Why does Chennai get most of its rain in October–December?

Why India Looks This Way

At the India Meteorological Department’s Nungambakkam observatory in Chennai, 856.3 mm of the 1,376.8 mm annual rainfall normal falls in October, November and December. That is about 62%, calculated from the IMD’s 1991–2020 monthly figures. These long-term averages describe one station, rather than every neighbourhood or a forecast for this year. They show Chennai’s unusual rhythm: its wettest months arrive as the summer monsoon withdraws from much of India.

Monthly normal rainfall at Chennai Nungambakkam, 1991–2020, in millimetres. October–December totals 856.3 mm; June–September totals 439.5 mm. Source: India Meteorological Department. Chart: BharatContext.

Why the summer monsoon gives Chennai less rain

During the southwest monsoon, broadly June to September, the Arabian Sea branch meets the Western Ghats, the mountain chain along the western side of the peninsula. Moist air rises over those slopes, cools and sheds much of its rain on the west-facing side. The land farther east receives less of this rain: the rain-shadow effect. The Bay of Bengal branch also travels roughly parallel to the Tamil Nadu coast rather than pushing directly across it. NCERT’s climate chapter describes both features of the summer monsoon.

The rain shadow does not leave Chennai rain-free in summer. The same IMD station normals add up to 439.5 mm from June through September; August and September average 132.8 mm and 146.2 mm respectively. The difference is in when the rain peaks.

What changes around October?

Around October, the broad pressure and wind patterns that drive the monsoon shift. Air flowing from the northeast and east can cross the Bay of Bengal before reaching India’s southeast coast. Winds are named for the direction they come from: northeasterly flow can begin over land, cross the Bay, pick up moisture and bring rain toward coastal Tamil Nadu. This later rainy period is often called the northeast monsoon. NCERT describes the seasonal change and Bay crossing in its account of winter rain and the retreating monsoon.

A schematic of two seasonal routes, not a geographic map. In summer, the Ghats receive rain on their western slopes; later, moisture-bearing winds cross the Bay toward the southeast coast. Based on NCERT’s climate chapter. Diagram: BharatContext.

Chennai lies on that southeast coast. In the station normal, October averages 300.3 mm and November 373.6 mm; even December is wetter than any June–September month. The city’s coastal location and shifting seasonal circulation together explain the later peak.

Why one northeast monsoon season can look different from another

The broad wind pattern sets the season, while weather systems over the Bay shape individual wet spells. Low-pressure areas and depressions can organise moisture into sustained rain; some systems grow into cyclones. NCERT identifies Bay depressions and cyclones as important sources of rain along the southeast coast during the retreating monsoon. A season can bring ordinary showers as well as a few much heavier spells.

The 1991–2020 normal shows the average seasonal pattern, but rainfall can vary sharply between years and across the region. A downpour over the city and rain over the catchments that fill its reservoirs are related concerns, yet they measure different places. A historical Chennai rainfall study published by the Indian Meteorological Society’s Chennai chapter discusses this variability.

This timing matters for Chennai’s water supply and flood preparation. Reservoir planning must account for much of the rain arriving late in the year, while drainage must cope with intense spells within that season. The Bay’s weather systems can change how much rain falls in any particular year, even though the long-term peak comes in October–December.