Q.1. Write a brief note on the south-west monsoon and its influence on the climate of India. (Important)
Ans. The word ‘monsoon’ is derived from the Arabic word ‘mausim’ which means season. It is marked by complete reversal of wind system according to seasons. The summer period is marked by the south-west monsons.
The low pressure centre over northern India intensifies by early June. It attracts the southeasterly trade winds from the Southern Hemisphere which become south-westerly winds after crossing the equator due to the coriolis effect. As they originate over warm sub-tropical areas of Southern Oceans they bring abundant moisture to India’s mainland as southwest monsoon. The southwest monsoon strikes the southern parts of the peninsula in early June and by mid-July has its sway over entire India. The duration of the monsoon is between 100-120 days from early June to mid-September. This period is known as the Rainy Season. The two branches of southwest monsoons – the Arabian Sea branch and the Bay of Bengal branch–advance through the country and merge over the Northern Plains.
They provide heavy torrential rainfall in the West Coast and Northeastern states. Even Rajasthan receives some amout of rain. The bulk of the rainfall in India (75% to 90%) is caused by the advancing southwest monsoons and is concentrated within the Rainy Season. India has hot, wet tropical monsoon climate on account of it. The south-west monsoon is pulsating in nature marked by wet spells followed by dry spells termed as ‘breaks’. The south-west monsoons are uncertain and variable in nature. They may cause floods in some parts and droughts in other parts. The entire climatic regime of India is dependant upon the south-west monsoons.
Q.2. Why does the rainfall decrease from east to west in Northern India? Write a brief note explaining why. (Important)
Ans. The bulk of rainfall in India comes from the southwest monsoons. The Arabian Sea branch of south-west monsoon approaches from the western side of the peninsula and moves in a southwesterly direction and reaches the Northern Plains by first week of July. The Bay of Bengal branch of southwest monsoon brings heavy rainfall to the northeastern states. It arrives in Assam in the first week of June. The lofty mountains of the Purvanchals causes the monsoon winds to deflect towards the west over the Ganga Plains. They cause heavy rainfall on the eastern parts of the Northern Plains as the moisture content of the onshore winds are high. As the winds move westwards their moisture content decreases and bence, the rainfall decreases.
The Arabian Sea branch of monsoons approach the Northern Plains from the west. But the highlands, the Aravalis, in the western part of the country, lie parallel to the direction of the onshore winds. Hence, very less or no rainfall is experienced in the western limits of the plains. They provide more rainfall on the central and eastern parts of the plains when the rain bearing winds strike the Himalayas.
Both the branches merge over the northwestern part of the Ganga Plains. Delhi receives its showers from either of the branches. The location of the monsoon trough over the Northern Plains also determines the amount of rainfall. When the axis of the trough lies over the plains, rainfall is good in these parts, especially in the eastern parts where tropical depressions from Bay of Bengal approach. When the monsoon trough moves northwards towards Himalayas, there are dry spells over the Northern Plains and heavy rainfall occurs in the Himalayas.
Hence, due to the above reasons, rainfall decreases from east to west in Northern India.
Q.3. Distinguish between southwest monsoon and northeast monsoon. (CBSE 2010)
Ans. The monsoon type of climate is marked by complete reversal of wind system according to seasons. The summer period is marked by the southwest monsoons while the winter period is marked by northeast monsoons. The points of difference between them are as follows :
(a) The southwest monsoons blow in a southwesterly direction to the mainland of India from June to September. While, the northwest monsoons blow in a northeasterly direction across India from mid-November to February.
(b) The southwest monsoons are seasonal winds influenced by differential heating of land and water. The southeasterly trade winds are attracted by low pressure over northern India. They are deflected to southwest after crossing the equator due to the Coriolis force while the northeast monsoons are the northeasterly trade winds.
(c) The southwest monsoons are onshore winds as they blow from the Indian Ocean as Arabian Sea and Bay of Bengal branches to land areas. Hence, they are moisture laden winds and bring heavy rainfall. While the northeast monsoons are offshore winds as they blow from land to sea. As a result they are mostly devoid of moisture.
(d) The southwest monsoons are responsible for the bulk of the rainfall in India (75% to 90%). They make Mawsynram in Meghalaya the rainiest station in the world. During the northeast monsoons some rainfall is brought by occassional western disturbances in north India. The northeast monsoons provide some rainfall in eastern coastal plain of Tamil Nadu after passing over Bay of Bengal.
(e) The southwest monsoons are warm winds. While the northeast monsoons are cold winds.
Q.4. Explain the major factors influencing climate in India.
Ans. The important factors that influence the climate of India can be identified as follows :
(a) Latitudinal Location. India lies in the Northern Hemisphere with the Tropic of Cancer (23�30′N) passing almost through the middle of India. Areas to its south have tropical type of climate, while areas to its north have characteristics of sub-tropical climate.
(b) Altitude and Relief Features. The mountainous areas to the north of the Himalayas have average altitude of upto 6000 metres. Stations located here are cool even in summer and receive snowfall during winter. The lofty Himalayas along the northern borders of India act as a mighty barrier protecting India from the cold winds from Central Asia in winter. As a result India experiences milder winters as compared to Central Asia. It also concentrates the monsoon rains within the mainland of India. The compact physical setting of India with the mountains in the north and Indian Ocean to the south of the peninsula, lends a broad common climatic framework to India.
(c) Pressure and Winds. The pressure and wind conditions over India are unique. They result in seasonal reversal of the wind system and monsoon winds dominate the climate of India. The bulk of the rainfall in the country is brought by the southwest monsoons. Upper air circulation and the position of jet streams in upper troposphere influence the monsoons. Western cyclonic disturbances in winter and tropical cyclones during retreating monsoon season bring rainfall and affect the climate.
(d) Distance from the sea. Places at coastal locations, e.g. Mumbai, Chennai, Kolkata, have maritime or equable climate due to the moderating influence of the sea. But places in the interior of the country, far from the sea, experience extreme climate due to continentality, e.g. Dehli.
Q.5. Write briefly about the Arabian Sea branch of the monsoon. (Important)
Ans. The Indian Peninsula divides the southwest advancing monsoons into two branches – the Arabian Sea branch and the Bay of Bengal branch. The Arabian Sea branch of monsoons approaches the western coast of India from the Arabian Sea in a southwesterly direction. It arrives at Thiruvananthapuram on the 1st of June. By 10th of June it reaches Mumbai. The onshore moisture laden winds strike the Western Ghats and provide heavy rainfall on its windward side. As a result Mumbai in the windward side receives much more rainfall than Pune which is located on the leeward side of the Western Ghats. The Deccan Plateau lies in the rainshadow area and hence receives less rainfall. By mid-June the Arabian Sea branch of the monsoon arrives over Saurashtra-Kuchchh and the central part of the country. In the western and northwestern parts of the country, the highlands, the Aravalis, lie parallel to the direction of the incoming Arabian Sea branch of monsoons. Hence, these areas do not receive much rainfall because the rainbearing winds do not strike any barrier. By the first week of July, western Uttar Pradesh, Punjab, Haryana and eastern Rajasthan experience the monsoons.
The Arabian Sea branch of monsoons merges with the Bay of Bengal branch over the northwestern part of the Ganga plains. Delhi receives its rain from either of the two branches.
Q.6. Write in brief about the mechanism of the monsoons. (Important)
What is meant by the term 'monsoon'? Give the main characteristics of monsoons.
Ans. The mechanism of monsoons can be explained by the following facts :
(a) The differential heating and cooling of land and water creates low pressure on the landmass in summer which attracts moisture-bearing winds from the high pressure centres over the sea.
(b) The shift of the position of Inter – Tropical Convergence Zone (ITCZ) in summer, over the Ganga plain from the equatorial region, creates a monsoon trough that attracts winds. The shifting of the axis of the trough effects the duration and intensity of monsoons.
(c) The movement of the westerly jetstream to the north of the Himalayas and the presence of the tropical easterly jetstream over Indian Peninsula during summer bring in the tropical depressions and cyclones associated with monsoons.
(d) The intensity and position of the high pressure area east of Madagascar, approximately at 20�S over the Indian Oceans, affects Indian monsoon.
(e) The Tibetan plateau gets intensely heated during summer, which results in strong vertical air currents. High pressure is formed over the plateau at about 9 km above sea level. Periodic change in pressure conditions over southern oceans, known as Southern Oscillation and effect of El Nino also influences the monsoons.
Q.7. Write a brief account of the conditions and characteristics of the retreating monsoons.
Ans. The months of October-November mark a period of transition from hot rainy season to cold dry winter conditions. With the apparent movement of the sun southwards, the low pressure monsoon trough over the Northern Plains weakens and is gradually replaced by a high pressure system. The low pressure conditions shift to the Bay of Bengal. As a result, the southwest monsoon winds weaken and start withdrawing gradually from India. By the beginning of October it withdraws from the Northern Plains. This period is known as the Retreating Monsoon or Transition season because the monsoons withdraw or retreat from India. The Retreating Monsoon season is marked by clear skies and rise in temperature. While days are warm, nights are cool and pleasant. High rate of evaporation from the land that is still moist results in high humidity andoppressive weather conditions during the day. This phenomenon is known as ‘October heat’. With the shift of low pressure zone to over the Bay of Bengal in early November, cyclonic depressions originating over the Andaman Sea moves over the Eastern Coast. The tropical cyclones and associated heavy rainfall cause heavy destruction in the thickly populated deltaic areas of Tamil Nadu and Andhra Pradesh and sometimes Orissa, West Bengal and Bangladesh.
The bulk of the rainfall of the Coromandel coast is derived from these cyclones and depressions. The retreat or withdrawl of monsoon is a gradual process and takes a long time.
Q.8. Describe how the Himalayas affect the climate of India.
Ans. The lofty Himalayas stand as a mighty barrier along the entire northern boundary of India. They have a profound influence on India’s climate. The high mountains with an elevation of 6000 metres act as an effective climatic divide. In winter they prevent the cold winds from Central Asia from entering India. As a result, India experiences comparatively milder winters as compared to Central Asia.
The mountain wall blocks the monsoon winds, preventing their escape out of India. It concentrates rainfall from the monsoon winds within India. When the moisture laden monsoon winds strike the Himalayas, heavy rainfall is caused in the foothills region. The Northern Plains of India receive rainfall from the Bay of Bengal branch of monsoons becaused the eastern mountains deflect these winds over the Ganga Plains.
The Himalayas are responsible for giving the subtopical areas in northern India a touch of the tropical climate with hot, wet summer and mildly cold, dry winters. The alignment of the Himalayas also influences climate, So Ladakh on the leeward side of the Himalayas is a cold desert while Mawsynram on the southern ranges of Purvanchal receives the highest rainfall in the world.
Q.9. Give an account of the weather condition and characteristics of the cold season.
Ans. The period between mid-November to February is the cold weather season in India. December and January are the coldest months in the northern part of India. The cold weather season is characterised by the following features :
(a) A high pressure region develops in the northern part of the country with the apparent movement of the sun southwards.
(b) The northeast trade winds prevail over the country. As they blow from land to sea, for most parts of the country it is a dry season.
(c) Light winds move outwards from the high pressure area. Under the influence of relief, these winds blow from the west and northwest in the Ganga Valley.
(d) Days are warm and nights are cool during the season. Weather is marked by clear sky, low temperature and low humidity and feeble variable winds mainly in the north.
(e) Temperature is lower in the northern parts, ranging between 10� to 15�C. Temperature is higher in southern India, between 24� to 25�C, as in Chennai.
(f) Frost is common in northern parts.
(g) The higher slopes of the Himalayas, e.g. places like Srinagar, Leh and Shimla, experience snowfall.
(h) The western cyclonic disturbances are characteristic weather phenomena of the winter months. Their inflow from the west and northwest is brought in by the westerly flow from the Mediterranean Sea region. They cause the much needed winter rains over the plains and snowfall in the mountains in the northwestern India.
(i) The peninsular region does not have a well-defined cold season due to moderating influence of the sea. Rainfall occurs on Tamil Nadu coast during winter because the northeast winds blow from sea to land here.
Q.10. Give a brief account of the hot weather season in India. (Important)
Ans. The period between April to June is the hot weather season or summer in India.
The characteristic features of the hot weather season are as follows :
(a) The global heat belt shifts northward due to the apparent northward movement of the sun.
(b) The temperature recordings taken during April to June of stations at different latitudes, show the influence of the shifting of the heat belts. In March temperature in Deccan is about 38�C. In April, temperatures in Madhya Pradesh and Gujarat are around 42�C. In May, temperature of 45�C is common in northwestern India. Peninsular India has moderate temperature due to influence of sea.
(c) The rising temperatures lead to fall in air pressure in the northern parts. Toward the end of May, an elongated low pressure area develops in the north extending from the Thar desert in the northwest to Patna and Chhota Nagpur platean in the east.
(d) Strong, gusty, hot, dry winds, locally called ‘loo’ blow during the afternoon over north and northwestern India. They may continue till late evening.
(e) Duststorms are common during May in Punjab, Haryana, Delhi, Eastern Rajasthan and Western Uttar Pradesh. Sometimes they bring light rain and pleasant cool breeze that provide temporary relief from the heat.
(f) High temperature during the day causes violent, localised thunderstorms by the evening. These thunderstroms are associated with violent winds and torrential downpour, often accompanied with rain. They are known as ‘Kal Baisakhi’ or calamity of the month of Baisakh in West Bengal.
(g) In late May pre-monsoon showers occur, especially in Karnataka and Kerala. They are known as mango showers as they help in the early ripening of mangoes.