Detailed Chapter Notes - (Part-1) - Climate, SST, CBSE Class 9 | EduRev Notes

Social Studies (SST) Class 9

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Class 9 : Detailed Chapter Notes - (Part-1) - Climate, SST, CBSE Class 9 | EduRev Notes

The document Detailed Chapter Notes - (Part-1) - Climate, SST, CBSE Class 9 | EduRev Notes is a part of the Class 9 Course Social Studies (SST) Class 9.
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Climate
Weather and Climate

Weather

Climate

(i) Weather is the state atmosphere at any point in time and space, it changes every moment.

(i) Climate refers to the sum total of weather conditions and variations over a large area for a long period of time. it is the sum total of average weather conditions of 30 years. 

(ii) Elements of weather are temperature, atmospheric pressure, wind, humidity, and precipitation.

(ii) Elements of climate are the same as those of water.

(iii)Example: Cloudy, dry, windy, wet weather.

(iii) Example: Monsoon, equatorial desert, cold climate etc.

(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.

Detailed Chapter Notes - (Part-1) - Climate, SST, CBSE Class 9 | EduRev Notes

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, the temperature at Drass in Jammu and Kashmir may be as low minus 45o C. Thiruvananthapuram, 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 from 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 is 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 moderating influences on climate: As the distance from 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 the mountains remains dry.

Factors Affecting India’s Climate

  • Latitude: Indian is situated roughly between 8o N and  37o N latitudes. India is divided into 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.
  • 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.
  • 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, 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 the sudden outbreak of monsoons in Northern India. a subtropical westerly jet stream brings 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.

  1. 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.
  2. 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).
  3. 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 affect the Indian Monsoon.
  4. 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.
  5. 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.
  6. 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 (the Pacific Ocean, 18o S/149o W) and Darwin in northern Australia (the 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.

 Detailed Chapter Notes - (Part-1) - Climate, SST, CBSE Class 9 | EduRev NotesDetailed Chapter Notes - (Part-1) - Climate, SST, CBSE Class 9 | EduRev Notes

The Monet 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 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 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-Kachchh 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 reasons can be identified in India –

  1. Cold weather season  --  December to February
  2. Hot weather season  --  March to May
  3. Advancing monsoon season  --  June to September
  4. Retreating monsoon season  -- October and November

(A) The Cold Weather Season (Winter):

  1. The cold weather season begins from mid-November in India and stays till February.
  2. December and January are the coldest months in the northern part of India. The temperature decreases as one move from the south to the north.
  3. Days are warm and the nights are cold. Frost is common in the north and the higher slopes of the Himalayas experience snowfall.
  4. The northeast trade winds prevail over the country. They blow from land to sea and hence, for the most part of the country, it is a dry season.
  5. In the northern part of the country, a feeble high-pressure region develops, with light winds moving outwards from this area.
  6. The weather is normally marked by clear sky, low temperatures, and low humidity and feeble variable winds.
  7. 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 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.
  8. The northeast trade winds cause a fair amount of rainfall in Chennai or the Coromandel Cast in winter.

(b) Hot weather season (Summer):

  1. 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.
  2. Temperature increases from south to north. In peninsular India, temperatures remain lower
  3. High temperature between 38o C and 48C in the plains.
  4. Local dust storms accompanied by light rains.
  5. Hot dry winds, ‘loo’is common in May and June.
  6. Kerala and Karnataka coast receivers pre-monsoon showers. (Mango showers)
  7. West Bengal and Assam are affected by northwesterly winds. (Kalbaisakhi).

(c) Advancing Monsoon (The Rainy Season):

Detailed Chapter Notes - (Part-1) - Climate, SST, CBSE Class 9 | EduRev Notes

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.

Detailed Chapter Notes - (Part-1) - Climate, SST, CBSE Class 9 | EduRev Notes

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