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The Universe and the Earth
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Geomorphic Processes
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Movement of Ocean Water
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Oceans and its Properties
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Climate of a Region
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Indian Geography - introduction, Geology
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Physiography of India
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Indian Climate
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Indian Drainage
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Chapter 15: Indian Climate
Chapter Test20 topics•Estimated reading: 60 minutes
Indian Climate – Features of Monsoon
Key Point
The Monsoon is not just rain; it is a 'Seasonal Reversal of Winds'. Imagine a giant sea breeze that blows from sea to land in summer (bringing rain) and land to sea in winter (bringing dry weather). It is the lifeline of the Indian economy.
The Monsoon is not just rain; it is a 'Seasonal Reversal of Winds'. Imagine a giant sea breeze that blows from sea to land in summer (bringing rain) and land to sea in winter (bringing dry weather). It is the lifeline of the Indian economy.
Detailed Notes (15 points)
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1. What is 'Monsoon'?
The word comes from the Arabic word 'Mausim', meaning season. In geography, it specifically refers to a unique climate type where the wind direction completely reverses every six months.
The Pendulum Analogy: Think of the wind like a pendulum. In Summer, it swings from the Sea towards the Land (bringing moisture and rain). In Winter, it swings back from the Land towards the Sea (bringing dry, cold air).
2. The Mechanism: Why does it happen?
For a beginner, understanding the 'Why' is crucial. It happens due to a combination of three main factors:
A. Differential Heating (The Vacuum Cleaner Effect):
In summer, the land gets very hot, much faster than the ocean. Hot air rises, creating a Low Pressure zone over India. The ocean remains cool (High Pressure). Nature hates a vacuum, so wind rushes from the High Pressure ocean to fill the Low Pressure void on land. These winds carry water, causing rain.
In summer, the land gets very hot, much faster than the ocean. Hot air rises, creating a Low Pressure zone over India. The ocean remains cool (High Pressure). Nature hates a vacuum, so wind rushes from the High Pressure ocean to fill the Low Pressure void on land. These winds carry water, causing rain.
B. Shifting of ITCZ (The Rain Magnet):
The Inter-Tropical Convergence Zone (ITCZ) is a belt of low pressure near the equator where the sun shines brightest. It acts like a magnet for winds. In summer, as the Sun moves north, this 'Rain Magnet' shifts northwards over the Ganga plains, pulling the moisture-laden winds from the ocean up into India.
The Inter-Tropical Convergence Zone (ITCZ) is a belt of low pressure near the equator where the sun shines brightest. It acts like a magnet for winds. In summer, as the Sun moves north, this 'Rain Magnet' shifts northwards over the Ganga plains, pulling the moisture-laden winds from the ocean up into India.
C. Coriolis Force (The Steering Wheel):
Once the winds cross the equator to rush towards India, the rotation of the Earth (Coriolis Force) twists them to the right. This makes them blow from the South-West direction. That is why it is called the 'South-West Monsoon'.
Once the winds cross the equator to rush towards India, the rotation of the Earth (Coriolis Force) twists them to the right. This makes them blow from the South-West direction. That is why it is called the 'South-West Monsoon'.
3. Unique Characteristics of Indian Rainfall
1. Pulsating Nature (The Burst):
The monsoon doesn't arrive slowly like a drizzle. It often starts with a sudden, violent thunderstorm. This dramatic start is called the 'Burst of Monsoon' (usually hitting Kerala on June 1st).
The monsoon doesn't arrive slowly like a drizzle. It often starts with a sudden, violent thunderstorm. This dramatic start is called the 'Burst of Monsoon' (usually hitting Kerala on June 1st).
2. Orographic Rain (The Mountain Wall):
The mountains decide who gets rain. When the monsoon winds hit the Western Ghats, they are forced to rise and cool down, dropping all their rain on the coast (Mumbai). By the time they cross to the other side (Pune), they are empty. This dry side is called the Rain Shadow Region.
The mountains decide who gets rain. When the monsoon winds hit the Western Ghats, they are forced to rise and cool down, dropping all their rain on the coast (Mumbai). By the time they cross to the other side (Pune), they are empty. This dry side is called the Rain Shadow Region.
3. Uneven Distribution:
Rainfall is not fair. Mawsynram (Meghalaya) gets over 1000 cm of rain because the hills there (Garo, Khasi, Jaintia) are shaped like a funnel, trapping the clouds. In contrast, Jaisalmer (Rajasthan) gets less than 10 cm because the Aravalli hills run parallel to the wind and don't stop it.
Rainfall is not fair. Mawsynram (Meghalaya) gets over 1000 cm of rain because the hills there (Garo, Khasi, Jaintia) are shaped like a funnel, trapping the clouds. In contrast, Jaisalmer (Rajasthan) gets less than 10 cm because the Aravalli hills run parallel to the wind and don't stop it.
4. Important Phases
The Break in Monsoon:
It does not rain continuously for 4 months. There are dry spells (days without rain) in between July and August. This is called a 'Break'. It usually happens when the rain-bearing belt shifts towards the Himalayas.
It does not rain continuously for 4 months. There are dry spells (days without rain) in between July and August. This is called a 'Break'. It usually happens when the rain-bearing belt shifts towards the Himalayas.
The Retreat (The Return Journey):
By October, the land cools down, and the low pressure disappears. The winds start withdrawing (going back) from India. This is called the Retreating Monsoon. Unlike the summer rain, this withdrawal is gradual.
By October, the land cools down, and the low pressure disappears. The winds start withdrawing (going back) from India. This is called the Retreating Monsoon. Unlike the summer rain, this withdrawal is gradual.
Comparison: SW Monsoon vs. NE Monsoon
| Feature | South-West Monsoon (The Giver) | North-East Monsoon (The Retreat) |
|---|---|---|
| Season | Summer (June to September) | Winter (October to December) |
| Direction | Blows from Sea to Land (Wet Winds) | Blows from Land to Sea (Dry Winds) |
| Rainfall Source | Arabian Sea Branch & Bay of Bengal Branch | It is dry initially but picks up moisture from the Bay of Bengal while crossing it. |
| Who Benefits? | Whole of India (75% of total annual rain) | Primarily Tamil Nadu and Coastal Andhra Pradesh |
Mains Key Points
Impact on Agriculture: 55% of India's net sown area is rain-fed. A good monsoon lowers food inflation and boosts rural income (demand for tractors, FMCG goods increases).
Cultural Significance: The monsoon rhythm defines the Indian calendar. Festivals like Onam (arrival of rain/harvest), Teej, and Ganesh Chaturthi are closely linked to the rainy season.
Climate Change Threat: We are observing a trend of 'Extreme Weather Events'—short periods of very heavy rain leading to flash floods (e.g., in Himachal, Uttarakhand) followed by long dry spells, which is bad for crops.
Prelims Strategy Tips
Retreating Monsoon Anomaly: While the rest of India is dry in winter, Tamil Nadu (Coromandel Coast) receives floods/rain. This is due to the North-East Monsoon picking up moisture from the sea.
Two Branches: The SW Monsoon hits India in two branches. The Arabian Sea branch is much stronger (3 times) than the Bay of Bengal branch.
El Nino vs La Nina:
- El Nino (warming of Pacific Ocean) usually means a Weak Monsoon (Droughts).
- La Nina (cooling of Pacific Ocean) usually means a Strong Monsoon (Floods).
- El Nino (warming of Pacific Ocean) usually means a Weak Monsoon (Droughts).
- La Nina (cooling of Pacific Ocean) usually means a Strong Monsoon (Floods).
Date of Onset: Traditionally, the monsoon hits the Kerala coast on June 1st. However, it reaches the Andaman & Nicobar Islands earlier (around May 20-25).
Salient Features of Indian Climate
Key Point
India's climate is a land of extremes but united by the Monsoon. It is defined by the 'Unity in Diversity' principle—while the whole country has a Tropical Monsoon climate, local conditions vary wildly from freezing Leh to scorching Rajasthan.
India's climate is a land of extremes but united by the Monsoon. It is defined by the 'Unity in Diversity' principle—while the whole country has a Tropical Monsoon climate, local conditions vary wildly from freezing Leh to scorching Rajasthan.
Detailed Notes (26 points)
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1. Seasonal Reversal of Winds (The Monsoon Pulse)
The most distinct feature of Indian climate is that the wind changes direction completely twice a year. Think of it like a pendulum swinging back and forth.
Summer (South-West Monsoon): From June to September, winds blow from the Sea towards the Land. Since they come from the ocean, they are full of moisture and bring heavy rain to the entire country.
Winter (North-East Monsoon): From October to December, winds blow from the Land towards the Sea. Since they come from the land (Himalayas/Siberia), they are dry and bring cold weather.
2. Formation of Pressure Zones (The Engine)
Wind always flows from High Pressure (Cool area) to Low Pressure (Hot area). India acts like a giant thermal engine.
Summer Mechanism: The Sun shines brightly over North India, heating the land immensely. Hot air rises, creating a Low Pressure vacuum. The Indian Ocean remains cool (High Pressure). Nature rushes to fill this vacuum, causing winds to rush from the ocean to the land.
Winter Mechanism: The land cools down rapidly, creating High Pressure. The ocean remains warmer (Low Pressure). So, winds blow outwards from India to the ocean.
3. Rainfall: The '4-Month' Phenomenon
In many Western countries, it rains a little bit throughout the year. In India, rainfall is highly concentrated.
Concentration: India receives about 80% to 90% of its total annual rainfall in just 4 months (June, July, August, September).
Consequence: This makes water management very difficult. We have floods during these 4 months and water scarcity during the rest of the year. This is why dams and tanks are crucial in India.
4. Unity of Indian Climate (The Himalayan Wall)
Despite being a large country, the whole of India is climatically 'United'. This is largely due to the Himalayas.
Protection from Cold: The Himalayas stand like a tall wall in the North. They block the freezing cold winds from Central Asia and Siberia. Without them, North India would be a freezing cold desert like the Gobi Desert. Because of this wall, even North India remains warm enough for agriculture.
Trapping the Rain: The Himalayas also stop the moisture-laden Monsoon winds from escaping into Tibet, forcing them to shed their rain within India.
5. Diversity of Indian Climate (Local Extremes)
While the general climate is 'Monsoon', the local experience varies drastically depending on where you are.
Temperature Extremes: In Summer, places like Churu (Rajasthan) can touch 55°C, while in Winter, Drass (Ladakh) can drop to -45°C.
Rainfall Extremes: Mawsynram (Meghalaya) receives over 1000 cm of rain (wettest place on Earth), while Jaisalmer (Rajasthan) receives less than 10 cm (drier than some deserts).
Coast vs Interior: Places near the sea (Mumbai, Chennai) have a 'Moderate Climate' (neither too hot nor too cold). Places far from the sea (Delhi, Punjab) have an 'Extreme Climate' (Very hot summers, very cold winters). This is called Continentality.
6. Natural Calamities
The Indian climate is often described as 'Moody'. It is prone to natural disasters.
Floods: Occur when the monsoon is too strong (common in Assam, Bihar).
Droughts: Occur when the monsoon fails or comes late (common in Vidarbha, Rayalaseema).
Cyclones: Occur mostly during the transition months (October-November) and affect the Eastern Coast (Odisha, Andhra Pradesh).
Salient Features of Indian Climate
| Feature | Description |
|---|---|
| Wind Reversal | NE trade winds in winter; SW monsoon winds in summer |
| Pressure Zones | Seasonal high-low pressure shifts over land & ocean |
| Rainfall | 80% annual rainfall in 4 months (SW monsoon) |
| Seasons | Broadly five; main three – Summer, Winter, Monsoon |
| Unity | Overall tropical; Himalayas shield from cold winds |
| Diversity | Temp: -45°C (Leh) to 55°C (Rajasthan); varied rainfall & humidity |
| Calamities | Floods, droughts, famines, epidemics due to variability |
Salient Features at a Glance
| Feature | Key Description | Example/Impact |
|---|---|---|
| Seasonal Reversal | Winds change direction twice a year. | Summer (Wet) vs Winter (Dry). |
| Rainfall Concentration | Rain is not spread out; it pours heavily in a short time. | 80% rain in June-Sept leads to water scarcity in summer. |
| Himalayan Shield | Protects from cold winds; Traps monsoon winds. | Ensures North India has a tropical climate, not temperate. |
| Continentality | Interior regions get extreme heat/cold (far from sea). | Delhi has very hot summers and very cold winters. |
| Maritime Effect | Coastal regions have moderate climate (near sea). | Mumbai/Chennai have almost same temp all year. |
Mains Key Points
Agriculture Dependence: Since rainfall is concentrated in 4 months, Indian agriculture is a 'Gamble with Monsoons'. Failure leads to farmer distress and food inflation.
Disaster Management: The variability means we need specific plans for both floods (East India) and droughts (West/Central India).
Cultural Unity: The rhythm of seasons dictates the festivals (Pongal, Baisakhi, Onam) and food habits of India, creating a cultural bond.
Prelims Strategy Tips
Tropic of Cancer: Divides India into two halves. But due to Himalayas, the northern half is also considered 'Tropical' climatically.
Diurnal Range: The difference between day and night temperature is Highest in Thar Desert (Hot day, Cold night) and Lowest in Kerala (Same temp).
Coldest Place: Drass (Ladakh) is the second coldest inhabited place in the world (after Siberia).
Wettest Place: Mawsynram (Meghalaya) receives highest rainfall due to the funnel shape of Garo, Khasi, and Jaintia hills.
Factors Determining India’s Climate
Key Point
Why is Kashmir freezing while Kerala is hot? Why does Mumbai have sticky weather while Delhi has dry heat? The answer lies in 6 key factors. Use the acronym LANDFORMS to remember them: L atitude, A ltitude, N earness to Sea, D irection of Wind, F orests, O cean Currents, R elief, M ountains, S oil.
Why is Kashmir freezing while Kerala is hot? Why does Mumbai have sticky weather while Delhi has dry heat? The answer lies in 6 key factors. Use the acronym LANDFORMS to remember them: L atitude, A ltitude, N earness to Sea, D irection of Wind, F orests, O cean Currents, R elief, M ountains, S oil.
Detailed Notes (21 points)
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1. Latitude (The Sun's Angle)
Latitude refers to how far a place is from the Equator. The Tropic of Cancer passes right through the middle of India. This divides the country into two climatic halves:
Northern Half (Sub-Tropical): This area is far from the equator. It experiences extreme heat in summer and extreme cold in winter (e.g., Delhi, Punjab).
Southern Half (Tropical): This area is closer to the equator. It remains warm throughout the year and has no distinct winter season (e.g., Kerala, Tamil Nadu).
2. The Himalayan Mountains (The Great Wall)
The Himalayas act like a giant climate control wall for India. They play a double role:
Shield against Cold: They block the freezing cold winds coming from Central Asia and Siberia. Without the Himalayas, North India would have been a cold desert.
Trap for Rain: They stop the moisture-laden Monsoon winds from escaping northwards into Tibet, forcing them to shed their rain within India.
3. Distance from the Sea (Continentality)
This factor explains why Mumbai and Delhi have different climates. Water heats up slowly and cools down slowly, while land heats up fast and cools fast.
Coastal Areas (Maritime Effect): Places near the sea (like Mumbai, Chennai) benefit from the sea breeze. The sea keeps the temperature moderate—neither too hot in summer nor too cold in winter.
Interior Areas (Continentality): Places far from the sea (like Delhi, Jaipur) do not get this moderating effect. Hence, they get scorching hot in summer and freezing cold in winter.
4. Altitude (Height above Sea Level)
As you go higher up a mountain, the air becomes thinner and cooler. For every 165 meters you climb, the temperature drops by 1°C. This is called the Normal Lapse Rate.
Example: Agra and Darjeeling are on the same latitude (receive same sunlight), but in January, Agra is 16°C while Darjeeling is 4°C because Darjeeling is located at a much higher altitude.
5. Relief (Physical Features)
Relief refers to the shape of the land (mountains, valleys). It decides the distribution of rainfall.
Windward Side: The side of the mountain that faces the incoming wind gets heavy rain (e.g., the Mumbai side of Western Ghats).
Leeward Side (Rain Shadow): The opposite side remains dry because the winds have lost their moisture by the time they cross the peak (e.g., the Pune side of Western Ghats).
6. Distribution of Land and Water
India is unique because it is surrounded by water on three sides (Indian Ocean) and has a huge landmass in the north. This contrast creates Pressure Differences. Since land heats/cools faster than water, air rushes back and forth between them, creating the Monsoon winds.
Factors and Their Impact (Quick Revision)
| Factor | How it works? | Real-life Example |
|---|---|---|
| Latitude | Distance from Equator determines heat. | Kerala (near equator) is hotter than Kashmir (far). |
| Altitude | Height reduces temperature (Lapse Rate). | Shimla is cooler than Ludhiana despite being close. |
| Distance from Sea | Sea creates moderate climate (Maritime effect). | Mumbai has no extreme winter/summer. |
| Relief | Mountains block wind/rain. | Mahabaleshwar rains heavily; Pune is dry (Rain Shadow). |
Mains Key Points
Unity in Diversity: Explain how the Himalayas provide a unified 'Tropical' climate to the whole subcontinent despite latitudinal differences that should make the north colder.
Monsoon Mechanism: Discuss how the differential heating of land (India) and sea (Indian Ocean) triggers the pressure differences that pull monsoon winds.
Urban Planning Relevance: Understanding local climate factors (like wind direction and relief) is crucial for planning cities to avoid creating heat islands.
Prelims Strategy Tips
Lapse Rate Value: The rate at which temperature drops with height is specifically 6.5°C per 1000 meters (or 1°C per 165m).
Tropic of Cancer States: It passes through 8 states. Remember the trick: 'Mitra Par Gamcha Jhar' (Mizoram, Tripura, Paschim Bangal, Rajasthan, Gujarat, MP, Chhattisgarh, Jharkhand).
Why Shillong has less rain? Even though it is near Cherrapunji, Shillong gets less rain because it lies on the leeward side (rain shadow) of the Khasi hills.
Jet Streams: Apart from surface factors, upper air circulation (Jet Streams) also determines when the monsoon arrives.
Climatic Regions of India – Köppen’s Classification
Key Point
Wladimir Köppen divided the world's climate based on Temperature and Rainfall. For India, he identified 9 distinct climatic zones ranging from the freezing Himalayas to the tropical coasts.
Wladimir Köppen divided the world's climate based on Temperature and Rainfall. For India, he identified 9 distinct climatic zones ranging from the freezing Himalayas to the tropical coasts.
Detailed Notes (21 points)
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1. Beginner's Guide: Decoding the Code
Köppen used symbols to describe climate. If you understand the symbols, you don't need to memorize the list.
Capital Letters (Broad Zone):
- A = Tropical (Hot all year).
- B = Dry (Desert/Semi-arid).
- C = Warm Temperate (Mild winters).
- D = Cold Forest (Cold winters).
- E = Polar (Freezing).
Small Letters (Rainfall Pattern):
- w = Winter Dry (It rains in Summer).
2. The 9 Climatic Regions of India
A. Tropical Regions (Group A)
1. Amw (Monsoon Type with Short Dry Season):
Found in the Western Coast (Konkan, Malabar). Here, rainfall is very heavy (monsoon) and the dry season is very short. Vegetation is Evergreen.
Found in the Western Coast (Konkan, Malabar). Here, rainfall is very heavy (monsoon) and the dry season is very short. Vegetation is Evergreen.
2. Aw (Tropical Savanna):
Found in most of the Peninsular Plateau (Maharashtra, Telangana, Andhra). 'Savanna' means grasslands. It rains in summer but has a long dry winter.
Found in most of the Peninsular Plateau (Maharashtra, Telangana, Andhra). 'Savanna' means grasslands. It rains in summer but has a long dry winter.
3. As (Monsoon with Dry Summer):
Found in the Coromandel Coast (Tamil Nadu). This is unique because it remains dry in summer (June-Sept) and gets rain in winter (Oct-Dec) from the retreating monsoon.
Found in the Coromandel Coast (Tamil Nadu). This is unique because it remains dry in summer (June-Sept) and gets rain in winter (Oct-Dec) from the retreating monsoon.
B. Dry Regions (Group B)
4. BWhw (Hot Desert):
Found in the extreme West Rajasthan (Thar Desert). 'W' stands for Wasteland (Desert). Rainfall is very low (<25 cm).
Found in the extreme West Rajasthan (Thar Desert). 'W' stands for Wasteland (Desert). Rainfall is very low (<25 cm).
5. BShw (Semi-Arid Steppe):
Found in areas surrounding the desert (East Rajasthan, Punjab, Haryana) and the Rain-shadow region of the Peninsula (Interior Karnataka/Maharashtra). 'S' stands for Steppe (Semi-dry grass).
Found in areas surrounding the desert (East Rajasthan, Punjab, Haryana) and the Rain-shadow region of the Peninsula (Interior Karnataka/Maharashtra). 'S' stands for Steppe (Semi-dry grass).
C. Warm Temperate Regions (Group C)
6. Cwg (Monsoon with Dry Winter):
Found in the Gangetic Plain (UP, Bihar, West Bengal). It has hot summers and mild (dry) winters. 'g' stands for Gangetic plain type.
Found in the Gangetic Plain (UP, Bihar, West Bengal). It has hot summers and mild (dry) winters. 'g' stands for Gangetic plain type.
D. Cold Climate Regions (Group D)
7. Dfc (Cold Humid Winter):
Found in the North-Eastern states (Arunachal, Sikkim, Assam). Winters are cold and humid, while summers are short and cool.
Found in the North-Eastern states (Arunachal, Sikkim, Assam). Winters are cold and humid, while summers are short and cool.
E. Polar Regions (Group E)
8. E (Polar Type):
Found in the higher reaches of Jammu & Kashmir and Himachal. The temperature of the warmest month is less than 10°C.
Found in the higher reaches of Jammu & Kashmir and Himachal. The temperature of the warmest month is less than 10°C.
9. ET (Tundra Type):
Found in the very high altitudes of Uttarakhand and Ladakh. It is freezing cold with mosses and lichens.
Found in the very high altitudes of Uttarakhand and Ladakh. It is freezing cold with mosses and lichens.
Köppen’s Climatic Regions of India
| Type | Features | Regions |
|---|---|---|
| Aw (Savanna) | Hot summers (~40°C), winter >18°C, monsoon rains | Jharkhand, Odisha, Maharashtra, AP |
| Amw (Monsoon) | Short dry winter, evergreen forests | Konkan, Malabar, Mizoram, Tripura |
| As (Moist) | Annual temp ~18°C, rainfall 75–100 cm | Coromandel Coast |
| BShw (Steppe) | Semi-arid, >18°C, summer rains | E. Rajasthan, TN rain-shadow, Gujarat |
| BWhw (Desert) | Rain <25 cm, hot summers, cold winters | Thar Desert (W. Aravallis) |
| Cwg (Mesothermal) | Dry winter, hot summer | Gangetic Plains |
| Dfc (Cold Humid) | Cold winters, short summers | Sikkim, Arunachal (hills) |
| E (Polar) | Warmest month <10°C | High reaches of J&K, HP, Uttarakhand |
| ET (Tundra) | Warmest month 0–10°C | Kashmir, Ladakh, HP, Uttarakhand |
Mains Key Points
Köppen’s classification highlights India’s climatic diversity within monsoon climate framework.
Regional variations explained by relief, latitude, altitude, distance from sea, monsoon pattern.
Aw (savanna) dominates Peninsular India → monsoon-dependent agriculture.
BWhw (desert) explains arid conditions in Thar; Cwg (Gangetic Plains) explains dense population base.
Polar & Tundra types show high-altitude influence of Himalayas.
Prelims Strategy Tips
Köppen’s classification = based on temperature & rainfall.
Tropical Savanna (Aw) covers most of Peninsular India.
Hot Desert (BWhw) → Thar Desert west of Aravallis.
Cwg climate → Gangetic Plains.
Tundra type (ET) → Ladakh, higher Himalayas.
Indian Climatic Calendar
Key Point
The Indian Meteorological Department (IMD) officially divides the year into four distinct seasons. Unlike the West which has Spring/Autumn, India's calendar is dominated by the arrival and departure of the Monsoon.
The Indian Meteorological Department (IMD) officially divides the year into four distinct seasons. Unlike the West which has Spring/Autumn, India's calendar is dominated by the arrival and departure of the Monsoon.
Detailed Notes (22 points)
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1. The Cold Weather Season (Winter)
Duration: Mid-November to February (January is the coldest month).
What happens? The sun shines on the Southern Hemisphere, so India (North Hemisphere) receives slant rays and stays cold.
Key Phenomenon - Western Disturbances: Beginners often ask, "Why does it rain in Delhi in January?" The answer is Western Disturbances. These are shallow cyclonic storms coming all the way from the Mediterranean Sea. They bring rain to the plains (Punjab, Haryana) and Snowfall to the Himalayas. This rain is like gold for the Rabi crops (Wheat).
Wind Direction: Winds blow from Land to Sea (North-East Trade Winds), so they are dry and don't bring rain to most parts, except Tamil Nadu.
2. The Hot Weather Season (Summer)
Duration: March to May.
What happens? The Sun starts moving North towards the Tropic of Cancer. The land heats up intensely.
Key Phenomenon - The 'Loo': In North India (May/June), strong, hot, and dry winds blow during the day. Exposure to these can cause heatstroke.
Pre-Monsoon Showers: Towards the end of summer, local thunderstorms occur. They have interesting local names:
- Mango Showers: Occur in Kerala/Karnataka.
3. The Southwest Monsoon Season (Rainy Season)
Duration: June to September.
What happens? This is the most important season. The land is so hot that it pulls moisture-laden winds from the Indian Ocean.
Two Arms of Monsoon: The wind hits the tip of India and splits into two:
- Arabian Sea Branch: Hits the Western Ghats first (Kerala, Mumbai).
The Break: It doesn't rain every single day. There are dry spells called 'Monsoon Breaks'.
4. The Retreating Monsoon Season (Autumn)
Duration: October and November.
What happens? The land cools down, and the low pressure moves to the sea. The monsoon winds become weak and start 'Retreating' (going back) from North India.
October Heat: The month of October is uncomfortable because the temperature is still high and the land is wet (high humidity). This sticky weather is called 'October Heat'.
Cyclones: As the winds shift to the Bay of Bengal, they pick up heat and moisture, creating severe Cyclones that hit Odisha and Andhra Pradesh during this time.
Local Storms of Summer (Prelims Favorite)
| Name | Region | Significance/Impact |
|---|---|---|
| Mango Showers | Kerala & Karnataka | Helps mangoes to ripen early; occurs in late summer. |
| Blossom Showers | Karnataka & Kerala | Helps coffee flowers to bloom. |
| Nor Westers (Kalbaisakhi) | Bengal & Assam | Violent storms; good for Tea, Jute, and Rice cultivation. |
| Loo | Punjab, Bihar, Delhi | Hot, dry, dust-laden winds; harmful to health. |
| Bardoli Chheerha | Assam | Local thunderstorm in Assam. |
Mains Key Points
Winter Rain Importance: Explain how 'Western Disturbances' are crucial for India's food security (Wheat production in Punjab/Haryana).
Changing Pattern: Discuss how climate change is affecting the calendar—Summer is becoming longer, and rainfall is becoming intense but short duration (Urban Floods).
Disaster Management: The Retreating Monsoon season (Oct-Nov) is the 'Cyclone Season'. Coastal states need high alertness during this specific time.
Prelims Strategy Tips
Retreating Monsoon Rain: While the rest of India is dry in Oct-Nov, Tamil Nadu gets its main rainfall during this season.
Coldest Place: Drass (Ladakh) is the coldest inhabited place in India.
Driest Place: Jaisalmer (Rajasthan) gets the least rainfall.
First Rain: The South-West Monsoon hits the Kerala Coast on June 1st (Standard Date).
El-Nino Effect: If the exam asks "Why was the monsoon weak this year?", the answer is often El-Nino (warming of Pacific Ocean).
The Winter Season in India
Key Point
Winter in India (mid-November to February) is characterized by clear skies, low temperatures, and low humidity. While North India shivers under a 'Cold Wave', South India enjoys pleasant tropical weather. The season is vital for the 'Rabi' crop (Wheat).
Winter in India (mid-November to February) is characterized by clear skies, low temperatures, and low humidity. While North India shivers under a 'Cold Wave', South India enjoys pleasant tropical weather. The season is vital for the 'Rabi' crop (Wheat).
Detailed Notes (16 points)
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1. Why do we have Winter? (Mechanism)
It happens because of the Apparent Movement of the Sun :
- By December, the Sun migrates to the Southern Hemisphere (directly over the Tropic of Capricorn).
- India (Northern Hemisphere) receives slant sun rays, reducing heat.
- High Pressure builds up over North-West India (Punjab/Rajasthan) because the land gets cold.
2. Temperature Patterns (North vs South)
India shows a distinct contrast during winter:
North India (The Freeze): Being far from the sea (Continentality) and closer to the Himalayas, temperatures drop drastically. The average is 10°C to 15°C. Places like Dras (Ladakh) can hit -45°C.
South India (The Warmth): Being near the Equator and surrounded by the sea (Maritime Effect), it stays warm. The average temperature remains above 20°C (e.g., Chennai, Kochi). They don't experience a 'real' winter.
3. Western Disturbances (The Winter Rain)
This is a crucial concept for exams. Beginners often wonder why it rains in Delhi/Punjab in January.
Origin: These are weak cyclones that form over the Mediterranean Sea (near Europe).
Journey: The high-altitude Westerly Jet Stream blows them across Iran, Afghanistan, and Pakistan into India.
Impact: They bring light rain to the plains (good for Wheat) and heavy snowfall to the Himalayas (recharging glaciers).
4. The Northeast Monsoon (Tamil Nadu's Rain)
While the rest of India is dry, Tamil Nadu receives its heaviest rainfall (Oct-Dec) during winter. This happens because the retreating trade winds pick up moisture from the Bay of Bengal and hit the Coromandel Coast.
5. Cold Wave (शीत लहर)
A 'Cold Wave' is declared when the minimum temperature drops significantly (usually below 4°C in plains). It is caused by icy winds blowing from the Caspian Sea and Turkmenistan into North India.
Winter Phenomena: North vs South
| Feature | North India | South India |
|---|---|---|
| Temperature | Distinct Winter (Mean < 21°C). Cold Waves common. | No real Winter (Mean > 20°C). Pleasant weather. |
| Rainfall Source | Western Disturbances (Mediterranean origin). | North-East Monsoon (Retreating Monsoon). |
| Wind Direction | Land to Sea (Dry winds). | Sea to Land (Wet winds for TN coast). |
| Pressure | High Pressure zone (creates outflow of wind). | Low Pressure zone (attracts wind). |
Mains Key Points
Economic Importance: Winter rain (Western Disturbances) determines the yield of Wheat (India's second most important crop). Failure of these rains causes agricultural distress in Punjab/Haryana.
Geopolitical Significance: The Western Disturbances travel through volatile regions (Iran/Pakistan). Changes in global climate patterns are making them erratic, affecting India's food security.
Air Pollution: Winter is characterized by 'Temperature Inversion' (cold air trapped near ground). This traps pollutants, leading to severe Smog in Delhi-NCR during this season.
Prelims Strategy Tips
Mawat: The local name for winter rainfall caused by Western Disturbances in Rajasthan/Haryana is 'Mawat'. It is excellent for Rabi crops.
Isotherm: The 20°C Isotherm runs roughly parallel to the Tropic of Cancer, dividing the cold North from the warm South.
Coldest Inhabited Place: Dras (Ladakh) is often called the 'Gateway to Ladakh' and is the coldest inhabited place in India.
Jet Stream Role: The 'Sub-Tropical Westerly Jet Stream' is the main driver that brings Western Disturbances into India.
Western Disturbances
Key Point
Western Disturbances are winter storms that originate in the Mediterranean region and travel thousands of kilometers to bring rain to North India. They are the reason why Delhi gets rain in January and why the Himalayas get heavy snow. They are the lifeline for India's wheat crop.
Western Disturbances are winter storms that originate in the Mediterranean region and travel thousands of kilometers to bring rain to North India. They are the reason why Delhi gets rain in January and why the Himalayas get heavy snow. They are the lifeline for India's wheat crop.
Detailed Notes (20 points)
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1. Decoding the Name
Why are they called 'Western Disturbances'? Let's break it down to understand better:
Western: This refers to the direction. unlike the Monsoon which comes from the South-West or North-East, these storms arrive in India from the West (travelling from Europe and the Mediterranean region).
Disturbance: In meteorology, an area of Low Pressure is called a disturbance because it disturbs the calm, established air. So, it is essentially a low-pressure storm coming from the West.
2. Origin and The Journey
Birthplace: They are born over the Mediterranean Sea (near Italy and Greece) as 'Extra-Tropical Cyclones'.
The Carrier (Jet Stream): How do they travel 9,000 km to India? They are carried by a fast-moving river of air high in the sky called the Sub-Tropical Westerly Jet Stream. Think of the Jet Stream as a 'Conveyor Belt' that picks up these storms from Europe and drops them over North India.
Moisture Recharge: On their long journey, they pick up more moisture from the Black Sea and the Caspian Sea before entering India via Pakistan.
3. Mechanism in India
When these winds reach India, they hit the massive wall of the Himalayas, which forces them to shed their moisture.
In Mountains (J&K, Himachal, Uttarakhand): The temperature is freezing, so the moisture falls as Snow. This snow accumulates and later melts in summer to feed rivers like the Ganga and Yamuna.
In Plains (Punjab, Haryana, Delhi, West UP): The temperature is slightly warmer, so the precipitation falls as Rain.
4. Significance for Agriculture
This winter rainfall is locally known as 'Mawat' in Rajasthan.
Rabi Crops: It is extremely beneficial for Wheat and Mustard crops. Wheat is a temperature-sensitive crop; light rain and cold weather help the grain grow bigger and healthier.
5. Negative Impacts
While usually beneficial, they can sometimes cause problems:
Hailstorms: Strong disturbances cause large hail (ice stones), which can destroy standing crops physically.
Cold Wave: Once the disturbance passes, the sky clears up, and freezing cold winds from the Himalayas blow into the plains, causing a severe drop in temperature known as a Cold Wave.
Fog and Smog: The moisture left in the air mixes with pollutants (especially in Delhi/NCR) to create dense fog and smog, reducing visibility.
Western Disturbances: Quick Summary
| Feature | Details |
|---|---|
| Origin Place | Mediterranean Sea (Extra-tropical region) |
| Transport Mechanism | Westerly Jet Streams (High altitude winds) |
| Key Season | Winter (November to March) |
| Good for | Rabi Crops (Wheat, Barley, Mustard), Apple orchards |
| Precipitation Type | Snowfall in Himalayas; Rainfall in Plains |
Mains Key Points
Food Security: Explain how Western Disturbances directly impact India's Wheat production and food security.
Climate Change Impact: Mention that recently WDs are becoming erratic—either too little rain (hurting crops) or sudden flash floods (like the Kedarnath disaster or Joshi Math sinking issues).
Water Security: Snowfall brought by WDs acts as a 'Water Bank' stored in glaciers, which melts in summer to keep Ganga/Yamuna flowing.
Prelims Strategy Tips
Not a Monsoon: Western Disturbances are NOT part of the Monsoon system. Monsoon brings rain in Summer; WD brings rain in Winter.
Caspian Sea Connection: Though they start in the Mediterranean, they pick up moisture from the Caspian Sea and Black Sea too.
Mawat: Remember the term 'Mawat' – strictly refers to winter rainfall in Rajasthan useful for agriculture.
Direction: They move West to East (Anti-trade wind direction).
Tropical Cyclones & Precipitation in Winter Season
Key Point
Winter in India is mostly dry, but it is not entirely rainless. There are two specific 'Rainy Pockets' during winter: North-West India (due to Western Disturbances) and South-East India (due to Retreating Monsoon/Cyclones).
Winter in India is mostly dry, but it is not entirely rainless. There are two specific 'Rainy Pockets' during winter: North-West India (due to Western Disturbances) and South-East India (due to Retreating Monsoon/Cyclones).
Detailed Notes (13 points)
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1. Tropical Cyclones: Why are they rare in Jan-Feb?
Tropical cyclones require warm sea water (above 27°C) to form. In peak winter (January-February), the sea surface temperature drops, and the ITCZ (Inter-Tropical Convergence Zone) shifts far south towards the equator. Hence, cyclone formation is lowest during these months.
The Exception (Oct-Dec): During the transition period (Retreating Monsoon), the Bay of Bengal is still warm. This causes severe cyclones to hit the coasts of Odisha, Andhra Pradesh, and Tamil Nadu. Example: Cyclone Fani, Cyclone Michaung.
2. Winter Rainfall: The Tale of Two Regions
While the rest of India is dry, two specific regions receive rainfall:
A. North-West India (The Wheat Belt)
Source: Western Disturbances (from Mediterranean Sea).
Pattern: The rainfall decreases from West to East. Punjab gets more rain than Uttar Pradesh, and Bihar gets very little. Note: This is the exact opposite of the Summer Monsoon pattern.
Significance: Essential for the Rabi crop (Wheat and Mustard).
B. South-East India (The Rice Bowl of TN)
Source: North-East Monsoon (Retreating Monsoon).
Mechanism: Dry winds blow from the land (India) towards the sea. However, when they cross the Bay of Bengal, they pick up moisture. These moisture-laden winds then hit the Eastern Ghats and cause heavy rain in Tamil Nadu and southern Andhra Pradesh.
Pattern: Rainfall decreases from Coast to Interior. Chennai (Coast) gets more rain than Salem (Interior).
Comparison: Winter Rain in North vs. South
| Feature | North-West India (Punjab/Haryana) | South-East India (Tamil Nadu/Andhra) |
|---|---|---|
| Source | Western Disturbances (Cyclonic) | North-East Monsoon (Trade Winds) |
| Direction of Rain | Decreases West to East (Punjab → Bihar) | Decreases East to West (Coast → Interior) |
| Major Crop | Wheat (Rabi) | Rice (Rabi/Winter Rice) |
| Nature | Light to Moderate Rain (Beneficial) | Heavy Rain (Often causes Floods in Chennai) |
Winter Cyclones & Rainfall in India
| Aspect | Details |
|---|---|
| Cyclone Activity | Lowest in winter; due to low SST & southward ITCZ |
| Bay of Bengal Cyclones | Strike Tamil Nadu; bring heavy rain |
| Arabian Sea Cyclones | Few; move north or west |
| Rainfall in TN & SE Coast | ~75 cm (Oct–Dec); due to NE monsoon |
| Rainfall in NW India | Western disturbances → light winter rain |
| Rainfall in NE India | Light showers in Assam, Arunachal |
Mains Key Points
Disaster Preparedness: The Eastern Coast (Odisha/Andhra) faces a 'Cyclone Season' right after the monsoon withdraws (Oct-Dec). This requires specific disaster management strategies distinct from flood management.
Cropping Pattern: The winter rain defines the agricultural map of India—Wheat in the North (temperate crop) and Rice in the South (tropical crop) during the same season.
Climate Change: Winter rains are becoming erratic. A lack of Western Disturbances leads to a drop in wheat yield (heat stress), while excess NE Monsoon rain leads to urban flooding in Chennai.
Prelims Strategy Tips
Opposite Trend: In Summer Monsoon, rainfall decreases from East to West (Kolkata to Delhi). In Winter (Western Disturbance), rainfall decreases from West to East (Delhi to Kolkata).
Cyclone Peak: The peak season for cyclones in the Bay of Bengal is October-November, NOT January-February.
Why Tamil Nadu? Tamil Nadu is the only major state that receives more rain in winter (via NE Monsoon) than in summer (via SW Monsoon).
Arunachal Exception: Apart from NW India, Arunachal Pradesh also receives some rain/snow in winter due to Western Disturbances.
Summer Season in India (Hot Weather Season)
Key Point
Summer season (March–May) is marked by the northward migration of the sun, intense heating of North India, and the formation of a low-pressure trough. It sets the stage for the monsoon by shifting the ITCZ and splitting jet streams.
Summer season (March–May) is marked by the northward migration of the sun, intense heating of North India, and the formation of a low-pressure trough. It sets the stage for the monsoon by shifting the ITCZ and splitting jet streams.
Detailed Notes (25 points)
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1. Causes & Timing (Why and When)
Begins after the March Equinox (21st March) and continues until the Summer Solstice (21st June).
The Sun moves northwards (Uttarayan) towards the Tropic of Cancer.
This shift causes the region of intense heat to move north progressively:
- March: Heat peaks in Deccan Plateau (approx. 38°C).
- April: Heat peaks in Gujarat & Madhya Pradesh (approx. 42°C).
- May: Heat peaks in North/North-West India (can exceed 45°C).
Note: South India remains moderate due to the cooling influence of the oceans (Maritime Effect).
2. Pressure & Wind Patterns
High solar radiation creates a Thermal Low-Pressure Trough over the land.
This trough creates a suction effect, extending from the Thar Desert (Punjab plains) in the NW to the Chota Nagpur Plateau (Bengal delta) in the East.
As pressure drops over the land, winds begin to be drawn inwards from the surrounding high-pressure seas.
Loo: Hot, dry, and gusty winds blow during the day over North India (Punjab, Haryana, UP, Bihar). Direct exposure can be fatal.
3. Role of Jet Streams & ITCZ
ITCZ Migration: The Inter-Tropical Convergence Zone (a low-pressure belt) shifts north towards the Tropic of Cancer, attracting moisture.
Jet Stream Splitting (April): The Sub-Tropical Jet Stream (STJ) splits into two branches:
- Northern Branch: Moves to the north of the Tibetan Plateau.
- Southern Branch: Disappears from the Indian plains, removing the 'lid' that was holding back the monsoon.
This shift is the critical trigger for the sudden arrival (burst) of the Monsoon later.
4. Pre-Monsoon Local Storms
As the ITCZ shifts, the contact between hot land air and moist sea air causes violent local storms.
Mango Showers: Kerala & Karnataka. They help in the early ripening of mangoes.
Blossom Showers: Kerala & Karnataka. Beneficial for coffee flowers.
Nor Westers (Kalbaisakhi): Bengal & Assam. Dreaded evening thunderstorms useful for tea, jute, and rice cultivation.
Andhi (Dust Storms): NW India. They bring temporary relief by dropping temperatures.
Summer Season Overview
| Aspect | Details |
|---|---|
| Duration | March – May (Equinox to Solstice) |
| Temperature Pattern | Heat shifts North; Highs in Deccan (March) → NW India (May) |
| Pressure System | Low-pressure trough extends from Punjab to Bengal |
| Jet Streams | STJ splits; Southern branch leaves Indian plains |
| Winds | Loo (Hot/Dry) in North; Sea breeze near coasts |
Local Storms (High Yield for Prelims)
| Local Name | Region | Significance |
|---|---|---|
| Mango Shower | Kerala / Karnataka | Helps ripen mangoes |
| Blossom Shower | Kerala / Karnataka | Helps coffee flowers bloom |
| Kalbaisakhi | Bengal / Assam | Good for Tea, Jute, Rice |
| Loo | North Plains (UP/Bihar) | Hot dry wind (Harmful) |
| Bardoli Chheerha | Assam | Local thunderstorm |
Mains Key Points
Dependence on Global Circulation: Summer highlights how Indian climate relies on global factors like the shifting ITCZ and Jet Streams.
Monsoon Preparation: The intense heating creates the thermal low needed to pull moisture-laden winds from the Indian Ocean.
Agriculture Impact: While the season is dry, pre-monsoon showers are economically vital for crops like Tea (Assam), Coffee (Karnataka), and Mangoes.
Prelims Strategy Tips
Temperature Gradient: Remember that temperature increases from South to North during summer (unlike winter when it increases North to South).
Storm Matching: UPSC often asks to match the storm name with the state (e.g., Kalbaisakhi = Bengal).
Jet Stream Rule: The withdrawal of the Westerly Jet Stream from the southern slopes of the Himalayas is the immediate cause of the Monsoon burst.
Indian Monsoon – Mechanism, Theories & Variability
Key Point
The Indian Monsoon is a large-scale seasonal reversal of winds. It is not just a simple land-sea breeze but a complex interaction of the shifting ITCZ, Jet Streams, Tibetan heating, and ocean phenomena like El Niño. It acts as the 'economic lifeline' of India.
The Indian Monsoon is a large-scale seasonal reversal of winds. It is not just a simple land-sea breeze but a complex interaction of the shifting ITCZ, Jet Streams, Tibetan heating, and ocean phenomena like El Niño. It acts as the 'economic lifeline' of India.
Detailed Notes (32 points)
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1. What is the Monsoon? (Beginner Concept)
The word 'Monsoon' comes from the Arabic word 'Mausim' meaning season.
It is essentially a seasonal reversal of winds:
- Summer: Winds blow from Sea to Land (South-West Monsoon) → Brings Rain.
- Winter: Winds blow from Land to Sea (North-East Monsoon) → Dry (except Tamil Nadu).
2. The Two Main Branches
When the SW Monsoon hits the tip of India (Kerala), it splits into two branches:
A. Arabian Sea Branch:
- Hits the Western Ghats causing heavy rainfall in Kerala/Konkan coast.
- Moves towards Gujarat and Rajasthan but produces little rain there because the Aravalli Hills run parallel to the winds.
B. Bay of Bengal Branch:
- Moves towards North-East India (Meghalaya) causing world-record rainfall.
- Deflects westwards towards the Ganga Plains (Bihar/UP/Delhi).
3. Theories: Why does it happen?
A. Thermal Theory (The Simple View):
Land heats up faster than the sea. Hot air over land rises (Low Pressure), and cool air from the sea rushes in (High Pressure) carrying moisture.
B. Dynamic Theory (ITCZ Shift):
The Inter-Tropical Convergence Zone (ITCZ) acts like a 'Low-Pressure Magnet'. In summer, it shifts north over the Ganga plains, pulling the Trade Winds across the equator.
C. Jet Stream Theory (The 'Lid' Concept):
- Winter: The Westerly Jet Stream sits over North India, acting like a 'lid' that prevents air from rising.
- Summer: This Jet Stream moves north of the Himalayas. The 'lid' is removed, allowing a sudden burst of winds.
4. Complex Global Factors (Teleconnections)
The monsoon is not local; it is connected to the world ocean systems:
1. El Niño (The Villain): Warming of the Pacific Ocean coast (Peru). It weakens the Indian Monsoon (causes droughts).
2. La Niña (The Hero): Cooling of the Pacific. It strengthens the Indian Monsoon (causes floods/good rain).
3. Indian Ocean Dipole (IOD): The 'Indian Niño'.
- Positive IOD: Western Indian Ocean is warmer = Good for Monsoon.
- Negative IOD: Eastern Indian Ocean is warmer = Bad for Monsoon.
5. Critical Phases
Onset (The Burst): Usually hits Kerala on June 1st. It is sudden and accompanied by thunder.
Break in Monsoon: A dry spell of 1-2 weeks during the rainy season (July/Aug) when the monsoon trough shifts to the foothills of the Himalayas.
Retreating Monsoon: By September, the land cools down, and winds start withdrawing from Punjab towards the south.
SW Monsoon vs. NE Monsoon
| Feature | SW Monsoon (Summer) | NE Monsoon (Winter) |
|---|---|---|
| Direction | Sea to Land (SW to NE) | Land to Sea (NE to SW) |
| Rainfall Source | Evaporation from Arabian Sea & BoB | Moisture picked up from Bay of Bengal |
| Region Benefited | Entire India (~75% rain) | Mainly Coromandel Coast (Tamil Nadu) |
| Timing | June to September | October to December |
Complete Breakdown of Monsoon Determinants
| Factor / Theory | Mechanism (How it works) | Impact on India |
|---|---|---|
| Thermal Theory (Halley) | Differential heating of Land (hot) vs Sea (cool). | Creates the basic Low Pressure needed for winds to blow inwards. |
| Dynamic Theory (ITCZ) | Northward shift of ITCZ pulls SE Trade winds across equator. | Coriolis force deflects winds to become South-West (direction). |
| Jet Stream Theory | Westerly Jet withdraws north; Easterly Jet appears over peninsula. | Burst of Monsoon (Onset) and steering of cyclones. |
| Tibetan Plateau | Intense heating at high altitude creates a thermal engine. | Strengthens the low pressure; Stronger vacuum = Better Rain. |
| ENSO (Pacific Ocean) | El Niño: Warm Central Pacific water. La Niña: Cool Central Pacific water. | El Niño = Weak Monsoon (Droughts). La Niña = Strong Monsoon (Floods). |
| Indian Ocean Dipole (IOD) | Positive: Warmer Arabian Sea. Negative: Warmer Eastern Indian Ocean. | Positive IOD can neutralize El Niño effect and bring good rain. |
| Madden-Julian Oscillation (MJO) | Eastward moving pulse of clouds/rainfall near equator. | Causes intra-seasonal variation (Active spells vs. Breaks). |
| Western Disturbances | Low-pressure systems from Mediterranean Sea (Westerlies). | Brings Winter Rainfall to NW India (Vital for Rabi/Wheat crops). |
Mains Key Points
Impact on Economy: Describe Monsoon as the 'Finance Minister of India'. A good monsoon lowers inflation (food prices) and boosts rural demand.
Climate Change: Mention that Global Warming is making the monsoon 'erratic'—fewer rainy days but more intense rainfall (leading to urban floods).
Water Crisis: Connect monsoon failure to the need for river interlinking and rainwater harvesting.
Prelims Strategy Tips
Date of Onset: 1st June (Kerala), 25th May (Andaman & Nicobar).
Rain Shadow Area: The Leeward side of Western Ghats (Pune, Vidarbha) receives less rain because winds lose moisture after crossing the peaks.
Winter Rain: Tamil Nadu receives most of its rain from the North-East Monsoon (Retreating Monsoon) in Nov-Dec.
El Niño Modoki: A special type of El Niño that affects India differently (Advanced topic, keep in mind).
Dynamic Concept of Monsoon (ITCZ Theory)
Key Point
Proposed by H. Flohn (1951) , this theory moves beyond simple land-heating. It explains the Monsoon as a global phenomenon resulting from the seasonal shifting of pressure belts and the ITCZ (Inter-Tropical Convergence Zone) under the influence of the Sun.
Proposed by H. Flohn (1951) , this theory moves beyond simple land-heating. It explains the Monsoon as a global phenomenon resulting from the seasonal shifting of pressure belts and the ITCZ (Inter-Tropical Convergence Zone) under the influence of the Sun.
Detailed Notes (17 points)
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1. What is the Dynamic Theory? (Beginner's View)
Unlike the Classical Theory which viewed Monsoon as a local 'Land-Sea Breeze', Flohn argued that the Monsoon is actually a modification of the Planetary Wind System .
It is driven by the seasonal movement of the ITCZ (Inter-Tropical Convergence Zone).
Analogy: Think of the ITCZ as a 'Vacuum Cleaner' of the atmosphere. Wherever the Sun goes, the ITCZ follows, sucking in winds from both hemispheres.
2. Understanding the ITCZ
The ITCZ is a low-pressure zone near the equator where the Trade Winds from the Northern and Southern Hemispheres collide (converge) and rise.
It is also called the Thermal Equator because it is the zone of maximum heat.
NITC & SITC: The northern boundary is the Northern Inter-Tropical Convergence (NITC), and the southern is the SITC. Between them lies the 'Doldrums' (calm zone).
3. Summer Mechanism (The Onset)
Step 1: As the Sun moves north (Summer Solstice), the ITCZ shifts northward towards the Ganga Plains (20°N - 25°N) .
Step 2: This shift creates a massive Low-Pressure trough over North India (often called the Monsoon Trough).
Step 3: The South-East Trade Winds (from the Southern Hemisphere) cross the Equator to fill this vacuum.
Step 4 (The Twist): Due to Coriolis Force (Ferrel’s Law), these winds turn RIGHT after crossing the equator, becoming the South-West Monsoon .
4. Winter Mechanism (The Retreat)
As the Sun moves south towards the Tropic of Capricorn (Winter Solstice), the ITCZ shifts southward.
The low-pressure zone over India disappears and is replaced by high pressure.
The winds revert to their normal planetary direction: North-East Trade Winds (blowing from Land to Sea → Dry Winter).
Halley's Thermal vs. Flohn's Dynamic Theory
| Feature | Classical (Halley, 1686) | Dynamic (Flohn, 1951) |
|---|---|---|
| Core Concept | Simple Land vs. Sea heating contrast. | Seasonal shift of Global Pressure Belts (ITCZ). |
| Scope | Localized (Focused only on India). | Global (Applies to Africa, Australia, etc.). |
| Wind Origin | Local winds generated by thermal gap. | Planetary Trade Winds crossing the equator. |
| Status | Outdated (cannot explain sudden burst). | Widely accepted (but needs Jet Stream for full picture). |
Mains Key Points
Evolution of Thought: Use this theory to show how geographical thought evolved from local thermal observations to global atmospheric dynamics.
Connecting Concepts: When writing about the Monsoon, always link the ITCZ shift with the 'Tibetan Heating' (which intensifies the low pressure) and 'Jet Streams' (which trigger the burst).
Global Similarity: Mention that the same ITCZ shift causes monsoons in Northern Australia and West Africa, proving Flohn's global perspective.
Prelims Strategy Tips
Flohn's Contribution: Remember that Flohn replaced the idea of 'Local Breeze' with 'Planetary Winds'.
NITC Position: In July, the NITC (Northern boundary of ITCZ) extends up to 20°-25° N latitude (over the Gangetic Plain).
The Driving Force: The primary driver here is the Apparent Movement of the Sun.
Correction: While ITCZ explains the direction , it struggles to explain the intensity and irregularity (Breaks) of the monsoon.
Jet Streams: Types, Theory & Impact on India
Key Point
Jet Streams are narrow bands of high-speed winds in the upper troposphere (7.5–14 km) that steer global weather. In India, the Sub-Tropical Westerly Jet (STJ) brings winter rain (Western Disturbances), while the Tropical Easterly Jet (TEJ) drives the summer monsoon.
Jet Streams are narrow bands of high-speed winds in the upper troposphere (7.5–14 km) that steer global weather. In India, the Sub-Tropical Westerly Jet (STJ) brings winter rain (Western Disturbances), while the Tropical Easterly Jet (TEJ) drives the summer monsoon.
Detailed Notes (18 points)
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1. What are Jet Streams? (Concept)
They are like 'Highways of Air' flowing from West to East at high altitudes.
Discovery: Found during World War II by pilots who noticed their planes were flying slower against strong headwinds.
Formation: They form where cold polar air meets warm tropical air. The greater the temperature difference, the faster the wind blows.
Seasonality: They are stronger and wider in winter because the temperature contrast between the poles and equator is maximum.
2. Key Properties
Meandering Path: They do not move in a straight line but in wavy paths called Rossby Waves.
Velocity: Ranges from 108 km/h to over 480 km/h (fastest in the core).
Location: Upper Troposphere (Tropopause), generally between 20°–60° latitudes.
3. The Jet Stream Theory of Monsoon
Proposed by P. Koteswaram and H. Flohn. It links the Monsoon to the movement of these jets.
A. Winter Scenario (The Barrier): The STJ flows south of the Himalayas (over North India). It prevents air from rising, keeping North India dry and cold (except for Western Disturbances).
B. Summer Scenario (The Trigger): As the sun moves north, the STJ shifts North of the Himalayas. This removal acts like opening a valve, allowing the Southwest Monsoon to 'burst' onto the Indian landmass.
C. The Enhancer (TEJ): Simultaneously, the intense heating of the Tibetan Plateau creates the Tropical Easterly Jet (TEJ), which actively pulls the monsoon winds inward.
4. Significance for India
Rabi Crops: The STJ carries moisture from the Mediterranean Sea (Western Disturbances) to Punjab/Haryana, which is vital for Wheat cultivation.
Monsoon Strength: A delay in the northward shift of the STJ can delay the monsoon. A weak TEJ results in a weak monsoon (droughts).
Aviation: Flights from Delhi to London (Westward) take longer because they fly against the Jet Stream. Flights from London to Delhi (Eastward) are faster.
Comprehensive List of Jet Streams (Updated)
| Type | Location / Altitude | Nature & Characteristics | Role / Impact on India |
|---|---|---|---|
| Polar Front Jet | 40°–60° Latitude (Upper Troposphere) | Formed by irregular convergence of polar & tropical air. Meandering path. | Indirectly affects global weather; can steer cold waves. |
| Sub-Tropical Westerly Jet (STJ) | 30°–35° Latitude (Upper Troposphere) | highly regular; flows West to East throughout the year. | Winter: Brings Western Disturbances (Rain/Snow). Summer: Its withdrawal triggers Monsoon onset. |
| Tropical Easterly Jet (TEJ) | Over Peninsular India (Summer Only) | Seasonal; flows East to West. Driven by Tibetan heating. | Crucial for Monsoon rain. Steers tropical depressions/cyclones into India. |
| Polar Night Jet | Stratosphere (~30km height) | Forms only in Winter around poles. Very high speed due to thermal gradient. | Minimal direct impact on Indian Monsoon; part of global stratospheric circulation. |
| Local Jet Streams | Regional / Small scale | Caused by local thermal contrasts (e.g., coastal winds). | Short-term impact; e.g., Low-level Somali Jet aids monsoon flow. |
Mains Key Points
Mechanistic View: Use the Jet Stream theory to explain the 'Burst of Monsoon' (dynamic cause) which the classical thermal theory failed to explain.
Economic Impact (Winter): Highlight the positive role of STJ in bringing winter rain for Rabi crops (Wheat) in Punjab and Haryana.
Global Linkage: Discuss how Jet Streams connect Indian weather to global events (e.g., disturbances from the Mediterranean, heating in Tibet).
Prelims Strategy Tips
Koteswaram's Role: The Jet Stream theory regarding the Indian Monsoon was largely developed by the Indian meteorologist P. Koteswaram.
Altitude Distinction: Note that the Polar Night Jet exists in the Stratosphere, while STJ and TEJ are in the Troposphere.
Western Disturbances: They are extratropical storms originating in the Mediterranean, but the STJ is the 'carrier' that brings them to India.
Speed Rule: Jet streams are fastest in winter because the temperature difference between the equator and poles is highest.
Impact of Jet Streams: Weather, Aviation & Monsoon
Key Point
Jet Streams act as the 'Steering Wheel' of the atmosphere. They move weather systems (storms/cyclones) around the globe, save time/fuel for airlines, and act as the 'Gatekeeper' for the Indian Monsoon.
Jet Streams act as the 'Steering Wheel' of the atmosphere. They move weather systems (storms/cyclones) around the globe, save time/fuel for airlines, and act as the 'Gatekeeper' for the Indian Monsoon.
Detailed Notes (18 points)
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1. Impact on Weather & Cyclones
The Conveyor Belt: Jet streams move air masses. If they slow down, weather gets 'stuck' (causing prolonged heatwaves or floods).
Cyclogenesis: They pump air upwards, creating low pressure on the ground. This intensifies Temperate Cyclones and storms.
Ridges & Troughs (Rossby Waves):
- Ridges (High Pressure): Bring clear skies and heatwaves.
- Troughs (Low Pressure): Bring cloudy weather and rainfall.
2. Impact on Aviation (The Tailwind Effect)
Fuel & Time Savings: Pilots use Jet Streams like a 'moving walkway' at an airport.
- West to East (e.g., New York to London): Flying with the Jet Stream makes the flight faster (Tailwind).
- East to West (e.g., London to New York): Flying against the Jet Stream takes longer (Headwind).
The Danger (CAT): Jet Streams cause Clear Air Turbulence (CAT). This is dangerous because it occurs in cloudless skies and cannot be detected by standard weather radar.
3. Impact on Indian Monsoon (Critical)
The Switch: The shift of the Sub-Tropical Westerly Jet (STJ) from south of the Himalayas to the north is the 'switch' that starts the monsoon.
The Engine: The Tropical Easterly Jet (TEJ) acts as an engine that pulls the moist winds deeper into India.
Winter Rain: The STJ brings Western Disturbances (Mediterranean storms) to North India, which is crucial for Wheat (Rabi crops).
4. Impact on Agriculture
Kharif Crops (Rice/Millet): Depend on the timely formation of TEJ for summer rain.
Rabi Crops (Wheat/Mustard): Depend on the STJ for winter rain/snowfall (which melts later to feed rivers).
Jet Streams: Benefits vs. Challenges
| Sector | Positive Impact (Benefit) | Negative Impact (Challenge) |
|---|---|---|
| Aviation | Saves time/fuel on West-to-East flights (Tailwinds). | Causes dangerous Clear Air Turbulence (CAT); Delays East-to-West flights. |
| Monsoon | TEJ strengthens the monsoon rainfall (Good for Rice). | Delayed withdrawal of STJ causes drought/delayed monsoon onset. |
| Winter Weather | Brings rain for Wheat crops (Western Disturbances). | Can bring severe Cold Waves by pushing polar air downwards. |
| Cyclones | Helps ventilate storms (making them move faster). | Can intensify storms into super-cyclones by increasing suction from above. |
Mains Key Points
Aviation Economics: Analyze how knowledge of Jet Streams helps airlines save millions in fuel costs (Economic Geography).
Climate Change: Discuss how Global Warming is slowing down jet streams (making them more wavy), leading to 'Stuck Weather' patterns (floods staying in one place for days).
Food Security: Explain the dual role: STJ for Food Security (Wheat/Winter) and TEJ for Food Security (Rice/Summer).
Prelims Strategy Tips
Clear Air Turbulence (CAT): Always associated with Jet Streams. It is invisible to weather radars.
Flight Duration: A flight from Delhi to Tokyo (Eastward) is faster than Tokyo to Delhi (Westward) due to Jet Streams.
Ozone Depletion: Jet streams can trap ozone-depleting substances near the poles (Polar Stratospheric Clouds).
Heat Domes: When a Jet Stream creates a large meandering loop (Ridge), it can trap heat, causing extreme heatwaves (Example: Canada/USA heatwaves).
Jet Stream Theory: The Engine of Indian Monsoon
Key Point
The Jet Stream Theory provides the most accurate explanation for the Monsoon's sudden 'burst'. It explains how the Tibetan Plateau (The Heater) , the STJ (The Gatekeeper) , and the Somali Jet (The Feeder) work together to drive rain over India.
The Jet Stream Theory provides the most accurate explanation for the Monsoon's sudden 'burst'. It explains how the Tibetan Plateau (The Heater) , the STJ (The Gatekeeper) , and the Somali Jet (The Feeder) work together to drive rain over India.
Detailed Notes (17 points)
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1. Sub-Tropical Westerly Jet (STJ) – The Gatekeeper
Winter (The Barrier): The STJ flows South of the Himalayas. It suppresses rising air, creating high pressure over North India. This prevents the monsoon winds from entering.
Summer (The Opening): As summer peaks, the STJ suddenly shifts North of the Himalayas (towards China).
Result: The removal of this 'lid' creates a sudden vacuum. The trade winds rush in to fill it, causing the Sudden Burst of Monsoon.
2. Tibetan Plateau – The Heat Engine
The Concept: The Tibetan Plateau is a massive landmass (4000m high). In summer, it gets hotter than the surrounding air at that altitude.
The Effect: It acts like a 'Hot Plate' in the sky. The air above it becomes warm and rises rapidly.
This rising air hits the upper atmosphere and spreads outwards, creating the Tropical Easterly Jet (TEJ).
3. Tropical Easterly Jet (TEJ) – The Rainmaker
Origin: Born from the heat of the Tibetan Plateau.
Direction: Flows from East to West over Peninsular India (roughly over Bengaluru/Chennai latitudes).
Function: It acts like a pump. It pushes air down over the Indian Ocean (High Pressure), which forces more moisture-laden winds back towards India (Low Pressure).
Equation: Stronger Heating of Tibet = Stronger TEJ = Heavier Rainfall.
4. Somali Jet (Findlater Jet) – The Moisture Feeder
Distinct Feature: Unlike STJ and TEJ which are high-altitude (Upper Troposphere), the Somali Jet is a Low-Level Jet Stream (moves near the surface).
Path: It originates near Madagascar, crosses the Equator, runs along the coast of Somalia, and crashes into India's West Coast.
Role: It acts like a 'Firehose', funneling massive amounts of moisture from the ocean directly into India.
Role of Atmospheric Players in Monsoon
| Factor | Type / Altitude | Key Function | Impact of Failure |
|---|---|---|---|
| STJ (Westerly Jet) | High Level (9-12 km) | Trigger: Its northward shift allows monsoon entry. | Delayed Shift = Delayed Monsoon Onset |
| TEJ (Easterly Jet) | High Level (upper air) | Engine: Intensifies the Low Pressure over India. | Weak TEJ = Droughts (Weak Rainfall) |
| Tibetan Plateau | Landmass (Heating Source) | Generator: Creates the heat low that forms the TEJ. | Less Snow/Heat on Plateau = Weak Monsoon |
| Somali Jet | Low Level (1-1.5 km) | Transporter: Carries moisture from ocean to land. | Weak Somali Jet = Dry Spell on West Coast |
| El Niño | Ocean Current (Pacific) | Disruptor: Sinks air over India (High Pressure). | Causes Weak TEJ and Weak Monsoon |
Mains Key Points
Integrative Answer: A good Mains answer must connect the Thermal factor (Tibet heating) with the Dynamic factor (Jet shift). You cannot explain the monsoon with just one.
Impact of Global Warming: If the Tibetan Plateau warms up less due to pollution/dust (Global Dimming), the thermal gradient weakens → Weaker TEJ → Weaker Monsoon.
Teleconnections: Mention how events in Antarctica (Mascarene High) pushes the Somali Jet, showing how connected the global climate system is.
Prelims Strategy Tips
Findlater Jet: This is another name for the Somali Jet. Do not get confused.
Altitude Trap: UPSC acts: 'TEJ is a low-level jet'. False. TEJ is High Level; Somali Jet is Low Level.
Duration: STJ is permanent (shifts position). TEJ is seasonal (only in summer).
Mawsynram Link: The Bay of Bengal branch is driven by the general monsoon circulation, but local hills (funnel shape) cause the extreme rain, not just the jets.
Monsoon in India – Mechanism, Branches & Variability
Key Point
India experiences two monsoon systems: the Southwest Monsoon (Summer) and the Northeast Monsoon (Winter). The mechanism involves a complex interaction of the ITCZ shift, Jet Streams, and global pressure belts like the Mascarene High.
India experiences two monsoon systems: the Southwest Monsoon (Summer) and the Northeast Monsoon (Winter). The mechanism involves a complex interaction of the ITCZ shift, Jet Streams, and global pressure belts like the Mascarene High.
Detailed Notes (27 points)
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1. Seasonal Wind Systems
India is the quintessential 'Monsoon Land'. The climate is dominated by:
- Southwest Monsoon (June–Sept): Blows from Sea to Land. Brings 75% of rainfall.
- Northeast Monsoon (Oct–Dec): Blows from Land to Sea. Brings rain mainly to Tamil Nadu.
2. The Onset Mechanism (The Burst)
The Trigger: In late May, the Subtropical Westerly Jet (STJ) suddenly shifts from the south of the Himalayas to the north.
The Vacuum: This shift removes the high-pressure 'lid' over North India. A massive Low-Pressure trough creates a suction effect.
The Rush: Winds from the Indian Ocean rush in to fill this void. They strike the Kerala coast around June 1st with thunder and lightning. This is called the 'Burst of Monsoon'.
3. The Engine: Mascarene High & Coriolis Force
The Push: The Mascarene High (High Pressure near Madagascar) pushes South-East Trade Winds towards the Equator.
The Turn: As these winds cross the equator, the Coriolis Force deflects them to the RIGHT. They become South-West winds.
4. The Two Branches of SW Monsoon
Due to the triangular shape of Peninsular India, the monsoon splits into two:
A. Arabian Sea Branch (More Powerful):
- Western Ghats: Hits perpendicular to the mountains, causing heavy orographic rain on the windward side (Mumbai/Kerala). The leeward side (Pune) remains in the Rain Shadow.
- North Movement: Moves towards Gujarat and Rajasthan. However, it causes little rain in Rajasthan because the Aravalli Hills run parallel to the wind direction, offering no barrier.
B. Bay of Bengal Branch:
- North-East India: Funneled into the Garo, Khasi, and Jaintia hills. The funnel shape traps winds, causing world-record rain in Mawsynram.
- Ganga Plains: Deflected by the Himalayas to the West, bringing rain to Bihar, UP, and Punjab.
5. Retreating Monsoon & October Heat
Withdrawal: Starts in September from Rajasthan. The ITCZ moves south, and the high-pressure system builds up over land.
October Heat: During the transition (October), the sky is clear, but the land is still moist. This results in high temperatures and high humidity, creating a sultry and oppressive weather condition known as 'October Heat'.
Northeast Monsoon: By November, dry winds blow from land to sea. They pick up moisture over the Bay of Bengal and dump it on the Coromandel Coast (Tamil Nadu).
6. Variability: Why do Monsoons Fail?
El Niño: Warming of the Pacific Ocean disrupts the Walker Cell, causing high pressure over India (Drought risk).
Break in Monsoon: A dry spell during the rainy season (July/Aug) when the monsoon trough shifts to the Himalayan foothills. It rains in the mountains (floods) but the plains are dry.
Indian Ocean Dipole (IOD): A 'Positive IOD' (warm Arabian Sea) can save the monsoon even during El Niño years.
Why Certain Regions are Dry?
| Region | Reason for Low Rainfall |
|---|---|
| Tamil Nadu (in Summer) | It lies in the Rain Shadow Area of the Western Ghats for the Arabian Sea branch. It receives rain only in Winter (NE Monsoon). |
| Western Rajasthan | The Aravalli Hills run parallel to the incoming wind direction, failing to stop the winds to cause rain. |
| Ladakh | It is located north of the Himalayas, which block moisture-laden winds (Cold Desert). |
| Central Maharashtra (Vidarbha) | Located on the leeward side of the Western Ghats (Rain Shadow effect). |
Phases of Indian Monsoon
| Phase | Features |
|---|---|
| Onset | ITCZ shift, STJ moves north, Kerala gets first rains (June 1) |
| Progression | Covers whole India by mid-July; Arabian & Bay branches spread rains |
| Break Monsoon | Temporary pause; rains confined to foothills |
| Withdrawal | Begins Rajasthan (Sept), completes by Oct |
| Northeast Monsoon | Oct–Dec; affects TN, AP, SE Karnataka, Kerala |
Mains Key Points
Geographical Determinism: Discuss how the shape of India (Peninsular) and the alignment of hills (Western Ghats vs Aravallis) dictates the rainfall pattern.
Economic Impact: Analyze how a 'Break in Monsoon' impacts the critical growth phase of Kharif crops.
Climate Resilience: Contrast the flood-prone nature of the Brahmaputra valley vs. the drought-prone nature of Marathwada within the same monsoon season.
Prelims Strategy Tips
Dates: Onset in Andaman & Nicobar is May 25th; Kerala is June 1st.
Direction Change: The SE Trade winds turn into SW Monsoon winds due to Ferrel’s Law (Coriolis Force).
October Heat: Refers to the oppressive humid heat experienced in October due to retreating monsoon transition.
Wettest Place: Mawsynram (Meghalaya) due to the funneling effect of hills, not just the monsoon winds.
Burst of Monsoon & Bay of Bengal Branch
Key Point
The 'Burst of Monsoon' marks the sudden onset of heavy rainfall, distinguishing it from pre-monsoon showers. The Bay of Bengal Branch is the lifeline for North and East India, getting deflected by the Arakan Yoma mountains and channeling rain into the Ganga Plains and Brahmaputra Valley.
The 'Burst of Monsoon' marks the sudden onset of heavy rainfall, distinguishing it from pre-monsoon showers. The Bay of Bengal Branch is the lifeline for North and East India, getting deflected by the Arakan Yoma mountains and channeling rain into the Ganga Plains and Brahmaputra Valley.
Detailed Notes (25 points)
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1. What is the 'Burst of Monsoon'?
Definition: It is the sudden onset of moisture-laden winds associated with violent thunder and lightning.
Key Difference: Unlike 'Pre-Monsoon Showers' (which are temporary and local like Mango Showers), the 'Burst' brings prolonged rainfall that lasts for days.
Trigger: It happens when the Sub-Tropical Westerly Jet Stream (STJ) leaves North India, creating a sudden vacuum that pulls the winds in.
2. The Bay of Bengal Branch: Path & Mechanics
This branch flows over the warm Bay of Bengal, picking up immense moisture.
The Deflection (Crucial Concept):
- Winds head towards Myanmar but hit the Arakan Yoma mountains.
- Since they cannot cross these high mountains, a large portion is deflected towards the Indian subcontinent (West Bengal and Bangladesh).
3. The Two Sub-Branches
Once it hits the Indian landmass, the Himalayas divide it into two:
A. The Northern Branch (Brahmaputra Valley):
- Flows into Northeast India.
- Hits the Garo, Khasi, and Jaintia hills.
- Funnel Effect: The hills form a funnel shape, trapping the wind. The air is forced to rise abruptly, causing heavy rainfall in Mawsynram (Meghalaya).
B. The Western Branch (Ganga Plains):
- Moves westward along the foothills of the Himalayas.
- Brings rain to West Bengal, Bihar, UP, and finally Punjab/Haryana.
- Merging: In Punjab/Haryana, this branch meets the Arabian Sea branch.
4. Why Rainfall Decreases East to West?
As the Ganga Plains branch moves from Kolkata to Delhi:
- Kolkata: Receives heavy rain (fresh moisture).
- Patna/Allahabad: Moderate rain.
- Delhi/Punjab: Low rain (moisture is exhausted).
Reason: The wind effectively 'empties its bucket' as it travels. By the time it reaches Rajasthan, it is almost dry.
Arabian Sea Branch vs. Bay of Bengal Branch
| Feature | Arabian Sea Branch | Bay of Bengal Branch |
|---|---|---|
| Volume | More powerful (3 times more volume). | Less volume compared to Arabian Sea branch. |
| First Landfall | Kerala Coast (Western Ghats). | Myanmar & Bangladesh Coast. |
| Coverage | West Coast, Maharashtra, Gujarat, MP. | NE India, West Bengal, UP, Bihar, Punjab. |
| Deflection | No major deflection (moves straight). | Deflected heavily by Arakan Yoma & Himalayas. |
| Rainfall Trend | Decreases from Coast to Interior. | Decreases from East to West. |
Bay of Bengal Branch – Key Features
| Branch | Impact |
|---|---|
| Myanmar Branch | Heavy rains over Arakan & Tenasserim ranges |
| West Bengal & Bangladesh | Deflected by Himalayas; monsoon enters India |
| Ganga Plains Branch | Rainfall from Bengal to Punjab |
| Brahmaputra Valley Branch | Heavy rains in Assam & Meghalaya |
| Khasi Hills | Mawsynram – highest average rainfall in world |
Mains Key Points
Physiographic Control: Explain how the shape of the Himalayas and Arakan Yoma determines the direction of the Bay of Bengal branch.
Agriculture: Analyze why the intensive rice cultivation (paddy) is concentrated in the path of the Bay of Bengal branch (West Bengal/Bihar/UP).
Urban Floods: Discuss how the funneling effect often leads to devastating floods in the Brahmaputra valley (Assam) every year.
Prelims Strategy Tips
Merge Point: The Arabian Sea branch and Bay of Bengal branch merge over the North-Western part of the Ganga Plains (Delhi/Punjab).
Rainfall Gradient: Remember the sequence of decreasing rainfall: Kolkata > Patna > Allahabad > Delhi.
Arakan Yoma: If these mountains didn't exist, the monsoon winds would have gone straight into Myanmar/China, leaving India dry.
Why Mawsynram? It is situated at the head of a funnel-shaped valley which traps moisture-laden air.
Arabian Sea Branch: The Powerhouse of Monsoon
Key Point
The Arabian Sea Branch is the stronger of the two monsoon arms (about 3 times stronger than the Bay of Bengal branch). It hits the Indian landmass in three distinct streams, shaping the wet climate of the West Coast and the dry climate of Rajasthan.
The Arabian Sea Branch is the stronger of the two monsoon arms (about 3 times stronger than the Bay of Bengal branch). It hits the Indian landmass in three distinct streams, shaping the wet climate of the West Coast and the dry climate of Rajasthan.
Detailed Notes (21 points)
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1. Why is this Branch Stronger?
Size Matters: The Arabian Sea is larger than the Bay of Bengal.
Streamlined: The entire airflow from the Arabian Sea enters India, whereas only a part of the Bay of Bengal airflow enters India (the rest goes to Myanmar/Thailand).
2. Sub-Branch 1: The Western Ghats Stream (The Barrier)
Mechanism: This stream hits the Western Ghats perpendicularly (at a 90-degree angle).
Orographic Rain: The winds are forced to climb the mountains. As they rise, they cool down and dump massive amounts of rain on the Windward Side (Coastal Karnataka, Konkan, Kerala).
The Rain Shadow Effect: Once the winds cross the peak and descend on the other side, they become dry and warm. This creates a dry zone called the Leeward Side or Rain Shadow Region.
Example: Mumbai (Windward) gets 200cm+ rain, while Pune (Leeward), just 150 km away, gets <70cm.
3. Sub-Branch 2: Narmada-Tapi Valley Stream (The Channel)
The Gap: This branch finds a natural gap between the mountains north of Mumbai.
The Funnel: It travels through the Rift Valleys of Narmada and Tapi rivers.
Since these valleys run East-West, there are no mountain barriers to stop the wind. So, it carries moisture deep into Central India (Madhya Pradesh/Chhattisgarh) without shedding much rain on the coast.
4. Sub-Branch 3: Saurashtra & Rajasthan Stream (The Failure)
The Parallel Problem: This branch passes through Gujarat and Rajasthan.
Why no Rain in Rajasthan?
1. Direction: The Aravalli Range runs parallel to the direction of the wind. It does not block the wind to lift it up.
2. Heat: The scorching heat of the desert keeps the air warm (clouds don't condense easily).
Consequently, Rajasthan remains a desert despite the monsoon winds passing right over it.
5. The Great Merge
After crossing the desert, this third branch reaches the foothills of the Himalayas (Himachal/Uttarakhand).
Here, it meets the Bay of Bengal Branch. Their collision causes heavy rainfall in the Western Himalayas (often leading to cloudbursts).
Rainfall Contrast: City vs City
| Windward City (Wet) | Leeward/Rain Shadow City (Dry) | Reason |
|---|---|---|
| Mumbai | Pune | Mumbai blocks the wind; Pune is on the downward slope. |
| Mangalore | Bengaluru | Western Ghats block moisture from reaching Bangalore. |
| Mahabaleshwar | Satara | Steep orographic uplift vs. immediate descent. |
| Kozhikode | Coimbatore | Palghat Gap allows some wind, but mostly blocked by Nilgiris. |
Mains Key Points
Geographical Determinism: Use the 'Parallel Aravallis' vs 'Perpendicular Western Ghats' example to show how landforms dictate climate.
Water Crisis: Explain why Maharashtra faces a water crisis (Vidarbha/Marathwada) despite being in a state with a heavy monsoon coast—purely due to the Rain Shadow effect.
Inter-linking: Mention that the convergence of branches causes floods in Punjab/Haryana, while the rain shadow areas face drought.
Prelims Strategy Tips
Chotanagpur Plateau: This is the unique meeting point where the Arabian Sea branch (via Narmada) meets the Bay of Bengal Branch.
Shimla/Dharamshala: Receive rain from both branches, making them very wet.
No Barriers: The Narmada and Tapi valleys act as wind tunnels, allowing marine air to reach as far as Jabalpur.
Thermal Low: Even though Rajasthan doesn't get rain, the intense heat there creates the Low Pressure that pulls the entire monsoon system towards India.
Break in Monsoon and Retreating/Northeast Monsoon
Key Point
The Monsoon cycle is not just about arrival; it includes pauses (Breaks), withdrawal (Retreat), and a complete reversal of winds (Northeast Monsoon). This phase determines the agricultural fate of Rabi crops and brings the primary rainfall season for Tamil Nadu.
The Monsoon cycle is not just about arrival; it includes pauses (Breaks), withdrawal (Retreat), and a complete reversal of winds (Northeast Monsoon). This phase determines the agricultural fate of Rabi crops and brings the primary rainfall season for Tamil Nadu.
Detailed Notes (33 points)
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1. Break in Monsoon (The Pause)
Rainfall does not occur continuously for 3-4 months. It has wet spells and dry spells.
Definition: If rainfall stops for one or more weeks during the rainy season (July/August), it is called a 'Break in the Monsoon'.
The Mechanism (Why it happens):
- The Monsoon Trough (a belt of low pressure) usually sits over the Ganga plains.
- Sometimes, this trough shifts North towards the Foothills of the Himalayas.
The Consequence (Two Extremes):
- Plains (Dry): Punjab, Haryana, and UP experience a drought-like situation because the rain-bearing winds have moved away.
- Mountains (Floods): The Himalayan region (Nepal, North Bihar, Assam) receives heavy rainfall, causing massive floods in rivers like the Kosi and Brahmaputra.
2. Retreating Monsoon (The Withdrawal)
Timeline: The withdrawal is a gradual process. It starts in Rajasthan around September 1st and clears the entire country by mid-October.
Why does it retreat?
- The Sun moves towards the Southern Hemisphere (Dakshinayan).
- As a result, the landmass of North India cools down rapidly.
- The Low Pressure over North India weakens and is replaced by High Pressure.
- Winds always blow from High to Low Pressure, so they start pushing the moisture-laden winds back towards the sea.
3. October Heat (The Transition Phase)
The period of retreat (October) marks a transition from the hot rainy season to the dry winter season.
The Phenomenon: Although the rains have stopped, the ground is still soaked with water.
- Clear Skies: Clouds disappear, allowing intense sunlight to hit the ground.
- High Evaporation: The sun heats the wet land, causing rapid evaporation.
Result: High Temperature + High Humidity = Oppressive/Sultry Weather. This sticky, uncomfortable condition is famously known as 'October Heat'.
4. Northeast Monsoon (Winter Monsoon)
Reversal of Winds: By late October, the winds completely reverse their direction. Instead of blowing from Sea to Land (SW), they now blow from Land to Sea (NE to SW).
Dry for Most of India: Since these winds originate over land (Siberia/China/India), they contain no moisture and bring dry winters to North India.
Wet for Tamil Nadu:
- When these dry winds cross the Bay of Bengal, they pick up moisture.
- These moisture-laden winds then hit the Coromandel Coast (Tamil Nadu and Andhra Pradesh).
- This is why Chennai gets floods in November/December while Delhi is dry.
5. Tropical Cyclones (The Hazard)
The transition period (October-November) is the second peak season for cyclones in the Indian Ocean (the first being May-June).
The Bay of Bengal remains warm, and the shifting ITCZ provides favourable conditions for the formation of severe cyclones.
These cyclones frequently hit the coasts of Odisha, Andhra Pradesh, and West Bengal, causing immense damage.
Monsoon Phases in India
| Phase | Period | Rainfall Pattern | Regions Affected |
|---|---|---|---|
| Southwest Monsoon Active | June–Sept | Heavy, widespread | All India (esp. Western Ghats, NE, Ganga plains) |
| Break in Monsoon | July–Aug (5–10 days) | Rain ceases in plains, heavy in Himalayas | NW & Central India dry; Himalayas wet |
| Retreating Monsoon | Sept–Oct | Declining rains | NW India → Central → Deccan |
| Northeast Monsoon | Oct–Dec | Heavy coastal rains, cyclones | Tamil Nadu, Andhra, Odisha, SE Karnataka, Sri Lanka |
Mains Key Points
Impact on Crops: Explain how 'Break in Monsoon' negatively affects the paddy crop (needs standing water) but 'Retreating Monsoon' rains are beneficial for Rabi sowing.
Disaster Management: Discuss the dual challenge during the 'Break' phase: Drought management in plains and Flood management in foothills.
Urban Resilience: Analyze why Chennai faces water scarcity in summer (failed NE monsoon) and floods in winter (excess NE monsoon cyclonic rain).
Prelims Strategy Tips
Break Concept: A 'Break' in monsoon over North India means no rain in plains but HEAVY rain in Himalayas (leading to floods).
Withdrawal Dates: Starts on Sept 1 (Rajasthan) → Oct 15 (Peninsula). The retreat is much slower than the onset.
Cyclone Season: The post-monsoon season (Oct-Nov) sees intense cyclones because the sea surface temperature is still high (>27°C).
ENSO Connection: Interestingly, El Niño years (which are bad for SW monsoon) are often associated with a Good Northeast Monsoon for Tamil Nadu.
Distribution of Rainfall in India: Spatial Patterns & Variability
Key Point
India's rainfall is highly skewed—both seasonally (75% in 4 months) and spatially. While the West Coast and Northeast are water-surplus 'Evergreen Zones', the interior Deccan and North-West are water-deficit 'Dryland Zones'. The concept of Variability is key: areas with lower rainfall often face the highest unpredictability (drought risk).
India's rainfall is highly skewed—both seasonally (75% in 4 months) and spatially. While the West Coast and Northeast are water-surplus 'Evergreen Zones', the interior Deccan and North-West are water-deficit 'Dryland Zones'. The concept of Variability is key: areas with lower rainfall often face the highest unpredictability (drought risk).
Detailed Notes (39 points)
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1. The Big Picture: Isohyets & Seasonality
Isohyets: Maps use lines called Isohyets to connect places with equal rainfall.
Seasonality: India receives ~118 cm of annual rain, but 75% of it falls during the Southwest Monsoon (June-Sept).
The East-West Divide: A clear line can be drawn from Kanpur to Kanyakumari dividing India into the 'Wet East' (Rice culture) and 'Dry West' (Wheat/Millet culture).
2. Heavy Rainfall Regions (>200 cm)
The Evergreen Belt: These areas remain humid throughout the year.
Regions:
- Western Coast: Western slopes of Western Ghats (Mumbai to Kerala).
- North-East India: The 'Seven Sisters' (especially Meghalaya and Assam).
- Islands: Andaman & Nicobar, Lakshadweep.
Ecological Impact:
- Soil: Intense rain causes Leaching , leading to the formation of Laterite Soil (good for Cashew/bricks but poor for crops).
- Vegetation: Tropical Wet Evergreen Forests (Mahogany, Ebony).
3. Moderate Rainfall Regions (100–200 cm)
The Agricultural Heartland: These areas support India's food security.
Regions:
- Ganga Plains: Eastern UP, Bihar, West Bengal.
- East Coast: Odisha and Coastal Andhra.
- Uplands: Eastern slopes of Western Ghats.
Vegetation: Tropical Moist Deciduous Forests (Teak, Sal) – shed leaves in summer.
4. Low Rainfall Regions (50–100 cm)
The Dryland Farming Zone: Rainfall here is not just low, it is highly variable (unreliable).
Regions:
- Deccan Plateau: The interior parts of Maharashtra, Karnataka, and Telangana (Rain Shadow).
- North-West: Western UP, Haryana, Punjab.
Ecological Impact:
- Vegetation: Dry Deciduous & Grasslands (Savanna type).
- Agriculture: Requires drought-resistant crops (Millets, Pulses, Cotton).
5. Scanty Rainfall Regions (<50 cm)
The Desert Zone: Evaporation often exceeds precipitation here.
Regions:
- Hot Desert: Western Rajasthan (Thar) and Kutch.
- Cold Desert: Ladakh and Lahaul-Spiti.
- Rain Shadow Core: A small patch in Rayalaseema (Andhra) and North Karnataka.
Cause:
- Aravalli Range: Runs parallel to SW Monsoon winds (no obstruction).
- Himalayas: Block moisture from reaching Ladakh.
- Western Ghats: Block moisture from reaching the Deccan interiors.
Vegetation: Xerophytic (Thorny Bushes, Cactus).
Rainfall, Vegetation & Soil Nexus
| Rainfall Category | Vegetation Type | Soil Characteristic | Typical Crops |
|---|---|---|---|
| Very High (>200 cm) | Wet Evergreen | Leached (Laterite), Acidic | Plantation (Rubber, Coffee), Rice |
| Medium (100-200 cm) | Moist Deciduous | Alluvial (Fertile) | Rice, Sugarcane, Jute |
| Low (50-100 cm) | Dry Deciduous / Savanna | Black Cotton Soil / Coarse Alluvial | Wheat, Cotton, Pulses |
| Scanty (<50 cm) | Thorny / Scrub | Sandy / Saline (Desert Soil) | Millets (Jowar, Bajra), Guar |
Mains Key Points
Dryland Farming: In regions with 50-100cm rain, the strategy must shift from water-intensive crops (Sugarcane) to climate-smart crops (Millets/Pulses).
Interlinking of Rivers: Discuss this as a potential solution to balance the surplus in the East with the deficit in the South and West.
Watershed Management: In the Deccan Plateau, catching rain where it falls (Rainwater Harvesting) is more effective than building large dams.
Prelims Strategy Tips
Rule of Thumb: Variability of rainfall is inversely proportional to the amount of rainfall. (Lower rainfall = Higher risk of failure).
Coefficient of Variation (CV): Regions with CV > 30% (like Rajasthan & Rayalaseema) are prone to frequent droughts.
Islands: Lakshadweep and Andaman receive >200cm rain, making them biodiversity hotspots despite small land area.
Cloud Seeding: Artificial Rain Technology
Key Point
Cloud seeding is a weather modification technique that mimics nature by adding 'Microscopic Particles' (seeds) into clouds to encourage rainfall. It is not magic; it cannot create rain from a clear blue sky—it can only squeeze rain out of existing clouds.
Cloud seeding is a weather modification technique that mimics nature by adding 'Microscopic Particles' (seeds) into clouds to encourage rainfall. It is not magic; it cannot create rain from a clear blue sky—it can only squeeze rain out of existing clouds.
Detailed Notes (20 points)
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1. The Core Concept (How it works)
The Problem: Sometimes clouds have moisture (water vapor), but the droplets are too small and light to fall as rain. They float instead of falling.
The Solution: Cloud seeding provides a surface (Nuclei) for this moisture to latch onto.
Analogy: Think of it like making a pearl. An oyster needs a grain of sand to start making a pearl. Similarly, water vapor needs a particle (dust, salt, chemical) to freeze or condense upon to become heavy enough to fall.
2. Types of Seeding based on Temperature
A. Cold Cloud Seeding (Glaciogenic):
- Used when cloud temperature is below 0°C.
- Chemical: Silver Iodide (AgI).
- Logic: Silver Iodide's crystal structure is very similar to natural ice. Water vapor thinks it is ice, freezes onto it, grows heavy, and falls as snow/rain.
B. Warm Cloud Seeding (Hygroscopic):
- Used when cloud temperature is above 0°C (Common in India/Tropics).
- Chemical: Calcium Chloride or Common Salt.
- Logic: Salt attracts water (hygroscopic). It absorbs moisture, making the droplets larger and heavier until they fall.
3. Delivery Systems
Aircraft: Planes fly into the cloud base and fire flares containing the chemicals.
Ground Generators: Canisters fired from the ground (anti-aircraft guns or rockets) into the sky.
4. Applications Beyond Rain
Hail Suppression: In agriculture, large hailstones destroy crops. Seeding breaks large hail into smaller, harmless pieces.
Fog Dispersion: Used at airports (like Delhi IGI) to clear fog for safe landings.
Pollution Control: 'Artificial Rain' is often proposed in Delhi to wash down suspended particulate matter (PM 2.5) during severe smog.
Comparison: Warm vs. Cold Cloud Seeding
| Feature | Warm Cloud Seeding | Cold Cloud Seeding |
|---|---|---|
| Temperature | Above 0°C (Liquid water clouds) | Below 0°C (Supercooled water) |
| Chemical Used | Salts (Calcium Chloride, Sodium Chloride) | Silver Iodide, Dry Ice |
| Mechanism | Condensation: Particles absorb water and grow. | Freezing: Particles act as ice crystals. |
| Primary Region | Tropical regions (like India's Monsoon) | Temperate/Polar regions |
Mains Key Points
Geopolitics of Rain: Discuss 'Rain Stealing'—if one country/state seeds clouds, does it deprive the downwind neighbor of their natural share of rain?
Cost-Benefit: It is expensive. Discuss if the cost of renting planes validates the 15-20% increase in rainfall, or if funds should be used for irrigation canals instead.
Ecological Impact: Concerns about Silver accumulation in soil and water bodies affecting aquatic life.
Prelims Strategy Tips
Chemical Match: Remember: Silver Iodide = Cold Clouds; Salt = Warm Clouds.
Project Varshadhari: A cloud seeding project implemented by the Karnataka government (2017) to tackle drought.
IIT Kanpur: Recently conducted successful artificial rain tests in 2023.
Blue Sky Limit: Cloud seeding CANNOT create clouds. It requires at least 40% cloud cover to be effective.
Chapter Complete!
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