Quick Revision Notes: Geography (Class 11)

Chapter: Climate

Weather: It is the momentary state of the atmosphere at a given place and time. (मौसम - किसी स्थान विशेष पर वायुमंडल की अल्पकालीन अवस्था।)
Climate: It refers to the average of weather conditions over a long period of time, usually 30 years or more, for a larger area. (जलवायु - लंबी अवधि जैसे 30 वर्ष या उससे अधिक के मौसम की दशाओं का औसत।)
Monsoon: The word 'monsoon' is derived from the Arabic word 'Mausim', which means season. It refers to the seasonal reversal in the wind direction during a year.

The elements that determine the weather and climate of a place are the same:

India's climate is controlled by several major factors:

Latitude: The Tropic of Cancer passes through the middle of the country. The southern part lies in the tropical zone (hot), and the northern part lies in the sub-tropical/temperate zone.
Altitude: Temperature decreases with an increase in height. Mountains are cooler (e.g., Himalayas).
Pressure and Wind System: Controlled by surface winds, upper-air circulation (Jet Streams), and cyclonic disturbances.
Distance from the Sea (Continentality): Coastal areas experience a moderating effect from the sea, while interior areas experience extreme temperatures.
Relief Features: Physiography or relief affects the temperature, wind direction, and amount of rainfall (e.g., Western Ghats cause heavy rainfall on the windward side).

The onset and progression of the monsoon are driven by the following key factors:

Differential heating and cooling: Land and water heat up and cool down at different rates, creating low-pressure areas on land and high-pressure over seas during summer.
Shift of Inter Tropical Convergence Zone (ITCZ): In summer, the ITCZ shifts northwards over the Ganga plain (this is also known as the monsoon trough).
High-Pressure area over Madagascar: The presence of a high-pressure area east of Madagascar in the Indian Ocean.
Tibetan Plateau: Intense heating of the Tibetan plateau during summer creates strong vertical air currents.
Jet Streams: The movement of the westerly jet stream north of the Himalayas and the presence of the tropical easterly jet stream over the Indian Peninsula.
El Niño and La Niña:
- El Niño: Periodic warm ocean current along the coast of Peru that temporarily replaces the cold current. It negatively impacts the Indian Monsoon.
- La Niña: Cooling of the Peru current, which brings good rainfall to India.

India's climate is marked by a seasonal cycle. The India Meteorological Department (IMD) classifies the Indian year into four seasons:

Duration: Mid-November to February.
Temperature: Days are pleasantly warm, and nights are cold. Temperatures decrease from south to north.
Winds: Northeast trade winds prevail over the country, blowing from land to sea (dry winds).
Special Feature: Western Disturbances (cyclonic depressions originating in the Mediterranean Sea) bring winter rainfall to North-West India, which is vital for rabi crops (wheat).

Duration: March to May.
Temperature: High temperatures and low air pressure prevail over the northern plains.
Winds:
- Loo: Hot, dry, and gusty winds blowing during the day over north and northwestern India.
- Kaal Baisakhi: Localized thunderstorms in West Bengal associated with rainfall, beneficial for tea, jute, and rice.
- Mango Showers: Pre-monsoon showers in Kerala and Karnataka that help in the early ripening of mangoes.

Duration: June to September.
Mechanism: Low pressure develops over the Indian subcontinent, attracting moisture-laden winds from the Indian Ocean.
Branches of Southwest Monsoon:
1. Arabian Sea Branch: Causes heavy rainfall on the western slopes of the Western Ghats, then moves into central India and northern plains.
2. Bay of Bengal Branch: Causes rainfall in the North-East India and the Brahmaputra valley, deflected by the Himalayas towards the Ganga plain.
Monsoon Break: Rainy spells are interspersed with rainless intervals known as 'breaks' in the monsoon.

Duration: October and November.
Characteristics: Southwest monsoon winds weaken and start withdrawing.
Weather: Clear skies and rise in temperature. The land is still moist, leading to high heat and humidity, commonly known as "October Heat".
Rainfall: The retreating monsoons pick up moisture from the Bay of Bengal and bring heavy rainfall to the Tamil Nadu coast.

High Rainfall Areas (Over 200 cm): Western slopes of the Western Ghats, sub-Himalayan region of Northeast India, and the Meghalaya hills.
Medium Rainfall Areas (100–200 cm): Southern parts of Gujarat, East Tamil Nadu, northeastern peninsular region, and the middle Ganga valley.
Low Rainfall Areas (50–100 cm): Western Uttar Pradesh, Haryana, Punjab, parts of the Deccan plateau, and eastern Rajasthan.
Scanty Rainfall Areas (Less than 50 cm): Western Rajasthan, Ladakh, and the interior of the Deccan plateau.

Variability: Calculated as the percentage of deviation of the mean from standard deviation. Areas of low rainfall (like Rajasthan, Gujarat, and leeward Western Ghats) have high variability, making them prone to droughts.
Global Warming and Climate Change: Due to greenhouse gas emissions, global temperatures are rising, leading to erratic monsoon patterns, unseasonal rainfall, frequent floods, and prolonged droughts in India.