A large, slow-moving low-pressure system that forms within the summer monsoon, usually over the north of the Bay of Bengal, and drifts inland across India bringing three to five days of heavy, widespread rain. Together with weaker monsoon lows, it brings much of the monsoon rain over central India.
PhenomenaWhat is a monsoon depression?
A monsoon depression is a large, slow-moving low-pressure system that forms within the summer monsoon — most often over the north of the Bay of Bengal — and drifts inland across India, bringing several days of heavy, widespread rain. Together with weaker monsoon lows, it brings much of central India’s monsoon rain: a single depression can soak an area the size of several states for three to five days, and its circulation often spans 1,000–2,000 km.
The word depression has a precise meaning. On the India Meteorological Department (IMD) scale, low-pressure systems are graded by their sustained surface winds:
| Stage | Sustained wind |
|---|---|
| Low / well-marked low | below 31 km/h |
| Depression | 31–49 km/h |
| Deep depression | 50–61 km/h |
| Cyclonic storm (named) | 62 km/h and above |
Monsoon depressions rarely climb past deep depression. During the monsoon, strong easterly winds blow high in the atmosphere while the monsoon westerlies blow near the surface, and that difference — vertical wind shear — tears the top off any system trying to organise into a compact cyclone. The result is a system that cannot concentrate its energy into a violent core, but carries enormous amounts of moisture across a very wide area. That is precisely what makes it such an effective rain-producer.
How and where they form
Through the monsoon, a long line of low pressure called the monsoon trough stretches from north-west India across the Gangetic plain to the head of the Bay of Bengal. Depressions form mostly at its eastern end, over the warm waters of the north Bay near the coasts of Odisha, West Bengal and Bangladesh, often seeded by the remains of disturbances that drifted west from the South China Sea across Myanmar.
Once formed, a depression usually moves west-north-west along the trough — inland across Odisha, Jharkhand and Chhattisgarh toward Madhya Pradesh and Vidarbha, and in some cases as far as Gujarat and Rajasthan before it fills. A smaller number form over land, or over the Arabian Sea.
A feature forecasters rely on: the heaviest rain does not fall at the centre. It is concentrated in the south-west sector of the depression, so the belt of heavy rain runs on the left-hand side of the track as the system moves inland. Towns in that belt usually get the worst of it, even when the centre passes some distance away.
How often, and a changing pattern
A monsoon season always brings a series of low-pressure systems across central India, but how many reach full depression strength has changed. In the middle decades of the twentieth century a season typically brought several depressions; studies of the IMD record since then show a clear decline in systems reaching depression strength — in some recent seasons only one or two — while weaker lows have become more frequent.
The rain has not gone away; lows still deliver it. But researchers have also documented a threefold rise in widespread extreme rain events over central India between 1950 and 2015 (Roxy and colleagues, 2017), and together these findings point to more of the monsoon’s rain arriving in short, intense bursts rather than long, organised soakings. For flood forecasting and for farmers, that shift matters as much as the seasonal total.
What a depression brings
- Heavy rain along the track. Falls in IMD’s “very heavy” category (115.6–204.4 mm in 24 hours) are common near a depression’s path, and isolated “extremely heavy” totals above 204.4 mm are not unusual. Because the system moves slowly, those totals repeat day after day over neighbouring districts.
- River floods. Many of central India’s biggest floods — on the Mahanadi, Godavari and Narmada — follow a depression or a string of lows crossing the basin. The Mahanadi delta in Odisha and the lower Godavari are particularly exposed.
- Wind and sea near the coast. Squally winds and rough seas along the Odisha, Andhra and Bengal coasts while the system is offshore, with fishing warnings issued by IMD.
- A revived monsoon. A depression moving inland often ends a “break” in the monsoon. Arriving over a parched Vidarbha or Marathwada in a weak year, it can be the difference between a failed kharif crop and a saved one.
Depression, cyclone or western disturbance?
Three kinds of system bring rain to South Asia, and they are easily confused:
| Monsoon depression | Tropical cyclone | Western disturbance | |
|---|---|---|---|
| Main season | June–September | April–June and October–December | Mainly December–March |
| Forms | North Bay of Bengal, on the monsoon trough | Over the open Bay of Bengal or Arabian Sea | Far to the west, travelling east in the upper westerlies |
| Winds | 31–61 km/h | 62 km/h and above | Mostly high in the atmosphere |
| Main hazard | Days of widespread heavy rain, river floods | Wind, storm surge and rain at landfall | Winter rain, snow in the hills, hail |
Frequently asked questions
Is a monsoon depression the same as a cyclone? No. A depression is a weaker stage on the same scale — sustained winds of 31–49 km/h against 62 km/h and above for a named cyclonic storm. Monsoon depressions rarely strengthen further because wind shear during the monsoon prevents them from organising, but they often bring more total rain to inland India than a cyclone does.
How long does a monsoon depression last? Typically three to five days at depression strength. The rain-bearing low that remains after it weakens can drift on for several more days, so heavy rain may continue after the “depression” label has been dropped.
Why does the heaviest rain fall away from the centre? Because of how the system tilts and draws in moist monsoon air, the strongest rising motion — and so the heaviest rain — sits in its south-west sector. Forecast maps of heavy rain therefore run parallel to, but south of, the predicted track.
Why are there fewer depressions than there used to be? The decline is clear in the observational record, but its causes are still being studied, with changes in the large-scale monsoon circulation and ocean warming patterns both discussed. What matters in practice is that the rain now more often arrives through weaker lows and intense short spells.
Where can I follow the rain from a depression? Mausam Online shows the live conditions, hourly rain probability and seven-day outlook for cities across the usual depression tracks, including Bhubaneswar, Raipur, Ranchi, Jabalpur, Bhopal, Nagpur and Ahmedabad.