The annual wet monsoon, officially known as the Southwest Monsoon, is a seasonal wind reversal that brings the majority of India’s annual rainfall between June and September. This four-month period significantly influences the country’s agricultural output, water resource management, and economic health. The timing of the monsoon’s arrival dictates the schedule for planting summer-sown Kharif crops and is essential for replenishing reservoirs and groundwater tables. This system delivers approximately 75% of the country’s total annual precipitation, making its timing a subject of intense national focus.
The Atmospheric Engine Driving the Monsoon
The underlying cause of the monsoon is the difference in heating rates between the Indian landmass and the surrounding oceans. During summer, the subcontinent heats up much more rapidly than the adjacent Indian Ocean waters, creating a massive zone of low atmospheric pressure over the land. This phenomenon is often termed the thermal contrast between land and sea.
This intense low-pressure area pulls in moist air from the high-pressure zones over the ocean. The northward migration of the Inter-Tropical Convergence Zone (ITCZ), a belt of low pressure near the equator where trade winds converge, is a primary factor. As the ITCZ shifts northward over the Indo-Gangetic Plain, it draws moisture-laden southeast trade winds from the Southern Hemisphere across the equator.
Once these trade winds cross the equator, the Coriolis effect turns them to the right, transforming them into the southwesterly winds that define the Southwest Monsoon. The air, carrying abundant moisture from the warm ocean currents, travels toward the low-pressure zone over India. The Tibetan Plateau acts as an elevated heat pump, intensifying this circulation by creating strong vertical air currents high in the atmosphere.
General Timeline for the Monsoon’s Onset
The official start of the wet monsoon season is measured by its arrival on the Indian mainland’s southwestern tip. The India Meteorological Department (IMD) uses a climatological date of June 1st for the onset over Kerala. This date is an average, with the actual arrival varying by about seven to eight days, reflecting the natural variability of atmospheric conditions.
The arrival of the monsoon is not gradual but is marked by a sudden, intense shift in weather conditions known as the “monsoon burst.” This burst is characterized by a significant drop in temperature, often by 5 to 10 degrees Celsius, accompanied by strong winds, thunder, and lightning. The rapid introduction of these moisture-rich winds signifies the beginning of the four-month rainy period.
Once the monsoon establishes itself over Kerala, it advances progressively north and east across the peninsula in surges. The monsoon covers the entire country, including the Northwest, by the average date of July 8th. The initial phase, covering the southern and central parts, is generally rapid, paving the way for the agricultural season.
How Timing Varies Across India
The monsoon’s journey involves two primary branches that influence regional arrival times. The Arabian Sea branch moves northward along the Western Ghats, bringing heavy rainfall to the coastal areas first. The average arrival time for Mumbai is typically around June 10th.
The Bay of Bengal branch sweeps over the Bay, reaching the northeastern states and then turning westward up the Gangetic Plain. This branch generally causes the monsoon to arrive in Kolkata around June 7th. Progression inland means central regions like Hyderabad often experience the onset between June 5th and 10th, shortly after the Kerala arrival.
The northern and western parts of India are the last to receive the seasonal rains. Delhi typically sees the monsoon arrive toward the end of the month, with an average date around June 30th. The arid state of Rajasthan, particularly the western parts, is among the final areas covered before the monsoon covers the entire country by early July.
The Monsoon’s Retreat and Post-Season Cycle
The wet monsoon season concludes with a phase known as the withdrawal or retreat. This process begins in the Northwest of India, typically starting from western Rajasthan around September 17th. The withdrawal is a gradual process where the southwesterly winds weaken and reverse.
As the low-pressure system over the land begins to dissipate, the monsoon withdrawal line moves southward across the country. The official end of the Southwest Monsoon season for the entire country is typically marked around October 15th. This withdrawal phase is slower than the rapid advance at the start of the season.
Following the withdrawal of the Southwest Monsoon, the Northeast Monsoon takes hold in the southern peninsula. Occurring primarily between October and December, this cycle brings rainfall to states like Tamil Nadu and parts of Andhra Pradesh. The wind direction changes to northeasterly, making this the major rainy season for the southeastern coastal regions.