Chapter: Climate

• Introduction

  • Weather: State of atmosphere over an area at any point of time.
  • Climate: Sum total of weather conditions and variations over a large area for a long period (more than 30 years).
  • Weather vs Climate: Elements are the same (temperature, wind, pressure, humidity, precipitation), but time frame differs.
  • Monsoon: Seasonal reversal in wind direction during a year.

• Climatic Controls

  • Latitude: Distance from the equator; affects temperature (solar energy).
  • Altitude: Height above sea level; temperature decreases with height.
  • Pressure and Wind System: Influenced by latitude and altitude; controls rainfall and temperature.
  • Distance from the Sea (Continentality): Moderating effect of oceans; extreme weather in interiors.
  • Ocean Currents: Influence coastal temperatures along with onshore winds.
  • Relief Features: Barriers to winds; cause relief (or orographic) rainfall.

• Factors Affecting India's Climate

  • Latitude: Tropic of Cancer passes through the middle; country divided into tropical and subtropical zones.
  • Altitude: Himalayas in the north prevent freezing winds from Central Asia; high coastal elevations.
  • Pressure and Surface Winds:
    • Surface winds (Northeast and Southwest trade winds).
    • Upper air circulation (Jet streams).
    • Cyclonic disturbances (Western disturbances).

• The Indian Monsoon

  • Mechanism of Monsoon (Key Factors):
    • Differential heating and cooling of land and water.
    • Shift of the Inter Tropical Convergence Zone (ITCZ) in summer.
    • High-pressure area over Madagascar (East of Madagascar).
    • Intense heating of the Tibetan plateau in summer.
    • Movement of westerly jet stream to the north and tropical easterly jet stream.
  • The Onset of the Monsoon:
    • Sudden onset ("Burst" of the monsoon).
    • Reaches southern tip of the peninsula by early June.
    • Splits into two branches: Arabian Sea Branch and Bay of Bengal Branch.
  • Withdrawal of the Monsoon:
    • Gradual process beginning in northwestern states in September.
    • Completely withdrawn from the northern half by mid-October.

• Seasons in India

  • The Cold Weather Season (Winter):
    • Mid-November to February.
    • Low temperatures in northern India; warm in south.
    • Northeast trade winds prevail.
    • Mahawat: Small winter rains caused by Western Disturbances (crucial for rabi crops).
  • The Hot Weather Season (Summer):
    • March to May.
    • High temperatures; falling air pressure in northern plains.
    • Loo: Strong, gusty, hot, dry winds.
    • Kaal Baisakhi: Localized thunderstorms in West Bengal bringing pre-monsoon showers.
    • Mango Showers: Pre-monsoon showers in Kerala/Karnataka helping early ripening of mangoes.
  • Advancing Monsoon (Rainy Season):
    • Early June to September.
    • Inflow of southwest monsoon winds laden with moisture.
    • Heavy rainfall in windward sides and Northeast India (Mawsynram receives highest rainfall).
    • Monsoon Breaks: Wet and dry spells interspersed during the rainy season.
  • Retreating/Transition Season (Autumn):
    • October and November.
    • Clear skies and rise in temperature ("October Heat").
    • High humidity and oppressive weather.
    • Tropical cyclones originating in the Bay of Bengal hit eastern coasts.

• Distribution of Rainfall

  • High Rainfall Areas: (> 200 cm) Western coast and Northeastern India (e.g., Meghalaya, Western Ghats).
  • Moderate Rainfall Areas: (100 - 200 cm) Parts of Western Ghats, northeastern peninsula, and northern plains.
  • Low Rainfall Areas: (< 60 cm) Western Rajasthan, parts of Gujarat, Haryana, Punjab, and rainshadow area of Deccan Plateau.
  • Scarcity and Variability: Prone to droughts in low rainfall regions and floods in high rainfall regions.

• Monsoon as a Unifying Bond

  • Unifies the entire country in a single rhythmic cycle of weather.
  • Agricultural activities and festivals revolve around the monsoon.
  • Water availability for life, flora, and fauna across India's diverse landscapes.