Southwest Monsoon Causes Heavy Rainfall in Rajasthan

• 23 Jun 2025

Why in News?

Rajasthan has come under the influence of the active southwest monsoon, resulting in widespread and intense rainfall across various parts of the State.

• Niwai in the Tonk district recorded the highest rainfall in the State, measuring 165 mm.

Key Points

Rainfall Distribution in Rajasthan

• Rainfall Patterns: 
  • Rajasthan shows a marked variation in rainfall distribution across different regions.
    • The mean annual rainfall in East Rajasthan is about 64.9 cm.
    • In West Rajasthan, it drops to around 32.7 cm.
  • The eastern part of Rajasthan receives considerably more rainfall than the west, with the southwest monsoon accounting for about 91% of the state’s total annual rainfall.
  • Western Rajasthan is dominated by arid and semi-arid conditions, with the northwest being the driest.
    • Jaisalmer is Identified as the driest district, with an annual rainfall of less than 100 mm.
  • Southern Rajasthan: Records the highest rainfall in the state, particularly in districts like Jhalawar and Banswara.
    • Jhalawar records the highest average annual rainfall among all districts in Rajasthan.
  • Aravalli Range Influence: The western slopes of the Aravalli Range, such as Pali and Jalore districts, receive more rainfall than other parts of western Rajasthan.
• Seasonal Variation:
  • Monsoon Season (June to September): Accounts for about 90% of the total annual rainfall.
  • Cold Weather Season (January and February): Receives minor rainfall due to local weather conditions.
  • Post-Monsoon Season: Contributes a small fraction to the overall annual rainfall.

Southwest Monsoon

• About: The rainfall received from the southwest monsoon is seasonal and occurs mainly between June and September every year.
• Factors Influencing its Formation:
  • Differential Heating and Cooling: Land heats up faster than water, creating a low-pressure zone over India and high pressure over surrounding seas.
  • Intertropical Convergence Zone (ITCZ): A low-pressure belt where northeast and southeast trade winds converge.
  • High-Pressure Area East of Madagascar: Located around 20°S in the Indian Ocean.
  • Tibetan Plateau Heating: Intense summer heating creates strong upward air currents and a low-pressure zone at high altitudes.
  • Jet Streams: Westerly Jet Stream moves north of the Himalayas in summer.
  • Southern Oscillation (SO): Periodic reversal of air pressure between the Pacific Ocean and Indian Ocean affects monsoon patterns.
• Mechanism and Onset of Southwest Monsoon:
  • ITCZ Movement: Shifts north with the Sun’s movement
  • Wind Direction: Southeast trade winds cross the equator, deflect due to the Coriolis force, and blow as southwest monsoon winds.
  • Monsoon Trough: In July, ITCZ reaches 20°–25°N, located over the Indo-Gangetic Plain.
  • Two Main Branches: Arabian Sea Branch and Bay of Bengal Branch.
  • Breaks in Monsoon: Rainfall is not continuous; dry spells (breaks) occur due to shifts in the monsoon trough.