Monsoon onset in Sivakasi
India Meteorological Department — Tamil Nadu has two monsoons, and here the second one matters more. During the southwest monsoon, June to September, the state lies in a rain shadow and receives only about 35% of its annual rainfall. Its chief rainy season is the northeast monsoon, October to December, which brings about 48%. IMD gives 20 October as the normal date for the northeast monsoon's onset over coastal Tamil Nadu.
Why our calculation does not apply here
We do not publish a calculated monsoon onset date for Sivakasi. Our method looks for the first sustained wet spell on or after 1 May, which across most of India is the southwest monsoon arriving. Here it is not: the southwest monsoon brings Sivakasi comparatively little rain, and the first spell to pass our test comes at very different points of the year from one year to the next — sometimes in May, sometimes not until October or later. A date that can land in any of those months is not the arrival of either monsoon, so we give IMD's figures instead.
The rainfall record for Sivakasi
What the record does show clearly is how much rain falls and when. Sivakasi receives about 948 mm in a year, and most of it does not come with the southwest monsoon: 23% falls between June and September, 55% between October and December. November is the wettest month with 216 mm across roughly 19 days of rain.
Temperature turns with the rain: May averages a daily maximum of 37.1°C, 7.8°C above November.
The last 12 years
These are the dates our rainfall rule produced. The year-to-year spread is real information even where the average is not trustworthy — but read them as the start of sustained rain, not as the monsoon arriving.
| Year | First sustained rain |
|---|---|
| 2013 | 18 November |
| 2014 | 25 September |
| 2015 | 23 September |
| 2016 | 9 October |
| 2017 | 5 October |
| 2018 | 21 September |
| 2019 | 20 September |
| 2020 | 24 October |
| 2021 | 15 September |
| 2022 | 8 June |
| 2023 | 14 September |
| 2024 | 6 May † |
† The 2024 date is the earliest in this record, stands well apart from every other year here, and several distant cities report exactly the same date. It is what the rule produced; nothing independent confirms it.
Withdrawal dates are left out for Sivakasi. Our rule for them looks no further than ten-day periods starting on or before 30 November, so the latest date it can give is 9 December, and in 21 of the 32 years here it gave that date or the day before. A date set by where the rule stops looking is not the end of the monsoon, so neither the dates nor the season lengths are shown.
Where these numbers come from
Source: Open-Meteo's historical weather archive, whose default model blends ERA5 and ERA5-Land reanalysis with ECMWF IFS HRES from 2017 — 1991-2024 (34 years). Open-Meteo offers no rainfall from ERA5-Land, so the rain figures come from ERA5 and IFS. Temperatures are monthly averages of daily highs and lows; rainfall is the average monthly total. A grid estimate can differ from a local weather station. The grid point used for Sivakasi sits about 1 km from the city. Onset dates are worked out from the same source's daily rainfall.
Our definition of onset is the first day on or after 1 May on which the five days starting that day bring at least 20 mm of rain, the ten days after those bring at least 30 mm, and each of the three ten-day periods starting that day brings at least 20 mm. The last condition is there to screen out cyclones; it was added after an earlier version of the rule recorded cyclone Tauktae, in May 2021, as the monsoon's arrival in four cities hundreds of kilometres apart. It narrows that problem without removing it: where a cyclone's rain runs straight into a wet June, the rule can still return a May date. Withdrawal is the last day of the last ten-day period, starting between 1 June and 30 November, that brings at least 20 mm.