REPORT: The Indian Monsoon Season Date: October 26, 2023 Subject: Overview, Timing, and Regional Variations of the Indian Monsoon
1. Executive Summary The Indian monsoon is a seasonal prevailing wind system that brings the majority of the subcontinent's annual rainfall. It is not a single, uniform event but a complex, migratory weather system. The season is officially divided into two main phases: the Southwest Monsoon (Summer) and the Northeast Monsoon (Winter). The primary monsoon season generally spans from June to September , though exact onset and withdrawal dates vary significantly by region.
2. The Primary Season: The Southwest Monsoon (June – September) The Southwest Monsoon is the principal rainy season for the majority of India, contributing approximately 75% to 80% of the country's annual rainfall. It is characterized by moisture-laden winds blowing from the southwest (Indian Ocean) toward the landmass. Timeline of Onset and Progression The monsoon does not arrive simultaneously across the entire country. It follows a distinct progression:
Late May / Early June (Onset): The monsoon typically hits the Andaman and Nicobar Islands around May 20. It reaches the Indian mainland over the Kerala coast around June 1st (with a standard deviation of about 7 days). June (Advance): Following the Kerala landing, the winds move northward along the Western Ghats and into the Northeast. By mid-June, it typically covers Mumbai (approx. June 10) and Delhi (approx. June 29). July (Coverage): By mid-July, the monsoon covers most of central India and parts of northwest India. July – August (Peak): This is the peak rainy season for the majority of the country, including the agricultural heartlands of the Gangetic plains. September (Withdrawal): The monsoon begins its retreat from northwest India starting in early September, gradually receding southward.
Regional Breakdown (Southwest Monsoon)
Western Coast (Goa, Mumbai, Kerala): Receives heavy rainfall from June through August due to the direct hit of the Western Ghats. Northern Plains (Delhi, Uttar Pradesh, Bihar): The season is shorter but intense, typically running from late June through August. Northeast India: Places like Cherrapunji receive some of the heaviest rainfall in the world during this period.
3. The Secondary Season: The Northeast Monsoon (October – December) While the Southwest monsoon withdraws by October, a significant portion of Southern India relies on a secondary rainy season known as the Northeast Monsoon (or the Retreating Monsoon).
Timing: October to December. Mechanism: As the Southwest monsoon retreats, winds reverse, blowing from the northeast toward the ocean. These winds pick up moisture from the Bay of Bengal and deposit rain over the eastern coast. Impact: This season provides the bulk of the annual rainfall for:
Tamil Nadu (receiving ~48% of its annual rain in this period). Coastal Andhra Pradesh. Parts of Telangana and Karnataka.
Rest of India: During this time, North and Central India generally experience clear skies and a drop in temperature.
4. Regional Variations & Anomalies It is inaccurate to say "Monsoon season is June-September" for the entire country without nuance. | Region | Monsoon Type | Primary Season Window | Characteristics | | :--- | :--- | :--- | :--- | | Kerala & West Coast | Southwest | June – August | Extremely heavy rainfall; earliest onset. | | Tamil Nadu & Chennai | Northeast | Oct – Dec | The Southwest monsoon provides little rain here; the main season is winter. | | Northwest (Rajasthan) | Southwest | July – August | Very short season; some areas are arid and receive minimal rain. | | Northeast (Meghalaya) | Southwest | May – September | Earliest arrival; extremely high precipitation volumes. |
5. Factors Influencing Timing The exact timing of the monsoon varies annually based on several meteorological factors: