyclonic Depressions which originate over the Andaman Sea:

In October and November the shift of the low pressure from land to sea is far from smooth. the period is associated with occurrence of cyclonic depressions which  originate over the Andaman Sea.

The cyclones affect the eastern coasts of southern peninsular.

The cyclones affect the Sundarban Delta, Mahanadi, Godavari, Krishna and Kaveri deltas too.

These tropical cyclones cause heavy and widespread rain.

One adverse effect of the cyclones is, it is very often destructive. No year is found disaster free, it affects one or the other deltas of the Eastern Coast.

VARIETIES THE MONSOONS IN INDIA

At times the monsoons come in full swing, or it may fail altogether. Thus, causing the twin problems of floods and famines.

The alternation of dry and wet spells keeps on varying in intensity, frequency and the in duration.

Implications of vagaries of the monsoons:

(i) Due to the late arrival the crops dry up.

(ii)Due to excessive rains floods are caused, leading to destruction.

(iii) The amount of rain may vary causing drought conditions.

(iv) Sometimes the monsoons come early, normal or late.

(iv) The monsoons may retreat early, normal or late.

DISTRIBUTION OF PRECIPITATION IN THE COUNTRY

Areas of Heavy rainfall: Areas which get rainfall of 200 cms and above are Assam, the Ganga Delta, the Western Ghats and the Western Coastal regions and the mountainous regions of Himachal Pradesh.

areas of Moderate Rainfall: Areas which get annual rainfall between 100 cms to 200 cms are Madhya Pradesh, Orissa, Chhota Nagpur, Western Bengal, Bihar, Eastern U.P , North-Eastern Punjab; Eastern parts of Tamil Nadu and Eastern slopes of Western Ghats receive moderate rainfall.

Areas with Low Rainfall: Areas receiving annual rainfall between 50 cms to 100 cms are the Deccan Plateau, Western U.P, South-Eastern Punjab, Eastern Rajasthan and parts of Kashmir get low rainfall.

Areas with Scanty Rainfall: Areas which get less than 50 cms rainfall annually are western Rajasthan, Kutch, Southeastern parts of Haryana, Northeastern Kashmir get scanty rainfall.

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                                                   Figure : Seasonal Rainfall (June-September)

Consequences of the Uneven Precipitation in India :

(i)  If there is too much rain in certain areas they cause floods and havoc all around. many grown up crops,villages, railway lines are washed away resulting in great loss of men and money.

(ii) If there are not sufficient rains even then people are doomed because of drought and hunger. Many people begin to starve and die of hunger. Standing crops dry away thereby bringing doom to the farmers.

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Uneven distribution rainfall is due to:

(A) Relief / Orography                      (B) Wind direction

(C) Location                                      (D) Low pressure axis.

Relief / orography largely govern the distribution of rainfall. For instance, the windward side of the Western Ghats registers a rainfall of 250cms. on the other hand, the leeward side of this ghat is hardly able to receive 50cms.

Again, the heavy rainfall in the northwestern states can be attributed to their hilly ranges and the eastern Himalayas.

Western Rajasthan gets scanty rain because the Arabian Sea branch of the monsoon blows parallel to the Aravallis.

Rainfall in the north decreases from east to west. Kolkata situated near the sea receive about 120 cm, Patna 102cm, Allahabad 91cm, and Delhi 56 cm.

 MONSOON AS A UNIFYING BOND

(i)There is great diversity in the climatic conditions due to location, extent and relief features.

(ii) But these diversities are subdued by the monsoons, which prevails over the whole country.

(iii) This water scarcity is felt all over country.

(iv)Thus the arrival of the monsoon is most welcome; it changes the Indian landscape, gives impetus to agricultural activities, the total life of the Indian people revolves around the monsoons, including festivals.

 IMPORTANT POINTS TO REMEMBER

(a) Mango showers:

(i) Mango showers are pre-monsoon showers.

(ii)This phenomenon is observed in Kerala and coastal Kamataka.

(iii) The mango showers help in the early ripening of, mangoes.

(b) Kalbaisakhi:

Kalbaisakhi means the violent black clouds of the month of Baisakh. This is the name given to the north-westerly and northerly winds in Bengal and Assam. These winds cause very heavy rains and distraction.

(c) Chennai receives more rain in winter:

(i)  The northeast winds pick up moisture while crossing Bay of Bengal. these winds are onshore in the winter season.

(ii) In the summer Chennai lies in the rain-shadow of the Western Ghats, moreover the winds are offshore, therefore receives less rainfall.

(d) Break or burst of the monsoon:

The sudden approach of the moisture laden winds is associated with violent thunder and lightening. This is known as “break” or “burst” of the monsoons. The first ‘break’ of monsoons on the south-west-coast of India is around 1st June.

(e) Southern Oscillation:

The pressure systems of Pacific and Indian Oceans are interrelated. When the pressure is high in the pacific, there is low pressure in the Indian Ocean. The winds move from high pressure to low pressure and vice-versa. This causes shifting of winds across the equator in different seasons. this is known as the southern oscillation.

(f) EI Nino Southern Oscillations :

A feature connected with Southern Oscillation is the EI Nino, 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 EI Nino, the phenomenon is referred to as ENSO (EI Nino Southern Oscillations).

(g) Mawsynram receives the highest rainfall in the world:

(i)  Mawsynram is situated at the head of a funnel shaped valley in the Khasi hills.

(ii) Its unique topographical location together with wind direction is responsible for causing the heaviest rainfall in the world.

(h) The rainfall decreases from South to North :

(i)  The S.W. Monsoons originates from the Indian Ocean and divides into branches due to the shape of the Indian peninsular.

(ii) Trivandrum gets above 200 cm. 

(iii) Whereas Delhi situated in the interior gets only about 50 cm.

(iv) The Arabian Sea branch strikes the Western Ghats and causes heavy rainfall.

(v) Whereas another branch of the Arabian Sea monsoon blows parallel to the Aravallis. thus jodhpur gets less than 25cms of rain.

(i) The Western Ghats receive more rain from the southwest monsoons than the Eastern Ghats because:

(i) The Arabian Sea branch of the monsoon is on shore.

(ii) These winds are forced to rise and cause heavy rains.

(iii) By the time these winds reach the east coast, most of the moisture is lost.

(iv) The winds are offshore, so they given less rain.

(v) Eastern Ghats lies on the leeward/rain-shadow area, hence it gets less rain.

(j) Indian would have been an arid land or desert if there had been no phenomena of monsoons :

(i) Indian receives 75 to 90% of the rainfall from the monsoons.

(ii) These monsoons winds occur due to the uneven heating of land and sea.

(iii) The mighty Himalayas check the two branches of southwest monsoons, the Arabian Sea branch and the Bay of Bengal branch. These, cover the whole of India thus preventing it from becoming a desert.

DIFFERENCE BETWEEN

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