Geography: Chapter 4- Climate
The distribution of rainfall in India
- Areas of heavy rainfall: An annual rainfall of over 400 cm is received over the parts of western coast and north-eastern states of India. - Areas of low rainfall: An annual rainfall of less than 60 cm is received in western Rajasthan and adjoining parts of Gujarat, Haryana, Punjab, interior parts of the Deccan Plateau, and Leh in Jammu and Kashmir. - Areas of moderate rainfall: The annual rainfall is between 100 cm and 200 cm is received by Madhya Pradesh, Odisha, West Bengal, Bihar, eastern Uttar Pradesh, eastern parts of Tamil Nadu, and the eastern slopes of Western Ghats.
How does latitude affect climate?
- At the equator, the sun's rays are concentrated onto a smaller area. - Near the poles, the same amount of sunlight energy is spread out over a larger area.
Advancing Monsoon(Rainy Season)
- By early June, the low-pressure condition over the northern plains intensifies which attracts the trade winds(south-east trade winds) of the southern hemisphere which originate over the warm sub-tropical areas of the southern oceans. - When they cross the equator, it is diverted to its right and blows in a south westerly direction entering the Indian peninsula as the south-west monsoons. - They are very strong and blow at an average velocity of 30 km per hour. - Early in the season, the windward side of the Western Ghats receives very heavy rainfall, i.e., more than 250 cm. - The maximum rainfall of this season is received in the north-eastern part of the country. - Mawsynram in the southern ranges of the Khasi Hills receives the highest average rainfall in the world. - Rainfall in the Ganga valley decreases from the east to the west. Because monsoon winds strike on the Himalayas in the north-eastern states and diverted to west while coming they give rain from east to west, as a result they lose their moisture. - Rajasthan and parts of Gujarat get scanty rainfall. - Another phenomenon associated with the monsoon is its tendency to have 'breaks' in the rainfall.
Latitude
- Due to curvature of the Earth, the amount of solar energy received varies according to latitude. - Temperature decreases from equator to polar region. - Tropical region- HOT - Polar region- COLD - Temperate region- MODERATE
Retreating/Post Monsoon(The Transition Season)
- During October-November, with the apparent movement of the Sun towards the south, the low-pressure trough over the northern plains becomes weaker which is gradually replaced by a high-pressure system. - The south-west monsoon winds weaken and start withdrawing gradually. - By the beginning of October, the monsoon withdraws from the northern plains. - The months of October-November form a period of transition from hot rainy season to dry winter conditions. - The retreat of the monsoon is marked by clear skies and rise in temperature. - While day temperatures are high, nights are cool and pleasant. - Due to the conditions of high temperature and humidity, the weather becomes rather oppresive during the day which is commonly known as 'October Heat'. - In the second half of October, the mercury(temperature) begins to fall rapidly in northern India. - The low-pressure conditions, over north-western India, get transferred from Rajasthan to the Bay of Bengal by early November which is associated with the occurrence of cyclonic depression, that originate over the Andaman Sea. - These cyclones generally cross the eastern coasts of India causing heavy and widespread rain. - The thickly populated deltas of the Godavari, the Krishna, and the Kaveri are frequently struck by tropical cyclones, which cause great damage to life and property. - Sometimes, they arrive at the coasts of Orissa, West Bengal, and Bangladesh. - The bulk of the rainfall of the Coromandel Coast is derived from depressions and tropical cyclones.
The Hot Weather Season(Summer)
- Hot weather seasons extend from March to May in India. - Temperature increases gradually from south to north. - In peninsular India, temperature remains lower due to the moderating influence of the oceans. - A striking feature of the hot weather season is the 'loo' that means strong, gusty, hot, and dry winds blowing during the day over north and north-western India. - Dust storms are very common during the month of May in northern India. - The storms bring temporary relief as they lower the temperature and may bring light rain. - It is also the season for localized thunderstorms, associated with violent winds, torrential downpours, often accompanied by haul. In West Bengal, these storms are called 'Kaal Baisakhi'. - Mango showers help in the early ripening of mangoes. (This is why they are often referred to as 'mango showers'.)
The consequences of the uneven distribution of rainfall
- If there are too much rains, they cause floods and havoc all around. - If there are insufficient rains, the people, because of drought and famine.
'India has diverse climatic conditions.'
- In summer, the mercury occasionally touches 50°C in some parts of Rajasthan deserts whereas it may be around 20°C in Pahalgam in Jammu and Kashmir. - In winter, the temperature at Drass in Jammu and Kashmir may be -45°C whereas Thiruvananthapuram may have a temperature of 22°C. - The annual precipitation is less than 10 cm in Ladakh and Western Rajasthan whereas it increases to 400 cm in Meghalaya. - Most parts of the country receive rainfall from June to September, but some parts like the Tamil Coast get most of its rain during October and November. - In the Thar Desert, the day temperature may rise to 50°C and drop down to 15°C the same night whereas there is no difference in day and night temperature in the Andaman and Nicobar Islands.
Distance from the sea
- Sea exerts a moderating influence of climate. - If the place is located away from the sea, then the moderating influence of the sea decreases.
What creates wind?
- The Sun heats up the land and water UNEVENLY. - The air over the water heats up slower than the air over land. - The high pressure air moves towards the low pressure. (This is WIND!)
The Cold Weather Season(Winter)
- The cold weather season begins from mid-November in Northern India and stays till February. - December and January are coldest months in the northern parts of India. - The temperature decreases from south to north. - Days are warm and nights are cold. - The north-east trade winds prevail over the country, and blows from land to sea that leads to dry season in most part of the country. - Tamil Nadu coast receives rainfall from the north-east trade winds as they blow from sea to land. - The weather is normally marked by clear sky, low temperature, and low humidity. - A characteristic feature of the cold weather season over the northern plains is the inflow of cyclonic disturbances from the west and the north-west. - They originate over the Mediterranean Sea and western Asia and move into India along with westerly flow. - They cause for winter rain over the plains (Punjab, Haryana, and Himachal Pradesh) and snowfall in the mountains. - The total amount of winter rainfall in the north is locally known as 'Mahawat' which help for the cultivation of 'rabi' crops. - The peninsular region does not have a well-defined cold season due to the moderating influence of the sea.
'Monsoon acts as an unifying bond.' Explain.
- The seasonal alteration of the wind systems and the associated weather conditions provide a rhythmic cycle of seasons. - Even the uncertainties of rain and uneven distribution are very much typical of the monsoons. - The Indian landscape, its animal and plant life, its entire agricultural calendar, and the life of the people, including their festivities, revolve around the changes of the monsoon. - Year after year, people of India from north to south and from west to east eagerly await for the arrival of the monsoon. - Monsoon winds bind the whole country by providing water to set the agricultural activities in motion. - The river valleys which carry water also unite as a single river valley unit.
Monsoon
- The word monsoon is derived from the Arabic word 'mausim' which literally means season. - Meaning of monsoon- the seasonal reversal in the wind direction during a year
Why do north easterly winds bring little rain or no rain?
1. Because India lies in the north easterly winds which originate from the subtropical high-pressure belt of the northern hemisphere. 2. They blow south and move on towards the equatorial low-pressure area. 3. These winds carry very little moisture as they originate and blow over land.
Characteristic features of the monsoon trough
1. For various reasons, the trough and its axis keep on moving northward or southward, which determines the spatial distribution of rainfall. 2. When the axis of the monsoon trough lies over the plains, rainfall is good there. If the axis shifts closer to the Himalayas, there are longer dry spells in the plains. 3. The frequency and intensity of tropical depressions too, determine the amount and duration of monsoon rains which format the head of Bay of Bengal and cross over mainland. The depression follow the axis of the 'monsoon trough'.
'The pressure and wind conditions over India are unique.'
1. In winter, there is a high pressure area in the north of the Himalayas which blows towards the low-pressure areas over the oceans to the south. 2. In summer, a low-pressure area develops over interior Asia and over north-western India which causes a complete reversal of the direction of winds. 3. Air moves from the high-pressure area over the southern Indian Ocean, in a south-easterly direction, crosses the equator and turns right towards the low-pressure areas over the Indian subcontinent which are known as the Southwest Monsoon winds.
The main features of El Nino
1. It is a Spanish word meaning "the child" and refers to the baby Christ, as the current starts flowing during Christmas. 2. It leads to an increase in sea-surface temperature. 3. It weakens the trade winds in the regions. 4. It causes heavy rainfall, floods, or drought in different regions of the world. 5. Sometimes, it is responsible for the bad monsoon in India.
How do jet streams affect the climate of India?
1. Jet streams located at 27° 30' N are known as subtropical westerly jet streams. 2. Over India, they blow south of the Himalayas, all through the year except in summer. 3. North and north-western parts of India experiences the western cyclonic disturbances brought in by westerly flow during the months of December to January. 4. In summer, the subtropical westerly jet stream moves north of the Himalayas with the apparent movement of the sun. 5. An easterly jet stream, called the tropical jet stream, blows over peninsular India approximately 14° N in summer.
Factors affecting India's climate
1. Latitude- The Tropic of Cancer(TOC) passes through the middle of the country that divides it almost into two equal parts. The places lying south of the TOC receive direct sun's rays and experience tropical climate. The places lying north of the TOC receive slanting sun's rays and experience sub-tropical climate. So, India's climate has characteristics of tropical and sub-tropical climates. 2. Altitude- India has mountains in the north (The Himalayas) with an average height of 6,000 metres. Himalayas prevent the cold winds coming from Central Asia from entering into the Indian subcontinent. It is because of the Himalayas, Indian subcontinent experiences mild winter compared to Central Asia, and receives highest rainfall in the world (from South-West monsoon winds).
Elements of climate and weather
1. Temperature 2. Atmospheric pressure 3. Wind 4. Humidity 5. Precipitation
The monsoon is known for its uncertainties. Because
1. The alternation of dry and wet spells vary in intensity, frequency, and duration. 2. While it causes heavy floods in one part, it may be responsible for droughts in the other. 3. It is often irregular in its arrival and its retreat. Hence, it sometimes disturbs the farming schedule of millions of farmers all over the country.
The factors that help us to understand the mechanism of monsoon
1. The differential heating and cooling of land and water creates low pressure on the landmass of India while the sea experiences comparatively the high pressure. 2. The shift of the position of ITCZ(Inter Tropical Convergence Zone) in summer, over the Ganga Plain from the equatorial trough about 5° N of the equator. 3. The presence of the high-pressure area to the east of the Madagascar is approximately at 20° South over the Indian Ocean. 4. The Tibetan Plateau is heated during summer which results in the formation of low-pressure over the plateau that attract high-pressure. 5. The movement of the westerly jet stream to the north of the Himalayas and the presence of the tropical easterly jet stream over Indian peninsula during summer.
Onset of the monsoon in India
1. The monsoon arrives at the southern tip of the Indian peninsula by the first week of June and it proceeds into two branches like the Arabian Sea and the Bay of Bengal. 2. The Arabian Sea branch reaches Mumbai approximately by the 10th of June and further moves towards north while the Bay of Bengal branch advances rapidly and arrives in Assam in the first week of June. 3. By mid-June, the Arabian Sea branch of the monsoon arrives at the central part of India. 4. Both the Arabian Sea and the Bay of Bengal branches of the monsoon converge over the north-western part of the Ganga Plains. 5. As a result of it: a) Delhi receives the monsoon showers from the Bay of Bengal branch by the end of June (tentatively 29th of June). b) By the first week of July, western Uttar Pradesh, Punjab, Haryana, and eastern Rajasthan experiences the monsoon rainfall. c) By mid-July, the monsoon reaches Himachal Pradesh and the rest of the country.
Breaks in monsoon
1. The monsoon rains take place only for a few days of time. 2. They are interspersed with rainfall intervals. 3. They are related to the movement of the monsoon trough.
Characteristics of the Monsoon
1. The monsoon type of climate is characterized by a distinct seasonal pattern. 2. The weather conditions greatly change from one season to the other. 3. The monsoons are pulsating in nature. 4. They are affected by different atmospheric conditions prevailing in the region. 5. The duration of the monsoon is between 100-120 days.
3 types of ocean currents
1. Warm current 2. Cold current 3. Equatorial current(flows along the equator)
The withdrawal of the monsoon
1. Withdrawal or the retreat of the monsoon is a more gradual process. 2. The withdrawal of the monsoon begins in north-western states of India by early September. 3. By mid-October, it withdraws completely from the northern half of the peninsula. 4. By early December, the monsoon withdraws from the rest of the country. 5. In the islands, the withdrawal takes place progressively from north to south from the first week of December to the first week of January.
ITCZ(Inter Tropical Convergence Zone)
A broad trough of low pressure in equatorial latitudes where the north-east and the south-east trade winds converge that lies more or less parallel to the equator but moves north or south with the apparent movement of the sun IN THE NORTHERN HEMISPHERE: The winds that blow to the equatorial low pressure belt are called the North East Trade Winds. IN THE SOUTHERN HEMISPHERE: The winds that blow to the equatorial low pressure belt are called the South East Trade Winds. The line along which they converge(meet) is called the INTER-TROPICAL CONVERGENCE ZONE. This is often abbreviated to ITCZ.
El Nino
A name given to the periodic development of a warm ocean current along the coast of Peru as a temporary replacement of the cold Peruvian current that occurs every 2 to 5 years
Windward side
A side of a mountain which receives winds As winds blow towards the mountain, they carry moist air which is lifted up over the mountain. Lifting cools the air mass and causes rain.
Continentality
A situation under which a place is very hot during summer and very cold during summer
Coriolis force
An apparent force caused by the Earth's rotation Due to the Earth's rotation... - Wind deflects to the right in the Northern Hemisphere. - Wind deflects to the left in the Southern Hemisphere. It is also known as Ferrel's Law.
Burst of the monsoon
Around the time of monsoon's arrival, the normal rainfall increases suddenly and continues constantly for several days.
Altitude
As one goes from the surface of the earth to higher altitudes, the atmosphere becomes less dense, atmospheric pressure decreases, and the temperature decreases.
Why are the houses in Assam built on stilts?
In Assam, the houses are built on stilts because it does not get affected by the flood or earthquake.
Why do houses in Rajasthan have thick walls and flat roofs?
In Rajasthan, the houses have thick walls and flat roofs because thick walls prevent the heat from entering into the houses while the flat roofs help to retain a little water that has been collected during rainfall.
What type of climate does India have and why is it so?
India has a monsoon type of climate because... - The climate of India is strongly influenced by the monsoon winds. - India receives most of its rainfall from the monsoon winds.
Southern Oscillation
Normally when the tropical eastern south Pacific Ocean experiences high pressure, the tropical eastern Indian Ocean experiences low pressure. But, in certain years, there is a reversal in the pressure conditions and the eastern Pacific has low pressure in comparison to the eastern Indian Ocean. This periodic change in pressure conditions is known as Southern Oscillation(SO). A feature connected with the SO is the El Nino.
Relief features
Plays a major role in determining the climate of a place because Himalayas prevent the cold winds coming from Central Asia from entering into the Indian subcontinent and also it is because of the Himalayas, India receives highest in the world
Mango showers
Pre-monsoon showers that are common mainly in Kerala and Karnataka
Pressure and Wind System
Pressure and wind system of any area depends on the latitude and altitude of the place which influences temperature and rainfall pattern.
Tropical Cyclone
The cyclones which originate and travel in the tropical zone and are often very destructive
How is the intensity of the monsoon predicted?
The difference in pressure over Tahiti(Pacific Ocean, 18° S/149° W) and Darwin in northern Australia(Indian Ocean, 12° 30' S/131° E) is computed to predict the intensity of the monsoons. If the pressure differences are negative, it would mean below average and late monsoons.
Rain shadow area
The dry area on another side of a mountain (Leeward side) caused as winds lose their moisture travelling up a mountain
Why do the houses in Tarai region have sloping roofs?
The houses in Tarai region, Goa and Mangalore, have sloping roofs because these regions receive heavy rainfall. Therefore, when the houses have sloping roofs, it helps them to get rid of the collected rainwater that flows from the roof to the ground.
Monsoon trough
The pathway of the monsoon winds
Trade winds
The permanent winds which blow from sub-tropical high pressure belt towards the equatorial low pressure belt
Weather
The state of atmospheric conditions over an area at any point of time
Ocean currents
The streams which flow continuously on the surface of sea or ocean water - It is generated by a number of forces acting upon the water, like wind, the Coriolis force, temperature, and breaking waves. - Ocean currents along with onshore winds affect the climate of the area, for example, any coastal area with warm or cold currents flowing past it, will be warmed or cooled if the winds are onshore.
Climate
The sum total of weather conditions and variations over a large area for a long period of time (more than 30 years)
Jet Stream
The upper air circulation in India is dominated by a westerly flow which is known as the Jet Stream. Jet streams are a narrow belt of high altitude (above 12,000 m) westerly winds in the top of troposphere. Their speed varies from about 110 km/hr in summer to about 184 km/hr in winter.
Why are the world's deserts located in the western margins of continents in the sub-tropics?
The world's deserts are located in the western margins of continents in the sub-tropics because these regions are devoid of moisture and rainfall.
Climatic controls
There are mainly six factors affecting the climate of any place: 1. Latitude 2. Altitude 3. Ocean Currents 4. Relief Features 5. Pressure and Wind System 6. Distance from the Sea
Western Cyclonic Disturbances
Weather phenomenon of the winter months brought in by the westerly flow from the Mediterranean Sea They influence the weather of the north and north-western parts of India by giving rainfall during the months of December and January.
Four seasons can be identified in India
a) The Cold Weather Season(Winter) b) The Hot Weather Season(Summer) c) Advancing Monsoon(Rainy Season) d) Retreating/Post Monsoon(The Transition Season)