Chapter Notes and Summary
• Climate It refers to sum total of weather conditions and variations over a large area for a long period of time (more than 30 years).
• Weather It refers to state of atmosphere over an area at any point of time.
1. elements of weather and climate are same i.e., temperature, atmospheric pressure, wind, humidity and precipitation.
2. climate of India is described as ‘monsoon’ type. In Asia, this type of climate is found mainly in south and south-east.
3. word monsoon is derived from Arabic word
‘mausim’, which literally means season.
4. Monsoon refers to seasonal reversal in wind direction during a year.
5. Variations in climatic conditions are found in our country especially relating to two important elements–temperature and precipitation. They vary from place to place, and season to season.
1. There are variations not only in form and types of precipitation but also in its amount and seasonal distribution.
2. Precipitation is mostly in form of snowfall in upper parts of Himalayas and rains over rest of country.
3. annual precipitation varies from over 400 cm in Meghalaya to less than 10 cm in Ladakh and western Rajasthan.
4. Most parts of country receive rainfall from June to September. However, some parts like Tamil Nadu coast get most of its rain during October and November.
5. In general, coastal areas experience less contrasts in temperature conditions. Seasonal contrasts are more in interior of country.
• Climatic Controls There are six major controls of climate of any place. They are latitude, altitude, pressure and wind system, distance from sea (continentality), ocean currents and relief features.
1. Due to curvature of Earth, amount of solar energy received varies according to latitude.
2. Air temperature generally decreases from equator towards poles.
3. Tropic of Cancer passes through middle of country from Rann of Kachchh in west to Mizoram in east.
4. Almost half of country lying south of Tropic of Cancer belongs to tropical areas. All remaining area north of this lies in sub-tropics.
5. Therefore, India’s climate has characteristics of tropical as well as subtropical climates.
1. As one goes from surface of Earth to higher altitudes, atmosphere becomes less dense and temperature decreases. hills are therefore cooler during summers.
2. India has mountains to north, which have an average height of about 6000 metres. elevation in coastal areas is a maximum of 30 metres.
3. Himalayas prevent entry of cold winds from Centrol Asia. It is because of these mountains that this sub-continent experiences comparatively milder winters as compared to Central Asia.
• Pressure and Winds
1. pressure and wind system of any area depend on latitude and altitude of place.
2. These influence temperature and rainfall pattern.
3. climate and associated weather conditions in India are governed by following atmosphere conditions
4. Pressure and surface winds.
5. Upper air circulation.
6. Western cyclonic disturbances and tropical cyclones.
7. pressure and wind conditions over India are unique.
During winter, there is a high pressure area north of Himalayas.
8. Cold dry winds blow from this region to low pressure areas over oceans to south. In summer, a low pressure area develops over interior Asia as well as over north-western India. This causes a complete reversal of direction of winds during summer.
9. Air moves from high pressure area over southern Indian ocean, in a south easterly direction, crosses equator and turns right toward, low-pressure areas over Indian subcontinent. These are known as Southwest Monsoon winds.
10. These winds blow over warm oceans, gather moisture and bring widespread rainfall over mainland of India lasting for 100-120 days.
• Coriolis Force This is an apparent force caused by Earth’s rotation. Coriolis force is responsible for deflecting winds towards right in northern hemisphere and towards left in southern hemisphere. This is known as Ferrel’s law.
• Jet Stream
1. There are a narrow belt of high altitude (above 12,000 m)
westerly winds in troposphere.
2. Their speed varies from about 110 km/h in summer to about 184 km/h in winter.
3. A number of separate jet streams have been identified.
4. most constant are mid latitude and sub-tropical jet stream.
5. jet streams are located approximately over 27° – 30°
north latitude and therefore, they are known as subtropical westerly jet streams.
6. In summer, sub-tropical westerly jet stream moves north of Himalayas with apparent movement of Sun.
7. An easterly jet stream called sub-tropical easterly jet stream blows over peninsular India approximately over 14°
N during summer months.
• Western Cyclonic Disturbances
1. western cyclonic disturbances are weather phenomena of winter brought in by westerly flow from Mediterranean region.
2. They usually influence weather of north and north-western regions of India.
3. Tropical cyclones occur during monsoon as well as in October-November and are part of easterly flow. They affect coastal regions of country.
4. factors influencing mechanism of monsoons are
(a) differential heating and cooling of land and water creates low pressure on landmass of India while seas around experience comparatively high pressure.
(b) shift of position of Inter Tropical Convergence Zone (ITCZ) in summer over Ganga plains. This is equatorial through normally positioned about 5°N of equator. It is also known as monsoon trough during monsoon season.
(c) presence of high pressure area east of Madagascar approximately at 20°S over Indian ocean. intensity and position of high pressure area affects Indian monsoon.
(d) 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.
(e) movement of westerly jet stream to north of Himalayas and presence of tropical easterly jet stream over Indian peninsula during summer.
• Southern Oscillation (SO)
1. Normally, when tropical eastern south Pacific Ocean experiences high pressure, tropical eastern Indian ocean experiences low pressure. But in certain years, there is a reversal in pressure in comparison to eastern Indian ocean. This periodic change in pressure conditions is known Southern Oscillation (SO).
• El Nino
1. A feature connected with Southern Oscillation (SO) is El Nino event during which a warm ocean current that flows past peruvian coast in place of cold peruvian current every 2 to 5 years. change in pressure conditions is connected to El Nino. Hence, phenomenon is referred to as ENSO (El Nino Southern Oscillations).
2. El Nino is a Spanish word meaning ‘the child’ and refers to baby Christ, as this current starts flowing during Christmas.
3. presence of El Nino leads to an increase in sea surface temperatures and weakening of trade winds in region.
• Onset of Monsoon
1. monsoon winds, unlike trades are not steady winds but are pulsating in nature and are affected by different atmospheric conditions encountered by them on their way over warm tropical seas.
2. duration of monsoon is between 100-120 days from early June to mid September.
3. Around time of its arrival, normal rainfall increases suddenly and continues constantly for several days. This is known as ‘burst’ of monsoon and can be distinguished from premonsoon showers.
4. monsoon arrives at southern tip of Indian peninsula generally by first week of June.
Subsequently, it divides into two, Arabian Sea branch and Bay of Bengal branch.
5. Arabian Sea branch reaches Mumbai about ten days later on approximately 10th June.
6. This is a fairly rapid advance.
7. Bay of Bengal branch also advances rapidly and arrives in Assam in first week of June.
8. lofty Himalaya mountain cause monsoon winds to deflect towards west over Ganga plain.
9. By mid June Arabian Sea branch of monsoon arrives over Saurashtra-Kuchchh and central part of country.
10. Arabian Sea and Bay of Bengal branches of monsoon merge over north-western part of Ganga plains.
11. Delhi receives monsoon showers by end of June
(around 29th June). By first weak of July, western Uttar Pradesh, Punjab, Haryana and eastern Rajasthan experience monsoon. By mid July, monsoon reaches Himachal Pradesh and rest of country.
• Withdrawal or Retreat of Monsoon
1. Withdrawal or retreat of monsoon is a more gradual process.
2. withdrawal of monsoon begins in north-western states of India by early September.
3. By mid October, it withdraws completely from northern half of peninsula.
4. withdrawal from southern half of peninsula is fairly rapid. By early December, monsoon has withdrawn from rest of country.
5. islands receive very first monsoon showers, progressively from south to north from last week of April to first week of May.
6. withdrawal takes place progressively from north to south from 1st week of December to first week of January. By this time, rest of country is already under influence of winter monsoon.
• Seasons Four main seasons can be identified in India:
the cold weather season, hot weather season, advancing monsoon and retreating monsoon, with some regional variations.
1. Cold Weather Season (Winter)
(a) cold weather season begins from mid November in northern India and stays till February.
(b) December and January are coldest months in northern part of India.
(c) temperatures decreases from south to north.
(d) average temperature of Chennai on East coast is between 24°-25° Celsius, while in Northern plains it ranges between 10° – 15° Celsius.
(e) Days are warm and nights are cold
(f) Frost is common in North and higher slopes of Himalayas experience snowfall.
(g) In northern part of country, weather is normally marked by clear sky, low temperature, low humidity and feeble variable winds.
(h) winter rainfall locally known as ‘Mahawat’ is small, but it is of immense importance for cultivation of Rabi crops.
1. peninsular region does not have a well defined cold season. There is hardly any noticeable seasonal change in temperature pattern, during winter due to moderating influence of sea.
2. Hot Weather Season (Summer)
(a) summer season occurs from March to May.
(b) In May, temperatures between 42° to 45° are common.
(c) In peninsular India, temperatures remain lower due to moderating influence of ocean.
(d) ‘Loo’ these are strong gusty, hot dry winds blowing during day over north and north-western India. Direct exposure to these winds may even prove to be fatal.
(e) Dust storms are very common during month of May in northern India.
(f) This is also season for localised thunderstorms associated with violent winds and torrential downpours often accompanied by hail.
(g) In West Bengal, these storms are known as ‘Kaal Baisakhi’ or calamity of month of Baisakhi.
(h) In Kerala and Karnataka, premonsoon showers are common. They help in early ripening of mangoes and are often referred to as ‘mango showers’.
3. Advancing Monsoon (The Rainy Season)
(a) By early June, trade winds are attracted by low pressure condition over northern plains.
(b) They cross equator and blow in a south westerly direction. They enter Indian peninsula as south west monsoon.
(c) As these winds blow over warm oceans, they bring abundant moisture to subcontinent.
(d) Early in monsoon season windward side of Western Ghats receives very heavy rainfall, more than
(e) Deccan plateau and parts of Madhya Pradesh also receive some amount of rain inspite of lying in rain shadow area.
(f) maximum rainfall of this season is received in north-eastern part of country.
(g) Mawsynram in Southern ranges of Khasi Hills receives highest average rainfall in world.
(h) Rajasthan and parts of Gujarat get scanty rainfall.
1. Another phenomenon associated with monsoon is its tendency to have ‘breaks’, i.e., monsoon rains take place only for a few days at a time.
(j) These breaks in monsoon are related to movement of monsoon trough.
(k) When axis of monsoon trough lies over plains, rainfall is good in these parts. Whenever axis shifts closer to Himalayas, there are longer dry spells in plains.
(l) Widespread rain occurs in mountainous catchment areas of Himalayan rivers. These heavy rains bring in their wake devastating floods causing damage to life and property in plains.
(m) frequency and intensity of tropical depressions too determine amount and duration of monsoon rains.
4. Retreating Monsoon (The Transition Season)
(a) During October and November south west monsoon winds weaken and start withdrawing gradually. By beginning of October, monsoon withdraws from northern plains.
(b) 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 rise in temperature.
(c) Day temperatures are high, nights are cool and pleasant. land is moist.
(d) Owing to conditions of high temperature and humidity, weather becomes rather oppressive during day. This is commonly known as ‘October Heat.’ In second half of October, mercury begins to fall rapidly in northern India.
(e) low pressure conditions over north western India, get transferred to Bay of Bengal by early November. This shift is associated with occurrence of cyclonic depressions which originates over Bay of Bengal. Deltas of Krishna and Kaveri are struck by
cyclones, which are often very destructive and cause damage to life and property.
(f) Sometimes these cyclones arrive on coasts of Orrisa, Paschim Banga and Bangladesh.
(g) maximum rainfall on coromandal coast is derived from depressions and cyclones.
• Distribution of Rainfall
1. Parts of western coast and north-eastern India receive over about 400 cm of rainfall annually.
2. It is less than 60 cm in Western Rajasthan adjoining parts of Gujarat, Haryana and Punjab.
3. Rainfall is equally low in interior Deccan plateau and east of Sahyadris.
4. A third area of low precipitation is around Leh in Jammu and Kashmir.
5. rest of country receives moderate rainfall.
6. Snowfall is restricted to Himalayan region.
7. Owing to nature of monsoons, annual rainfall is highly variable from year to year.
• Monsoon As a Unifying Bond
1. India is a land of diversities. It has diverse food habits, languages, customs and festivals; also it has diverse climatic conditions.
2. However, monsoons have a unifying influence on Indian subcontinent.
3. Indian landscape, its animals and plant life, its entire agricultural calender and life of people including their festivals revolve around this phenomenon (monsoon).
4. Year after year people of India from north to south and from east to west eagerly await arrival of monsoon.
5. These monsoon winds bind whole country by providing water to set agricultural activities in motion.
6. river valleys which carry this water also unite as a single river valley unit.
Chapter Notes and Summary