CLASS 9 GEOGRAPHY LESSON 4 - CLIMATE

CLIMATE

What is Climate & Weather?

Climate refers to the sum total of weather conditions and variation 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.

The elements of weather and climate are the same. ie; temperature, atmospheric pressure, wind, humidity and precipitation.

CLIMATE OF INDIA

•      The climate of India is described as the ‘monsoon’ type.

•      There are perceptible (noticeable) regional variations in climatic conditions within the country.

•      For eg: In summer, the mercury occasionally touches 50 degree C in some parts of Raj. Desert, where as it may be around 20 degree C in Pahalgam in Jammu & Kashmir.

•      On a winter night, temperature at Drass in Jammu and Kashmir may be as low as minus 45 degree C, where as in Tvpm may have a temperature of 22 degree C.

CLIMATIC CONTROLS

•      There are 6 major controls of the climate of any place.

•      They are latitude, altitude, pressure and wind system, distance from the sea, ocean currents and relief features (mountains and hills).

•      Latitude:- Due to the curvature of the earth, the amount of solar energy received varies according to latitude. As a result, air temperature generally decreases from the equator towards the poles.

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

•      Pressure and wind system:- The pressure and wind system of any area depend on the latitude and altitude of the place. Thus it influences the temperature and rainfall pattern.

•      Distance from the sea:- As the distance from the sea increases, its moderating influence decreases and the people experience extreme weather conditions. This condition is known as continentality (ie.very hot in summers and very cold during winters)

•      Ocean currents:- Ocean currents (rivers of hot or cold water that flow within the oceans) along with onshore winds affect the climate of the coastal areas. For eg. any coastal area with warm or cold currents flowing past it, will be warmed or cooled if the winds are onshore.

•      Relief features:- High mountains act as barriers for cold or hot winds; they may also cause precipitation if they are high enough and lie in the path of rain-bearing winds.

FACTORS AFFECTING INDIA’S CLIMATE

•      u Latitude

•      As latitude increases, the intensity of solar energy decreases.

•      The tropical zone is between 23.5^o north (the tropic of Cancer) and 23.5^o south (the tropic of Capricorn) of the equator. The sun’s rays are most intense and the temperatures are always warm.

•      The temperate zones are between 23.5^o and 66.5^o north and between 23.5^o and 66.5^o south of the equator. The sun’s rays strike Earth at a smaller angle than near the equator.

•      The Tropic of Cancer passes through the middle of India from Rann of Kuchch (West) to Mizoram (East). Hence regions South of Tropic of Cancer experiences tropical climate and those towards its North experiences subtropical climate.

•      So India’s climate has the features of tropical as well as subtropical climates.

•      u Altitude

•      As the temperature decreases with increase in altitude.

•      In India, the mountains in the north have an average height of 6000 metres that prevents the cold winds from Central Asia from entering India, thus giving the country milder winters as compared to central Asia.

•      u Pressure and Winds

•      The climate & associated weather conditions in India are governed by Pressure and surface winds, Upper air circulation and Western cyclonic disturbances and tropical cyclones.

•      In winters, there is a high pressure area north of the Himalayas. Cold dry winds blow from this region to the low pressure areas over the oceans to the south.

•      •  In summers, a low pressure area develops over interior Asia as well as over northwestern India. Due to this, the reversal of the direction of winds during summers helps the wind to move from the high pressure areas over the Indian Ocean towards the low pressure areas. These are called South West Monsoon winds. These winds blow over the warm oceans, gather moisture and bring abundant rainfall to the Indian Subcontinent.

THE INDIAN MONSOON

•      Factors that affect the mechanism of Indian Monsoon

•      Differential heating & cooling of land & water leads to the creation of low pressure on land while the sea experience high pressure.

. Shifting of Inter Tropical Convergence Zone (ITCZ) over the Ganga plain during summers.

. The position and concentration of the high-pressure area towards the east of Madagascar affects the Indian monsoon.

. The heating up of the Tibetan plateau in summers leads to the creation of low pressure above the plateau.

. The shifting of the westerly jet stream towards North of the Himalayas and the presence of tropical easterly jet stream in the Indian peninsula during summer.

ONSET OF MONSOON AND WITHDRAWL

Duration of monsoon: 100-120 days (early June-mid September)

. The normal rainfall increases suddenly and continues constantly for several days. This is known as ‘burst’ of the monsoon.

. Branches of monsoon: the Arabian Sea branch and the Bay of Bengal branch.

. The Arabian Sea branch arrives in Mumbai roughly around 10^th June and the Bay of Bengal branch arrives in Assam around the first week of June.

. The lofty (high) mountains divert the monsoon winds towards the West over the Ganga plains. Then it reaches Saurashtra-Kuchch and Central India during mid-June.

. Delhi receives showers from the Bay of Bengal branch roughly by 29^th June and Punjab, Haryana, Eastern Raj & Western U.P receive the monsoons by the first week of July. By mid-July, it reaches Himachal Pradesh and the rest of the country.

•      The withdrawal of monsoon begins in northwestern states of India by early Sept.

•      By mid-Oct, it withdraws completely from the northern half of the peninsula. The withdrawal from the southern half of the peninsula is fairly rapid.

•      By early Dec, the monsoon has withdrawn from the rest of the country.

•      The islands receive the very first monsoon showers, progressively from south to north, from the last week of April to the first week of May. The withdrawal takes place progressively from north to south from the first week of Dec to the first week of Jan.

THE SEASONS

•      The Cold Weather Season (Winter)

Ø Begins from mid-Nov & stays till Feb in Northern India.

Ø Coldest months are Dec and Jan.

Ø Temperature –  of Chennai is between 24-25 degree C, it ranges between 10 to 15 degree C in the northern plains.

Ø Days are warm and nights are cold.

Ø The Himalayan slopes experience snowfall.

Ø TN coast receive some amount of rainfall due to northeast trade winds.

Ø The northern Plains experience cyclonic disturbances brought in from West and the Northwest directions. They begin in the Mediterranean Sea and West Asia and proceeds towards India. They cause rains in the plains and snowfall in the mountains.

Ø The total amount of winter rainfall locally known as ‘mahawat’ is small; they are of immense importance for the cultivation of ‘rabi’ crops.

•      The Hot Weather Season (Summer)

Ø March to May is the hot weather season in India.

Ø Temperature upto 48 degree Celsius experience in North India.

Ø However, Peninsular India experiences lower temperatures because of the influence of the ocean.

Ø A striking feature of the summer season is ‘loo’. They are strong, hot and dry wind that blows during the day in North and Northwestern regions. It originates from the deserts of Rajasthan and carries sands to the alluvial plains of Ganga.

Ø During May, dust storms are common in North India and they at times are of great relief, as they help in reducing the temperature and bringing light showers. In West Bengal these storms are known as the ‘Kaal Baisakhi’.

Ø Towards the end of the season, Kerala and Karnataka receive pre-monsoon showers, often called ‘mango showers’ as it helps in the early ripening of mangoes.

•      Advancing Monsoons (Rainy Season)

Ø A complete change in weather is experienced after the inflow of the Southwest monsoons into India.

Ø The windward side of the Western Ghats receive more than 250 cm. of heavy rainfall.

Ø The highest Avg. rainfall in the world received by Mawsynram, lying in the Southern range of Khasi Hills in Meghalaya.

Ø  Guj.and Raj.receive scanty rainfall.

Ø The monsoon rain has its tendency to have ‘breaks’ ie.occur in intervals. It has wet and dry spells. This dry and wet spells of the monsoons vary in strength, frequency and duration causing floods and droughts at the same time in different regions. As the monsoons are often irregular, they can cause damage to the agricultural crops.

Ø Retreating/Post Monsoons (The Transition Season)

Ø From Oct. to Nov., the movement of the sun towards South causes the monsoon trough in the Northern Plains to become weaker.

. The South west monsoon winds become weak and by October they withdraw completely from the Northern Plains.

. The transition from the warm rainy season to dry winters happens in October and November. It is commonly called ‘October Heat’ because of the rise in temperature and humidity.

. Cyclonic depressions occur over the Andaman Sea due to the shifting of low-pressure conditions from the Northwest to the Bay of Bengal. The cyclones are destructive and cause damage to life and property.

DISTRIBUTION OF RAINFALL

The western coast and Northeastern India receives approximately 400 cm of rainfall annually.

Ø Western Raj, Guj, Hary. and Punj. Receive 60 cm rainfall annually.

Ø Due to trend of monsoons, the system of rainfall is often different from year to year. While some areas get affected by floods due to heavy rainfall, other areas may experience droughts due to the lack of it.

Ø Monsoon as a unifying Bond

. Uncertain rainfalls and unevenness are the characteristics of the Indian monsoons. People in the North, East, West and South keenly wait for the monsoons for starting their agricultural activities.