Monsoon onset: how IMD declares it and what it means
The three tests the India Meteorological Department applies before announcing the monsoon over Kerala, and why that announcement is not a forecast for your district.
Every year the news says the monsoon "has arrived in Kerala", and officials, farmers and dam operators far away start asking what that means for them. The answer depends on knowing what the announcement is. Monsoon onset is the date on which the India Meteorological Department (IMD) judges that the southwest monsoon has begun over Kerala, the first Indian state it normally reaches. This article explains the monsoon onset criteria IMD uses, how onset differs from advance and withdrawal, and what the date does and does not tell you.
What is the onset of monsoon?
The onset of the monsoon is the transition from the pre-monsoon state to the monsoon state: a sharp, sustained rise in rainfall together with a change in the large-scale winds. IMD describes onset as occurring "in many stages" and as a significant transition in the large-scale circulation of the atmosphere and ocean in the Indo-Pacific region [1]. The southwest monsoon normally sets in over Kerala around 1 June and, in the normal pattern, covers the whole country around 8 July [1].
Onset is therefore a marker on the calendar, not a rainfall total. It records the moment when a seasonal change is judged to have happened at one place.
Monsoon onset criteria: how does IMD declare onset over Kerala?
IMD declares onset over Kerala only when three tests agree. The rainfall test is the trigger; the wind and radiation tests must support it. The criteria below follow IMD's published description [1][2].
| Test | What must be true |
|---|---|
| Rainfall | After 10 May, at least 60 % of 14 listed stations in and around Kerala report 2.5 mm or more on two consecutive days. Onset is declared on the second day. |
| Wind field | Westerly winds must reach up to the 600 hPa level (roughly 4 km above the sea) in a box from the equator to 10° N and 55° E to 80° E. Zonal wind at 925 hPa over 5–10° N, 70–80° E should be 15–20 knots. |
| Outgoing longwave radiation (OLR) | Satellite-derived OLR below 200 W/m² in a box over the Arabian Sea off southern India. |
The 14 stations are Minicoy, Amini, Thiruvananthapuram, Punalur, Kollam, Alappuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery, Kannur, Kasaragod and Mangaluru [2]. OLR is the heat radiation leaving the top of the atmosphere. Thick, high rain clouds are cold, so they radiate little, and a low OLR value is a satellite signature of deep cloud.
The wind and OLR tests exist so that a few isolated thundershowers cannot trigger a declaration. IMD's 2016 criteria emphasise "the sharp increase in rainfall over Kerala along with setting up of large-scale monsoon flow" [1]. IMD also issues a forecast of the Kerala onset date before the season, from statistical models built for its operational use [3].
A worked example (illustrative)
Sixty per cent of 14 stations is 8.4, so at least nine stations must qualify. Suppose that on 3 June nine stations record 2.5 mm or more, and on 4 June ten do. The rainfall test is met on 4 June. If the westerlies reach 600 hPa and OLR is below 200 W/m² by then, onset is declared on 4 June. If only eight stations had reached 2.5 mm on 4 June, the count would stay at 57 % and the test would not yet be met, however heavy the rain at those eight.
What happens after onset: advance and withdrawal
Once the monsoon is over Kerala, its progress north is tracked as the northern limit of the monsoon. For the rest of the country there is no fixed formula. IMD says advance is declared from rainfall over parts of subdivisions, keeping the northern limit continuous, and that elsewhere onset rests on a sharp rise in rainfall and its characteristic sustenance for a few days [1]. A subjective element therefore enters once the monsoon leaves Kerala.
Withdrawal is also defined. IMD considers withdrawal from the extreme northwest only after 1 September, and requires five days without rain, a lower-level anticyclone and reduced moisture. The monsoon is declared withdrawn from the whole country around 15 October in the normal pattern [1]. Between onset and withdrawal, rainfall comes in active spells and break spells. IMD defines a break as a spell of more than two days in which the surface monsoon trough is not seen and easterlies are practically absent in the lower atmosphere, mainly in July and August [1]. A monsoon declared "set in" can still have dry weeks.
What onset means for you
Onset does not tell you the season's rainfall. A study of onset over Kerala, using a regional climate model and IMD's observed dates, found the onset date only weakly correlated with all-India summer monsoon rainfall, with no one-to-one match between early, normal or late onset and the seasonal total [4]. An early date is neither good news nor bad news by itself.
It is not your local start date. In Himachal Pradesh, IMD's Shimla centre records onset dates from 9 June (2000) to 5 July (2010), and describes the 29 June arrival in 2022 as 5 to 7 days later than the normal date [5]. That puts the normal date more than three weeks after Kerala's, and the arrival in the hills depends on how the northern limit moves up the western Himalaya, not on what happened in Kerala.
It is not a sowing signal. Agro-meteorological advisories tie sowing to rain actually received. One national bulletin, for example, advised sowing kharif sorghum in a Maharashtra district "only after receipt of sufficient amount of rainfall (75-100 mm)" [6]. Check the advisory for your district rather than the onset headline.
It is not a flood forecast. Onset says nothing about extreme rainfall. In our study of the July 2023 floods in Himachal Pradesh, the flood peak came from extremely heavy rain falling on near-saturated soil, with snowmelt adding to the flow [7]. A conventional "normal" onset can still be followed by a flash flood. For a hill catchment, the useful question is what the soil, the river and the forecast look like this week; see flash floods vs river floods and river monitoring in hill and Himalayan catchments.
It is a cue to be ready. The months before onset are the time for maintenance and testing, and the pre-monsoon readiness checklist lists what to check. Once the monsoon is in, the question changes to how this season compares with normal, which is how IMD classifies it in normal, excess, deficient: how IMD classifies monsoon rainfall.
Key points
- IMD declares onset over Kerala when rainfall at 60 % of 14 stations reaches 2.5 mm on two consecutive days, and westerlies reach 600 hPa and OLR falls below 200 W/m².
- The normal Kerala onset date is around 1 June; the monsoon normally covers the country around 8 July and leaves around 15 October [1].
- Advance beyond Kerala is declared less formulaically, from sustained rainfall and a continuous northern limit.
- Onset date has only a weak link to seasonal rainfall, and none to a particular flood or dry spell.
- For a district, watch the local rainfall, the soil and the river, not the Kerala date.
Where ClimMaTech fits
Local rainfall is what turns an onset announcement into decisions for a district or a catchment. At ClimMaTech we are developing VARSHAQ, a weather station that will record rainfall and weather at the site. It is in development and not yet available. Our river-level and discharge devices already record the response of the river to that rain.
References
- India Meteorological Department, Frequently Asked Questions (FAQs) on Monsoon, Q34 (onset criteria) and Q47–48 (active and break spells), IMD Pune. imdpune.gov.in
- India Meteorological Department, Lecture Notes on Synoptic Meteorology, IMD Training Centre Pune, section "Objective criteria of onset of SW monsoon over Kerala". imdpune.gov.in
- D. S. Pai, Rajeevan M. Nair, Summer monsoon onset over Kerala: new definition and prediction, Journal of Earth System Science 118(2), 123–135, 2009. doi:10.1007/s12040-009-0020-y
- K. C. Pattnayak, S. K. Panda, V. Saraswat, S. K. Dash, How good is regional climate model version 4 in simulating the monsoon onset over Kerala?, International Journal of Climatology 39(4), 2197–2208, 2019. doi:10.1002/joc.5945
- India Meteorological Department, Meteorological Centre Shimla, Monsoon Report 2022 (onset on 29 June 2022 against the normal date) and Monsoon 2023 in Himachal Pradesh (earliest and latest onset on record). 2022 report; 2023 report
- AICRP on Agrometeorology, ICAR-CRIDA, National Agromet Advisory Services Bulletin, issued 2 July 2024 (Parbhani advisory). icar-crida.res.in
- V. Gupta, B. Syed, A. Pathania, S. Raaj, A. Nanda, S. Awasthi, D. P. Shukla, Hydrometeorological analysis of July-2023 floods in Himachal Pradesh, India, Natural Hazards 120(8), 7549–7574, 2024. doi:10.1007/s11069-024-06520-5