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Notes of Ch 4 Climate| Class 9th Geography
26 Sept, 2018
Study Material and Notes of Ch 4 Climate Class 9th Geography
• 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.
• Elements of Weather and Climate:
→ Temperature
→ Atmospheric pressure
→ Wind
→ Humidity
→ Precipitation
Climatic Controls
• There are six major controls of the climate of any place. They are:
→ Latitude
→ Altitude
→ Pressure and wind system
→ Distance from the sea (continentality)
→ Ocean currents
→ Relief features
Factors affecting India's Climate
→ Latitude
→ Altitude
→ Pressure and Winds
Latitude
• The Tropic of Cancer passes through the middle of the country. Half of the
country lying south of the Tropic of Cancer, belongs to the tropical area.
Altitude
• The Himalayas prevent the cold winds from Central Asia from entering the
subcontinent. making winter milder as compared to central Asia.
Pressure and Winds
• Atmospheric conditions that govern climate and associated weather conditions
in India are:
→ Pressure and surface winds
→ Upper air circulation
→ Western cyclonic disturbances and tropical cyclones.
Upper Air Circulation
• The higher level of the atmosphere is dominated by a westerly flow. An
important component of this flow is the jet stream.
• Jet Streams are a narrow belt of high altitude westerly winds in the troposphere
(transition between troposphere and stratosphere.
Western cyclonic disturbances and tropical cyclones
• The western 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.
• Tropical cyclones originate over the Bay of Bengal and the Indian ocean.
• Tropical cyclones occur during the monsoon as well as in October-November,
and are part of the easterly flow.
The Indian Monsoon
• Monsoon are seasonal winds which reverse their direction of flow with the
change of season.
• The climate of India is strongly influenced by monsoon winds.
Factors that affect the mechanism of Indian Monsoon
• Differential heating and cooling of land and water create low pressure on land
while the sea experiences high pressure.
• Shifting of Inter Tropical Convergence Zone (ITCZ) over the Ganga plain during
summers.
• The intensity and position of high-pressure area towards the east of
Madagascar approximately at 20°S over the Indian Ocean affects the Indian
Monsoon. affects the Indian Monsoon.
• The heating up of the Tibetan plateau in summers creates low pressure above
the plateau.
• 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.
• Southern Oscillation.
The Onset of the Monsoon and Withdrawal
• The duration of the monsoon is between 100-120 days from early June to mid-
September.
• When the monsoon arrives the normal rainfall increases suddenly and
continues constantly for several days. This is known as the ‘burst’ of the
monsoon.
• 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 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.
• By December, the monsoons retreat completely from the rest of India.
The Seasons
• Four main seasons can be identified in India
→ the cold weather season
→ the hot weather season
→ the advancing monsoon
→ the retreating monsoon
The Cold Weather Season (Winter)
• The season begins from mid-November and stays till February in northern
India.
• Coldest months are December and January in the northern part of India.
The Hot Weather Season (Summer)
• The hot weather season in India ranges from March to May.
• Temperatures upto 48 degree Celsius are experienced in North India.
• In peninsular India, temperatures remain lower due to the moderating
influence of the oceans.
Advancing Monsoon (The Rainy Season)
• By early June, the trade winds of the southern hemisphere get attracted to the
northen hemisphere due to the creation of low-pressure condition there.
• Rainfall in the Ganga valley decreases from the east to the west.
• Rajasthan and parts of Gujarat get scanty rainfall.
Retreating/Post Monsoons (The Transition Season)
• During October-November, the movement of the sun towards South causes the
monsoon trough in the Northern Plains to become weaker.
• By early November, the low-pressure conditions, over north-western India, get
transferred to the Bay of Bengal which cause cycloninc depressions.
Monsoon as a Unifying Bond
• There is great diversity in the climatic conditions due to location, extent and
relief features.
• These monsoon winds bind the whole country by providing water to set the
agricultural activities in motion.
