Chapter 4. Climate

Climate: It refers to sum total of weather conditions and variations over a large area for a long period of time [more than thirty years].
Weather: It refers to state of atmosphere over an area at any point of time.
• elements of weather and climate are same, i.e. temperature, atmospheric pressure, wind, humidity & precipitation. On basis of monthly atmospheric conditions, year is divided into seasons such as:
(1) Winter
(2) Summer
(3) Rainy season.

Climatic Controls
• climate of any place depends on following factors:
(1) Latitude: amount of solar energy received varies according to latitude due to curvature of earth. That’s why air temperature usually decreases from equator towards poles.
(2) Altitude: atmosphere becomes less dense and temperature decreases when we go to higher altitude from Earth’s surface. It is reason why hills are cooler during summers.
(3) Pressure and Wind System: pressure and wind system of any area depends on latitude and altitude of place. It influences temperature and rainfall pattern.
(4) Distance from Sea: sea exerts a moderating influence on climate. As distance from sea increases, its moderating influence decreases and people experience extreme weather conditions. This condition is called continentality i.e., very hot during summers and very cold during winters.
(5) Ocean Currents: Ocean currents along with onshore winds affect climate of coastal areas. For example, any coastal area with warm or cold currents flowing past it, will be warmed or cooled if winds are onshore.
(6) Relief Features: Relief plays a major role in determining climate of a place. High mountains act as barriers for cold or hot winds. They may cause precipitation if they are high enough and lie in path of rain-bearing winds. Precipitation is any form of moisture that falls to earth. It includes rain, snow, hail & sleet.

Factors Affecting India’s Climate Latitude
• Tropic of Cancer passes through middle of country from Rann of Kachchh in west to Mizoram in east. India’s climate has characteristics of tropical as well as subtropical climates.

• India has mountains to north and has a vast coastal area where maximum elevation is about 30 metres. Owing to mountains, subcontinent experiences comparatively milder winters as compared to central Asia.

Pressure and Wind
• climate and weather conditions in India are governed by following atmospheric conditions:
(1) Pressure and surface winds.
(2) Upper air circulation.
(3) Western cyclonic disturbances and tropical cyclones.
• Air moves from high-pressure area over southern Indian Ocean, in a south-easterly direction, crosses equator and turn right towards lowpressure areas over Indian subcontinent. These are called Southwest Monsoon winds. These winds blow over warm oceans, gather moisture and bring widespread rainfall over territories of India.
• Jet streams are fast flowing, narrow, meandering air currents in atmosphere.

Indian Monsoon
• climate of India is strongly influenced by monsoon winds.
• seasonal reversal of Wind system is called ‘monsoon’. monsoon is experienced in tropical area roughly between 20° N and 20° S. Go through following facts to understand mechanism of monsoons in a better way:
(1) differential heating and cooling of land and water create low pressure on landmass of India while seas around experience comparatively high pressure.
(2) shift of position of Inter-Tropical Convergence Zone [ITCZ] in summer, over Ganga plain, is called monsoontrough during monsoon season.
(3) presence of high-pressure area, east of Madagascar, approximately at 20°S over Indian Ocean affects Indian Monsoon.
(4) Tibetan plateau gets intensely heated during summer, which results in strong vertical air currents and formation of low pressure over plateau at about 9 km above sea level.
(5) movement of westerly jet stream to north of Himalayas and presence of tropical easterly jet stream over Indian peninsula during summer impact Indian Monsoon.
• changes in pressure conditions over southern oceans affect monsoons. irregularly periodic variation in winds and sea surface temperatures over tropical eastern Pacific Ocean that affects climate of tropics and subtropics is called Southern Oscillation or SO.

Onset of Monsoon and Withdrawal
• duration of monsoon is between 100-120 days from early June to mid-September. Around time of its arrival, normal rainfall increases suddenly and continues constantly for several days, which is called ‘burst’ of monsoon.

Arrival of Monsoon In Different Parts of India
• monsoon arrives at southern tip of Indian peninsula usually by 1st week of June. Subsequently, it proceeds into two – Arabian Sea branch and Bay of Bengal branch.
• Arabian Sea branch reaches Mumbai on approximately 10th of June.
• Bay of Bengal branch arrives in Assam in 1st week of June.
• By mid-June, Arabian Sea branch of monsoon arrives over Saurashtra-Kuchchh and central part of country.
• Arabian Sea and Bay of Bengal branches of monsoon merge over northwestern part of Ganga plains.
• Delhi receives monsoon showers from Bay of Bengal branch by end of June.
• By first week of July, western Uttar Pradesh, Punjab, Haryana & eastern Rajasthan experience monsoon.
• By mid-July, monsoon reaches Himachal Pradesh and rest of country.

Withdrawal of Monsoon
• withdrawal of monsoon begins in northwestern states of India by early September. By mid-October, it withdraws completely from northern half of peninsula. From north to south, withdrawal of monsoon takes place from first week of December to first week of January. By early December, monsoon has withdrawn from rest of country.

• 4 main seasons can be identified in India:
(1) cold weather season [Winter].
(2) hot weather season [Summer].
(3) advancing monsoon [Rainy Season].
(4) retreating monsoon with some regional variations [Transition Season].

Cold Weather Season [Winter]
• Winter begins from mid-November in northern India and stays till February.
• December and January are coldest months in northern part of India.
• temperature decreases from south to north.
• Days are warm and nights are cold.
• weather is marked by clear sky, low temperatures and low humidity and feeble, variable winds.
• This season is extremely important for cultivation of ‘the Rabi’ crops.

Hot Weather Season [Summer]
• From March to May, hot weather season is observed in India. summer months experience rising temperature and falling air pressure in northern parts of country. A striking feature of hot weather season is ‘loo’. Loo is a strong, gusty, hot, dry winds blowing during day over north and northwestern India.
• Towards end of summer season, pre-monsoon showers come which help in early ripening of mangoes, and are often referred to as ‘mango showers’.

Advancing Monsoon [Rainy Season]
• South-east trade winds originate over warm subtropical areas of southern oceans. They cross equator and blow in a southwesterly direction entering Indian peninsula as south-west monsoon. monsoon winds cover country in about a month. Mawsynram in southern ranges of Khasi Hills receives highest average rainfall in world.
• A phenomenon associated with monsoon is its tendency to have ‘breaks’ in rainfall. These breaks in monsoon are related to movement of monsoon trough. trough and its axis keep on moving northward or southward and determine spatial distribution of rainfall. Frequency and intensity of tropical depressions determine amount and duration of monsoon rains. monsoon is known for its uncertainties. alternation of dry and wet spells varies in intensity, frequency & duration.

Retreating/ Post Monsoon [Transition Season]
• months of October-November form a period of transition from hot rainy season to dry winter conditions. retreat of monsoon is marked by clear skies and a rise in temperature. While day temperatures are high, nights are cool and pleasant. Owing to conditions of high temperature and humidity, weather becomes rather oppressive during day. It is commonly called ‘October heat’. In second half of October, mercury begins to fall rapidly in northern India.

Monsoon as a Unifying Bond
• This is clear that monsoon brings people together in Indian subcontinent. rhythm of seasons is caused by way wind systems change with seasons and how that affects weather. Monsoons tend to have unpredictable rain and rain that falls in different places. Everyone in India, from north to south and from east to west, looks forward to monsoon every year. These monsoon winds tie whole country together by bringing rain to start farming.

Distribution of Rainfall
• Parts of western coast and northeastern India receive over about 400 cm of rainfall annually.
• Rainfall is less than 60 cm in western Rajasthan and adjoining parts of Gujarat, Haryana & Punjab.
• Rainfall is low in interior of Deccan plateau, and east of Sahyadris.
• Snowfall is restricted to Himalayan region.
• annual rainfall is highly variable from year to year.

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