Chapter 4. Climate

Unity and Diversity in Monsoon Climate
• The climate of India is not same everywhere, it differs from place to place.
• India has a monsoon type of climate.
• The climate of India has many regional variations expressed in pattern of winds, temperature and rainfall, rhythm of seasons and degree of wetness or dryness.
• There are regional variations in precipitation also.
• Variations are noticeable not only in type of precipitation but in its amount.
• While Cherrapunji and Mawsynram receive rainfall of over 1,080 cm in a year, Jaisalmer in Rajasthan rarely gets more than 9 cm of rainfall during same period.
• The Ganga delta and coastal plains of Odisha are hit by strong rain-bearing storms almost every third or fifth day in July and August.
• The Coromandel coast in south goes usually dry during monsoon months. Most parts of country get rainfall during June-September.
• The coastal areas of Tamil Nadu receive rain at beginning of winter season.

Factors Determining Climate of India Factors Related to Location and Relief
• Latitude is an important factor in determining climate of India.
• The Tropic of Cancer passes through central part of India in an east-west direction.
• The northern part of India lies in sub-tropical and temperate zone and part lying in south of Tropic of Cancer falls in tropical zone.
• The tropical zone being nearer to equator experiences high temperatures throughout year with a small daily and annual range of temperature.
• The northern area of Tropic of Cancer being away from equator experiences an extreme climate with a high daily and annual range of temperature.
• The Himalayan Mountains play an important role in determining climate of India.
• The lofty Himalayas in north along with their extensions act as an effective climatic divide.
• Distribution of land and water is another factor related to location and relief.
• This differential heating of land and sea creates different air pressure zones in different seasons in and around Indian subcontinent.
• Distance from sea affects climate of India.
• With a long coastline, large coastal areas have an equable climate.
• Areas in interior of India are far away from moderating influence of sea. Such areas have extremes of climate.
• Temperature decreases with height.
• The physiography or relief of India affects temperature, air pressure, direction & speed of wind and amount and distribution of rainfall.

Factors Related to Air Pressure and Wind
• The mechanism of air pressure and wind plays an important role in determining climate of India.
• Upper air circulation is caused by factors that control global weather and inflow of different air masses and jet streams.

Mechanism of weather in winter season and mechanism of weather in summer season
• Western disturbances create weather conditions favourable to rainfall in winter season.
• In winter months, weather condition over India is usually influenced by distribution of pressure in Central and Western Asia.
• A high-pressure centre in region lying to north of Himalayas develops during winter.
• This centre of high pressure gives rise to flow of air at low level from north towards Indian subcontinent, south of Himalayan mountain range.
• The pattern of air circulation discussed above is witnessed only at lower level of atmosphere near surface of earth.
• Winds blowing across Asian continent at latitudes north of Himalayas roughly parallel to Tibetan highlands are called jet streams.
• The western cyclonic disturbances which enter Indian subcontinent from west and northwest during winter months, originate over Mediterranean Sea and are brought into India by westerly jet stream.
• An increase in prevailing night temperature usually indicates an advance in arrival of these cyclonic disturbances.
• Tropical cyclones originate over Bay of Bengal and Indian ocean.
• In summer season, sun shifts northwards, and wind circulation over subcontinent undergoes a complete reversal at both, lower as well as upper levels.
• The maritime tropical air mass from southern hemisphere, after crossing equator, rushes to low-pressure area in general southwesterly direction. It is this moist air current that is popularly called southwest monsoon.
• The pattern of pressure and winds is formed only at level of troposphere. An easterly jet stream flows over southern part of peninsula in June.
• The easterly jet stream steers tropical depressions into India. These depressions play a significant role in distribution of monsoon rainfall over Indian subcontinent.

The Nature of Indian Monsoon Onset of monsoon, rain bearing system and rainfall distribution, break in monsoon
• A single theory is there which explains nature and causes of monsoon.
• Systematic studies of causes of rainfall in South Asian region help to understand causes and salient features of monsoon, particularly some of its important aspects, like onset of monsoon, rain-bearing systems, and breaks-in monsoon, etc.
• It is believed that differential heating of land and sea during summer months is mechanism that sets stage for monsoon winds to drift towards subcontinent.
• The southwest monsoon sets in over Kerala coast by 1st of June and moves swiftly to reach Mumbai and Kolkata between 10th and 13th June. By mid-July, southwest monsoon engulfs entire subcontinent.
• Bay of Bengal Branch and Arabian Sea Branch are two rain-bearing branches of monsoon in India.
• The Bay of Bengal Branch causes rain over plains of northern India.
• The Arabian Sea Branch brings rain to west coast of India.
• Much of rainfall along Western Ghats is orographic as moist air is obstructed and forced to rise along Ghats.
• The intensity of rainfall over west coast of India depends on offshore meteorological conditions and position of equatorial jet system along eastern coast of Africa.
• EI-Nino is a complex weather system that appears once every three to seven years, bringing drought, floods & other weather extremes to different parts of world.
• EI-Nino is used in India for forecasting long-range monsoon rainfall.
• Breaks-in monsoons are dry spells when there is no rain for a few days.

The Rhythm of Seasons
• The climatic conditions of India can best be described in terms of an annual cycle of seasons.
• The meteorologists recognise four seasons- cold-weather season, hot weather season, southwest monsoon season and retreating monsoon season.
• Usually, cold weather season sets in by midNovember in northern India.
• December and January are coldest months in northern plain.
• The Peninsular region of India, however, does not have any well-defined cold-weather season.
• Winter monsoons do not cause rainfall as they move from land to sea. It is because firstly, they have little humidity; and secondly, due to anticyclonic circulation on land, possibility of rainfall from them reduces.
• With apparent northward movement of sun towards Tropic of Cancer in March, temperatures start rising in northern India. April, May & June are months of summer in northern India.
• The hot weather season in southern India is mild and not so intense as of in northern India.
• The Peninsular situation of southern India with moderating effect of oceans keeps temperature lower than that prevailing in northern India.
• The summer months are a period of excessive heat and falling air pressure in northern half of country.
• During summer, in heart of ITCZ (Inter Tropical Convergence Zone) in northwest, dry & hot winds are called ‘Loo’, that blow in afternoon, and very often, they continue to well into midnight.
• Dust storms in evening are very common during May in Punjab, Haryana, Eastern Rajasthan and Uttar Pradesh.
• These temporary storms bring a welcome respite from oppressing heat since they bring with them light rains and a pleasant cool breeze.
• Mango Shower, Blossom Shower, Nor Westers, and loo are some famous local storms of hot weather season.

The Southwest Monsoon Season
• In India, Southwest Monsoons are caused by trade winds of Southern Hemisphere coming from Indian Ocean.
• After crossing equator, these trade winds follow a southwesterly direction and are called southwest monsoons.
• The monsoon winds originating over Arabian Sea further split into three branches.
• The Bay of Bengal branch strikes coast of Myanmar and part of southeast Bangladesh.
• Rainfall received from southwest monsoons is seasonal.
• Monsoonal rainfall is largely governed by relief or topography.
• Monsoon play a pivotal role in agrarian economy of India because over three-fourths of total rain in country is received during southwest monsoon season.
• The months of October and November are known for retreating monsoons.
• The retreating southwest monsoon season is marked by clear skies and a temperature rise.
• The weather in retreating monsoon is dry in north India but it is associated with rain in eastern part of Peninsula.

Traditional Indian Seasons
• In Indian tradition, a year is divided into six twomonthly seasons.
• This cycle of seasons, which common people in north and central India follow is based on their practical experience and age-old perception of weather phenomena.
• The average annual rainfall in India is about 125 cm, but it has great spatial variations.
• Areas are divided into areas of high rainfall, areas of medium rainfall, areas of low rainfall, and areas of inadequate rainfall.
• A characteristic feature of rainfall in India is its variability.
• Variations represent subtypes of monsoon climate and on this basis, climatic regions of India can be identified.
• A climatic region has a homogeneous climatic condition which is result of a combination of factors.
• Temperature and rainfall are two important elements that are considered to be decisive in all schemes of climatic classification.
• Koeppen’s classification of climate is based on monthly values of temperature and precipitation.
• Koeppen identified five major climatic types.
• Koeppen used letter symbols to denote climatic types. The agricultural prosperity of India depends very much on time and adequately distributed rainfall.
• Regional climatic variation in India is reflected in vast variety of food, clothes, & house types.

Global Warming
• Climate has witnessed a change in past at global as well as local levels.
• Many geological pieces of evidence suggest that in past large part of earth was under ice cover.
• Besides natural causes, human activities such as large-scale industrialization and presence of polluting gas in atmosphere are important factors responsible for global warming.
• The temperature of world is significantly increasing. Carbon dioxide produced by human activities is a major source of concern.
• Other gases like methane, chlorofluorocarbons, and nitrous oxide which are present in much smaller concentrations in atmosphere, together with carbon dioxide are called greenhouse gases.
• The mean annual surface temperature of earth in past 150 years has significantly increased.
• It is projected that by year 2100 global temperature will increase by about 2°C.

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