हिंदी
CISCEISC (Arts) Class 12
प्रश्न पत्र३७२
पाठ्यपुस्तक उत्तर१६६८१
महत्वपूर्ण प्रश्न एवं उत्तर५८७१
अवधारणा स्पष्टीकरण और वीडियो९५३
समय सारणी१५
पाठ्यक्रम

Overview of Climate

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Estimated time: 39 minutes
CISCE: Class 12

Key Points: Factors Affecting India's Weather and Climate

  • Monsoon Climate: India has a monsoon type climate. The word monsoon comes from Arabic word “mausim” meaning seasonal reversal of winds.
  • Latitude and Location: India lies between 6°N to 37°N. South India is hotter (near equator) while North India is cooler in winter (warm temperate zone).
  • Distance from Sea: Coastal areas have moderate (maritime) climate due to sea influence, while interior regions have extreme (continental) climate.
  • Himalayan Mountain Barrier: Himalayas protect India from cold Central Asian winds and also stop moisture-laden monsoon winds, acting as a climatic divide.
  • Physiography Influence: Relief affects rainfall. Western Ghats cause heavy rain on west coast and create rain shadow in parts of Deccan. Meghalaya hills make Mawsynram the rainiest place.
  • Monsoon Winds:
    Northeast Monsoon (winter): cold and dry, gives rain mainly on Coromandel coast.
    Southwest Monsoon (June–Sept): sea winds bring widespread rainfall, called rainy season.
  • Upper Air Circulation & Other Factors: Jet streams affect monsoon. Western disturbances bring winter rain and snowfall. Cyclones, El-Nino and Southern Oscillation also influence strength of monsoon and cause floods/droughts.
CISCE: Class 12

Key Points: The Concept of Monsoon

  • Meaning of Monsoon: Monsoon is a seasonal reversal of winds—winds blow from land to sea in winter and from sea to land in summer, so most rainfall occurs in summer.
  • Halley’s Thermal Theory: Sir Edmund Halley explained monsoon as a result of differential heating of land and sea, causing low pressure over land in summer and high pressure in winter.
  • Criticism by Flohn: Flohn argued that land–sea heating alone is insufficient and explained monsoon through seasonal shift of pressure and wind belts due to the sun’s vertical rays.
  • Role of ITCZ: In summer, ITCZ shifts north creating low pressure over northwest India, attracting southwest monsoons; in winter, ITCZ shifts south causing northeast monsoons.
  • Jet Streams Influence: Sub-tropical westerly jet strengthens winter monsoon, while tropical easterly jet in summer helps in the onset and variation of southwest monsoon.
CISCE: Class 12

Key Points: Mechanism of Indian Weather

  • Three Weather Mechanisms: Indian weather is controlled by surface pressure and winds, upper air circulation (jet streams), and rainfall from western disturbances and tropical depressions.
  • Winter Season – Surface Conditions: In winter, high pressure over Central and West Asia causes cold, dry north-westerly winds to blow over north-western India up to the middle Ganga Valley.
  • Winter Season – Upper Air Circulation: At higher altitudes, a westerly jet stream flows south of the Himalayas (around 25°N) and brings western disturbances, causing winter rain and snowfall.
  • Summer Season – Surface Conditions: In summer, the ITCZ shifts north (around 25°N) creating low pressure over India, attracting moist maritime winds from the southern hemisphere as south-west monsoons.
  • Summer Season – Upper Air Circulation: An easterly jet stream develops in the upper atmosphere, guiding tropical depressions, which control the distribution and intensity of monsoon rainfall.
CISCE: Class 12

Key Points: Indian Monsoonal Regime

  • Onset of Monsoon: The south-west monsoon begins due to intense heating of north-west India, northward shift of ITCZ, withdrawal of westerly jet stream, and development of easterly jet stream.
  • Advance of Monsoon: The monsoon sets in over Kerala by 1 June, reaches Mumbai and Kolkata by 10–13 June, and covers the whole country by mid-July.
  • Rain-bearing Systems & Distribution: Rainfall occurs in wet and dry spells.
    Bay of Bengal depressions bring rain to northern plains.
    Arabian Sea branch causes heavy rainfall on the west coast.
    Rainfall decreases inland from the coast.
  • Break in the Monsoon: A break is a dry spell lasting two or more weeks, caused by failure of tropical depressions or shifting of the monsoon trough (ITCZ).
  • Retreat of Monsoon: Retreat starts from western Rajasthan by 1 September, withdraws from most of India by mid-October, and the retreating monsoon brings winter rain to Tamil Nadu (October–December).
CISCE: Class 12

Key Points: Rhythms of Seasons > The Winter Season

  • Duration & Temperature: Winter lasts from mid-November to March; January is the coldest month. North India is cold, while Peninsular India remains warm.
  • Temperature Distribution: Mean temperature in north India falls below 21°C, sometimes below freezing; south India stays above 22–25°C with little seasonal change in coastal areas.
  • Pressure & Winds: High pressure develops over north-west India and low pressure over southern India and nearby seas, causing slow north-east winds.
  • Western Disturbances: Western disturbances from the Mediterranean region bring rainfall to north-west India and snowfall in the Himalayas during winter.
  • Rainfall Pattern:
    Tamil Nadu and south-east coast receive rain from northeast monsoon.
    North-west India gets light winter rain, important for rabi crops (wheat).
CISCE: Class 12

Key Points: Rhythms of Seasons > The Summer Season

  • Duration & Temperature: Summer season lasts from April to June. Temperatures rise sharply, reaching 41–42°C, and may go up to 45–47°C in north-west India.
  • Regional Temperature Variation: Northern plains experience extreme heat and hot winds (loo), while southern India remains cooler due to sea influence, with temperatures around 26–30°C.
  • Pressure & Wind Pattern: Low pressure develops over north-west India due to intense heating, while high pressure exists over southern India, causing winds to blow from south-west to north-east.
  • Local Hot Weather Storms: Important local storms include Mango showers, Blossom showers, Nor’westers (Kalbaisakhi), and loo, which affect agriculture and weather conditions.
  • Rainfall: Summer rainfall is generally low and uneven, caused by dust storms and thunderstorms; Kerala and Assam receive higher rainfall, while Rajasthan and Gujarat get very little.
CISCE: Class 12

Key Points: Rhythms of Seasons > The Rainy Season

  • Duration & Importance: Rainy season lasts from June to October and India receives about 80% of its total rainfall from the south-west monsoon.
  • Monsoon Burst: The rainfall begins suddenly, called “monsoon burst”, usually in early June on coasts and by early July in interior areas.
  • Temperature Drop: Due to rainfall, temperature falls by about 5°C to 8°C from mid-June to mid-July.
  • Two Main Monsoon Currents: The south-west monsoon has two branches:
    Arabian Sea Current and Bay of Bengal Current.
  • Rainfall Distribution:
    Arabian Sea current gives heavy rainfall on Western Ghats (over 250 cm) but little in rain-shadow areas (e.g., Bengaluru).
    Bay of Bengal current causes very heavy rain in Meghalaya (Mawsynram rainiest) and rainfall decreases from Kolkata to Delhi. Tamil Nadu remains mostly dry in this season.
CISCE: Class 12

Key Points: Rhythms of Seasons > The Season of Retreating Monsoon

  • Period of Retreat: The south-west monsoon starts retreating from north India in early September and withdraws from south India by early December.
  • Nature of Retreat: The retreat of monsoon is gradual, unlike the sudden onset of the advancing monsoon.
  • Weather Conditions: With retreat, clouds disappear, skies become clear, and humidity decreases.
  • Temperature Changes: Day temperatures rise slightly, while nights become cooler and pleasant, with minimum temperature around 20°C or lower.
  • Cyclonic Rainfall: Cyclonic storms from the Bay of Bengal move towards the north-west and bring heavy rainfall to the eastern coastal areas.
CISCE: Class 12

Key Points: Annual Rainfall

  • Uneven Rainfall: India receives uneven rainfall, with very large regional variations across the country.
  • Areas of Heavy Rainfall: Western Ghats, west coastal plain, north-eastern hills and Meghalaya receive more than 200 cm rainfall; Khasi–Jaintia hills get over 1000 cm annually.
  • Moderate Rainfall Regions: Areas like West Bengal, Odisha, Bihar, Jharkhand, Chhattisgarh, Madhya Pradesh, parts of UP and Himalayan states receive 100–200 cm of rainfall.
  • Low Rainfall Areas: Punjab, Haryana, Rajasthan, Kachchh and Kathiawar receive less than 60 cm, while western Rajasthan gets below 20 cm rainfall and depends heavily on irrigation.
  • Rainfall Trends:
    i. Rainfall decreases from east (Bengal–Odisha coast) towards west and north-west.
    ii. Rainfall also declines from coastal areas towards the interior plateau.
CISCE: Class 12

Key Points: Variability of Annual Rainfall

  • Meaning of Rainfall Variability: Rainfall varies from year to year and is measured using the Coefficient of Variation (C.V.) = (Standard Deviation / Mean) × 100.
  • General Range: In India, the coefficient of variation of annual rainfall generally ranges between 15% and 30%.
  • Low Variability Areas: West coast (Mangaluru), sub-Himalayan belt, and north-eastern hills have low variability (below 15%) and receive heavy rainfall (over 100 cm).
  • High Variability Areas: Interior plateau regions, Gujarat, western MP, central UP, and especially Rajasthan and Kachchh have high variability (30%–80%) with low rainfall.
  • Impact on Agriculture: Areas with high rainfall variability are prone to drought and water scarcity, while high rainfall areas have low variability.
CISCE: Class 12

Key Points: Incidence of Droughts and Floods > Droughts

  • Meaning of Drought: Drought is a condition caused by failure or deficiency of rainfall, leading to shortage of surface and ground water and affecting living organisms.
  • Causes of Droughts: Droughts occur due to weak south-west monsoon, late onset or early withdrawal, and long breaks in monsoon rainfall.
  • Rainfall Factors: Both the amount and timing of rainfall are important; drought depends on the effectiveness of rainfall, not just quantity.
  • Extent of Drought-Prone Areas: About 16% of India’s area and 12% of its population are affected by recurring droughts; nearly one-third of India’s land is drought-prone.
  • Characteristics of Drought Areas: Regions with less than 75 cm rainfall, high variability (over 40%), and poor irrigation are most vulnerable to drought.
  • Major Drought Regions:
    Desert and semi-desert areas (Rajasthan, Gujarat, parts of UP and Punjab).
    Large rain-shadow and interior plateau regions extending to southern India.
  • Scattered Drought-Prone Pockets: Areas like Saurashtra, Kachchh, Palamau (Jharkhand), Purulia (West Bengal), Kalahandi (Odisha) and parts of Tamil Nadu and UP frequently face droughts.
CISCE: Class 12

Key Points: Incidence of Droughts and Floods > Floods

  • Meaning – Flood is overflow of water over land due to heavy rainfall; it becomes a hazard when it causes damage.
  • Main Causes – Heavy monsoon rain, cyclones, silting of rivers, deforestation, change in river course, dam failure.
  • Flood-Prone Area – About 49.8 million hectares are flood-prone; around 24 million people are affected every year.
  • Most Affected Regions – Northern plains (Punjab, Haryana, UP, Bihar, West Bengal, Assam) suffer maximum damage.
  • Major Rivers – Ganga, Brahmaputra, Kosi, Yamuna, Mahanadi, Godavari, Krishna cause frequent floods.
  • Flood Control Programme (1954) – Immediate, short-term (embankments), and long-term (dams and reservoirs) measures.
  • Flood Forecasting – Started in 1958 by CWC; forecasts issued 12–48 hours in advance using modern technology.
CISCE: Class 12

Key Points: Temperature and Rainfall Graphs

  • Graph Type – Combined graph: temperature (line), rainfall (bar).
  • Use – Shows seasonal climate of a place.
  • Mumbai – Small temp range; heavy rain June–Sept; October heat.
  • Delhi – Hot summer, cold winter; monsoon rain July–Sept.
  • Chennai – Rain mainly in Oct–Nov (NE monsoon).
  • Jaisalmer – Very hot, very dry (desert climate).
  • Leh – Very cold, very low rainfall (cold desert).
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