Chennai’s normal annual rainfall is about 1,350 mm, and well over half of it arrives in just three months. The India Meteorological Department’s 1991-2020 average for Nungambakkam, the city’s reference observatory, is 1,376.8 mm a year. The state’s statistical handbook puts the district normal at 1,346.2 mm, of which 809.6 mm is expected during the northeast monsoon from October to December.
That concentration explains most of Chennai’s rain story: long dry spells, then a handful of days that decide whether the year is a drought or a flood. Here are the official normals, the records, and how 2015 and 2023 compare. This year’s northeast monsoon set in over Tamil Nadu on 15 October 2026.
The normal year, season by season
The Statistical Hand Book of Tamil Nadu 2022-23, which uses IMD Chennai data, splits Chennai district’s normal rainfall into four seasons. The table also shows what actually fell in 2022-23.
| Season | Normal (mm) | Actual 2022-23 (mm) |
|---|---|---|
| Southwest monsoon (Jun-Sep 2022) | 448.5 | 437.3 |
| Northeast monsoon (Oct-Dec 2022) | 809.6 | 924.6 |
| Winter (Jan-Feb 2023) | 39.3 | 2.2 |
| Hot weather (Mar-May 2023) | 48.8 | 102.5 |
| Annual (Jun 2022-May 2023) | 1,346.2 | 1,466.6 |
So roughly 60% of a normal year’s rain (809.6 of 1,346.2 mm) comes in the northeast monsoon. The southwest monsoon, which drenches Kerala and Mumbai, gives Chennai about a third, usually as evening thunderstorms in August and September.
Normals shift slightly between editions as IMD updates its base period. The 2020-21 handbook listed the northeast monsoon normal as 784.0 mm and the annual normal as 1,315.7 mm. That year was wet: 1,040.8 mm fell in October to December 2020 and 1,728.9 mm over the twelve months to May 2021.
Records that still stand
IMD’s Chennai centre publishes month-by-month record sheets. The October sheet for Nungambakkam holds two of the city’s best-known marks:
- Wettest month on record: 1,077.8 mm in October 2005, more than three times the October normal of 300.3 mm.
- Heaviest October day: 279.7 mm in 24 hours, in October 1969.
The same sheet’s data runs back to the 1880s for temperature, which is why Chennai’s rainfall history is often described as a century and more long. IMD does not publish a single public table of annual totals for every year, so claims about “the wettest year ever” should be checked against IMD’s station records before being repeated.
2015 and 2023 side by side
The two floods most residents remember were caused by very different rain. In 2015, the decisive deluge came on 1 December. In 2023, Cyclone Michaung parked off the coast and rained on the city for two days. Figures reported from IMD by The News Minute:
| Event | Nungambakkam | Meenambakkam |
|---|---|---|
| 1 December 2015 (one day) | More than 29 cm | Nearly 35 cm |
| 4-5 December 2023 (24 hours) | 24 cm | 19 cm |
| 3-5 December 2023 (48 hours) | 47 cm | 42 cm |
The 2015 day was more intense at Meenambakkam, the airport station. The 2023 event delivered more total water over two days in the city core. Both show the same lesson: one or two days can bring a third of the year’s normal rain.
The last two Octobers
Recent years show how far a single month can stray from the normal. IMD’s sheet records 390.0 mm at Nungambakkam in October 2024, with 142.9 mm on 16 October alone, and 324.3 mm in October 2025, with 97.9 mm on 22 October. Both beat the 300.3 mm October normal. By contrast, October 2023 brought only 109.5 mm, and the season’s heavy rain came later, in December.
How Chennai rain actually falls
A study in IMD’s journal MAUSAM analysed a decade of hourly rain at Nungambakkam and Meenambakkam. Its findings help explain why streets flood on some days and not others:
- About 55% of rain spells through the year bring less than 2 mm.
- In most months, 60 to 70% of spells fall at under 7 mm an hour.
- In June to September, more than 65% of rainy days have rain lasting under two hours; in October to December that share falls below half, meaning longer, steadier rain.
Long monsoon spells saturate the ground and fill lakes, while short, intense bursts overwhelm drains. Chennai gets both in the same season.
Quick answers
What is Chennai’s average annual rainfall?
1,376.8 mm at Nungambakkam (IMD 1991-2020 mean). The state handbook gives a district normal of 1,346.2 mm.
What is Chennai’s wettest month on record?
October 2005 at Nungambakkam, with 1,077.8 mm, per IMD’s record sheet. The October normal is 300.3 mm.
Was 2023 worse than 2015?
In two-day totals at Nungambakkam, yes (47 cm). The one-day downpour of 1 December 2015 was heavier at Meenambakkam.
What is the forecast for 2026?
IMD expects normal northeast monsoon rain over south peninsular India for October to December 2026.
Information checked against official sources on 18 October 2026. Fees, dates and rules can change; confirm on the linked official pages before acting.
Sources
- Extreme Weather Events in the Month of October, Chennai Nungambakkam (IMD Regional Meteorological Centre, Chennai)
- Statistical Hand Book of Tamil Nadu 2022-23, Climate and Rainfall (Department of Economics and Statistics)
- Statistical Hand Book of Tamil Nadu 2020-21, Climate and Rainfall (Department of Economics and Statistics)
- Chennai received more rainfall in 2023 compared to 2015: a look at rainfall statistics (The News Minute)
- An analytical study of rainfall pattern over Chennai (MAUSAM, IMD journal, Vol. 55 No. 4, 2004)
