Indian Monsoon – UPSC Indian Geography Notes

The term ” monsoon ” denotes the climate associated with the seasonal reversal of wind direction. The monsoons constitute a dual system of seasonal winds that journey from the sea to the land in the summer and from the land to the sea in the winter. Monsoons are prevalent in the Indian subcontinent, central-western Africa, Southeast Asia, and a few other locations, with the winds being most robust in the Indian subcontinent. The Indian Monsoon plays a pivotal role, contributing to approximately 70% of India’s annual rainfall. This article delves into the specifics of the Indian Monsoon, providing valuable insights for those preparing for Geography in the UPSC Civil Service Exam.

Indian Monsoon

The climate in India is characterized as hot monsoonal, a pattern also observed in South and Southeast Asia. Among the four prevailing seasons in the Indian subcontinent, two are dedicated to the monsoon. These are:

1. The Southwest Monsoon Season
2. The North-East Monsoon Season

Onset of the Monsoon

• Sun shines vertically over the Tropic of Cancer in April and May
• Large landmass north of the Indian Ocean becomes extremely hot
• Formation of an intense low-pressure system in the northwestern part of the subcontinent
• Low-pressure cell attracts southeast trades across the Equator
• Pressure in the Indian Ocean to the south is high due to slow heating of water
• Conditions facilitate the ITCZ northward shift in position

Intertropical Convergence Zone (ITCZ)

The ITCZ (Intertropical Convergence Zone) is a low-pressure belt that influences precipitation in the tropics through its northward and southward movements along the equator.

• Southwest monsoon continues from the southeast trades deflected towards the Indian subcontinent
• Winds cross the Equator between 40°E and 60°E longitudes
• Southwest monsoon reaches the Kerala coast on June 1st
• Progresses swiftly through Mumbai and Kolkata from June 10th to 13th
• Engulfs the entire subcontinent by mid-July

The Southwest Monsoon Season

• Rainy Season: June to September
• Rapid temperature rise over northwestern plains in May intensifies low-pressure conditions
• Intense low-pressure attracts Southern Hemisphere trade winds from the Indian Ocean by early June
• Southeast trade winds cross the equator and enter the Bay of Bengal and the Arabian Sea
• Winds become entangled in air circulation over India
• Carry substantial moisture from equatorial warm currents
• Travel in a southwesterly direction after crossing the equator
• Southwest monsoons named after this pattern

Southwest Monsoon Bursts

• Rain commences abruptly during the southwest monsoon season
• Initial rain significantly lowers temperature
• “Break” or “burst” of monsoons involves sudden onset of moisture-laden winds with thunder and lightning
• Coastal areas of Kerala, Karnataka, Goa, and Maharashtra may experience monsoon in the first week of June, while interior areas may see it in the first week of July
• Daytime temperature drops by 5°C to 8°C between mid-June and mid-July
• Relief and thermal low pressure over northwest India alter the southwesterly direction of approaching winds
• Monsoon divided into two branches approaching the landmass: Arabian Sea branch and Bay of Bengal branch

North-East Monsoon

• Northeast monsoon originates from the northeast
• Wind direction: sea to land
• Monsoon winds carry moisture from the Indian Ocean
• Limited to south India, affecting regions such as Tamil Nadu, Puducherry, Karaikal, Yanam, Andhra Pradesh, Kerala, Mahe, and south interior Karnataka
• Active from October to December
• Low-pressure systems, depressions, and cyclones induce associated rainfall, termed the winter monsoon
• Tamil Nadu’s primary rainy season, contributing 48% (447.4mm) of its annual rainfall during these three months

Monsoon winds of the Arabian Sea

The three branches of the monsoon winds originating over the Arabian Sea are delineated as follows:

1. The first stream impacts India’s west coast, contributing over 250 cm of rain to the country.
   • Strikes the Western Ghats perpendicularly, causing abundant Orographic Rainfall (annual rainfall of 400 to 500 cm on the windward side).
   • On the leeward side, rainfall diminishes to 30-50 cm.
   • Immediate leeward side of the Western Ghats experiences a narrow belt of severe aridity.
   • After passing, the air ascends, leading to increased rainfall further east.
2. The second stream traverses the Narmada—Tapi troughs (narrow rift valley) before reaching central India.
   • Lacks a major orographic obstacle, resulting in limited rain near the coast.
   • Provides rainfall to specific parts of central India (e.g., Nagpur).
3. The third stream parallels the Aravali Range and has minimal rain-producing capacity, rendering Rajasthan predominantly a desert state.
   • On the southeastern edge of the Aravali Range, there is some orographic effect.
   • Mt. Abu receives approximately 170 cm of rain, while surrounding plains receive only 60 to 80 cm.

Monsoon Winds of Bay of Bengal

The monsoon from the Bay of Bengal Branch exhibits a division into two distinct streams:

1. The first stream reaches Meghalaya after crossing the Ganga-Brahmaputra delta.
   • Orographic effect induces heavy rainfall, with Cherrapunji receiving an annual rainfall of 1,102 cm, mostly between June and August.
   • Mawsynram, situated at 1,329 metres above sea level and only 16 kilometres west of Cherrapunji, surpasses it with more annual rainfall (1,221 cm).
   • Both stations are located at the northern end of a deep valley on the southern slopes of the Khasi hills.
2. The second stream of the Bay of Bengal branch flows along the Himalayan foothills, deflected westward by the Himalaya, resulting in widespread rainfall in the Ganga plain.
   • Rainfall gradually decreases as we move westward up the plain.

The rain shadow effect of the Arabian Sea current and the Bay of Bengal current, flowing parallel to the coast, maintains relative dryness along the Tamil Nadu coast during the southwest monsoon season.

Characteristics of Monsoonal Rainfall

Rainfall during the southwest monsoons follows a seasonal pattern occurring between June, August, and September. This monsoon season’s rainfall is predominantly influenced by relief or topography.

• Windward side of the Western Ghats receives over 250 cm of rain.
• Hill ranges and the Eastern Himalayas contribute to heavy rainfall in the northeastern states.
• Monsoon rainfall tends to decrease with increasing distance from the sea.
• During the southwest monsoon, Kolkata receives 119 cm, Patna 105 cm, Allahabad 76 cm, and Delhi 56 cm of rain.
• Monsoon rains occur in short bursts, separated by periods of no rain known as ‘breaks.’
• Cyclonic depressions forming at the head of the Bay of Bengal and crossing into the mainland cause these rain breaks.
• Frequency, intensity, and the path of these depressions determine rainfall distribution.
• Summer rains can result in torrential downpours, causing significant runoff and soil erosion.
• The southwest monsoon, contributing over three-quarters of the country’s total rainfall, plays a critical role in India’s agrarian economy.
• Uneven spatial distribution of rainfall, ranging from 12 cm to more than 250 cm.
• The onset of rains can be significantly delayed, affecting the entire country or a specific region.
• Rains may end earlier than expected, leading to significant damage to standing crops and challenges in winter crop sowing.

Season of Retreating Monsoon

Monsoons are recognized for their retreat during October and November. As of end of September, the southwest monsoon weakens due to the southward movement of the low-pressure trough over the Ganga plain, responding to the sun’s southward progression.

• First week of September: Monsoon departs from western Rajasthan.
• End of September: Monsoon withdrawal extends to encompass Rajasthan, Gujarat, the Western Ganga plain, and the Central Highlands.

Retreating Monsoon in the Southern Half of India

• Low-pressure system encompasses northern Bay of Bengal by early October
• It progresses over Karnataka and Tamil Nadu by early November
• By middle of December, the center of low pressure entirely exits the Peninsula
• Clear skies and increasing temperatures mark the retreat of the southwest monsoon season
• Ground remains wet
• High temperature and humidity lead to oppressive weather, termed ‘October heat’

Retreating Monsoon in the Northern Half of India

• Temperature starts to decline swiftly in the second half of October, particularly in northern India
• North India experiences dry weather during the retreating monsoon, while rain is associated with it in the eastern part of the Peninsula
• October and November are the rainiest months in this area
• Rainy season is connected to the movement of cyclonic depressions originating in the Andaman Sea and reaching the southern Peninsula’s eastern coast

Effects caused by Retreating Monsoons

• Tropical cyclones pose extreme danger
• Preferential targets include densely populated deltas of the Godavari, Krishna, and Kaveri rivers
• Annual devastation occurs in this area due to these cyclones
• West Bengal, Bangladesh, and Myanmar coasts are also susceptible to cyclonic storms
• Depressions and cyclones contribute significantly to rainfall on the Coromandel coast
• In the Arabian Sea, such cyclonic storms are less common

Significance of Indian Monsoon

• Nearly 60% of India’s net arable land lacks irrigation, making the monsoon critical for agriculture
• Monsoon contributes to about 70% of India’s annual rainfall
• It plays a crucial role in determining the yield of various grains and pulses, such as rice, wheat, and sugarcane
• Over half of India’s population is engaged in agriculture, and monsoon rains directly impact their earnings and well-being
• Agriculture constitutes more than 15% of India’s Gross Domestic Product (GDP)
• Crop failure due to insufficient rain adversely affects the economy
• Monsoonal rainfall is instrumental in water storage for irrigation, electricity generation, and drinking

Conclusion

• The influence of the monsoon on local weather differs across locations, leading to varying amounts of rainfall.
• Some areas may experience a slight increase or decrease in precipitation.
• In certain regions, semi-deserts undergo a significant transformation, turning into lush green grasslands that foster a diverse array of plants and crops.
• The Indian Monsoon is instrumental in converting extensive portions of India from semi-desert conditions to abundant green landscapes.
• In such transformed areas, farmers must carefully time the sowing of seeds in their fields, ensuring optimal utilization of all available rain for crop growth.

FAQs on Indian Monsoon

Question: What causes the Indian monsoon?

Answer: The Indian monsoon is primarily caused by the differential heating of land and sea. During summer, the landmass of the Indian subcontinent heats up more rapidly than the surrounding oceans, creating a low-pressure area. This low pressure draws in moist air from the Indian Ocean, resulting in the monsoon winds and subsequent rainfall.

Question: What is the importance of monsoon?

Answer: The monsoon holds immense significance, particularly for agriculture. It provides the majority of India’s annual rainfall, influencing crop growth and yield. Over half of India’s population is engaged in agriculture, and the monsoon directly affects their livelihoods. Additionally, the monsoon contributes to water storage for irrigation, electricity generation, and drinking.

Question: Which country rains every day?

Answer: There isn’t a specific country that experiences rain every day. However, regions with tropical climates, such as some parts of Southeast Asia and the Amazon rainforest, may have a high frequency of rainfall throughout the year due to their geographical location and climate patterns.

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