The first organized stage of a tropical cyclone with sustained winds below 62 km/h. Many monsoon depressions never get stronger but still deliver torrential rain.
PhenomenaWhat is a Tropical Depression?
A tropical depression is the first formally organized stage of a tropical cyclone. It develops from a tropical disturbance when it acquires:
- A closed low-level circulation — winds blowing cyclonically around a defined center.
- Organized deep convection — thunderstorm activity concentrated near the center.
- Sustained winds of 31-61 km/h (17-33 knots).
In the North Indian Ocean (Bay of Bengal and Arabian Sea), IMD uses two sub-classifications:
| Stage | Sustained Wind (km/h) | Sustained Wind (knots) |
|---|---|---|
| Depression | 31–49 | 17–27 |
| Deep Depression | 50–61 | 28–33 |
| Cyclonic Storm (= Tropical Storm) | 62–88 | 34–47 |
A depression that intensifies further becomes a “cyclonic storm” and receives a name. A depression that stays below 62 km/h winds never gets named but can still produce major weather impacts.
Tropical depressions in South Asia
The Bay of Bengal generates most of the North Indian Ocean’s tropical depressions. During the monsoon (June-September), the whole North Indian Ocean averages 4.9 depressions or stronger systems per year. Many never intensify beyond the depression stage but still travel inland and deliver heavy rain.
Typical monsoon depression life cycle:
- Formation in the head of the Bay of Bengal (typically 17-22°N, 88-95°E).
- Westward / northwestward track at 10-25 km/h, steered by the monsoon trough.
- Landfall typically over Odisha, West Bengal or Bangladesh coast.
- Slow inland decay over 2-5 days, delivering rain to Bihar, Jharkhand, UP, MP, Chhattisgarh.
A typical monsoon depression delivers:
- 100-300 mm rainfall over 3-5 days
- Affects an area roughly 500 × 500 km
- Recharges reservoirs and aquifers
- But also causes localized flooding, especially in already-saturated soils
Why depressions matter even without becoming cyclones
Several reasons:
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Rainfall, not wind, is the main hazard. A slow-moving depression dumping 200-300 mm over 3 days causes more flooding than a single fast-moving severe cyclone.
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Frequency multiplies impact. The North Indian Ocean averages 4.9 depressions or stronger systems per monsoon; each affects a wide area, and together with weaker low-pressure areas they bring up to 57% of the monsoon rainfall in India’s monsoon core zone, though depressions alone bring at most 17%.
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They support kharif agriculture. Bihar, Odisha and West Bengal’s rice crop depends on the steady moisture supply from monsoon depressions.
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They can intensify suddenly. Some depressions intensify rapidly into severe cyclones when over warm water — e.g., Cyclone Hudhud (2014) intensified from a depression on 7 October to a very severe cyclonic storm by 10 October.
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They produce flash floods. Even moderate depressions in mountainous areas (Northeast India, Bangladesh hills) can produce flash floods.
Notable depressions and their impacts
- 2017 Bihar floods: A weak monsoon depression stalled over Bihar, producing 200+ mm rainfall in 3 days. 514 deaths.
- 2008 Bihar Kosi floods: The Kosi River embankment breached at Kusaha in Nepal while the river carried only about 15% of its design discharge; poor maintenance, not depression rainfall, was the main cause. Millions affected in Bihar.
- 2022 Pakistan super-floods: Multiple monsoon depressions stalled over Sindh and Balochistan. 33 million affected, $30+ billion damage.
- 2023 Sikkim GLOF: A mass of frozen lateral moraine collapsing into the lake, not depression rainfall, set it off. South Lhonak burst, dozens of deaths and many people missing.
Formation conditions
A tropical depression requires:
- Sea-surface temperature ≥ 26.5°C to depths of 50+ m.
- High lower-tropospheric humidity.
- Atmospheric instability supporting convection.
- Low vertical wind shear (under 10 m/s).
- Coriolis force (latitude > ~5°).
- Pre-existing disturbance — a low-pressure area or easterly wave to organize around.
In the Bay of Bengal during monsoon, all these conditions are routinely satisfied — which is why depressions are so frequent there.
Monsoon depressions vs typhoons
Two distinct types of tropical depressions affect the region:
Monsoon depressions:
- Form within the active monsoon trough.
- Slow-moving (10-25 km/h).
- Wide rain shield (500+ km).
- Less wind-driven, more rain-driven.
- Usually peak at depression or cyclonic-storm stage.
Pre/Post-monsoon cyclonic depressions:
- Form outside the monsoon trough.
- Can intensify more rapidly.
- Smaller, more intense circulation.
- Greater wind risk.
- Often reach severe-cyclone stage.
For South Asia, both types matter: monsoon depressions for water/flood risk, pre/post-monsoon storms for wind/surge risk.
How depressions are tracked
IMD’s Cyclone Warning Division at RSMC New Delhi monitors the Bay of Bengal and Arabian Sea continuously. They issue:
- Tropical Weather Outlook daily (at 0600 UTC), all year round.
- Special Bulletins when a depression forms.
- 3-hourly updates as a depression approaches landfall.
- Stage-specific colour-coded alerts: Green → Yellow → Orange → Red.
Pakistan’s PMD and Bangladesh’s BMD issue parallel warnings for their respective coastal areas.
Modern tools:
- Doppler radar networks track precipitation distribution.
- Geostationary satellites (INSAT-3D, Himawari) provide continuous cloud imagery.
- Numerical weather models (ECMWF, GFS, India’s NCMRWF) project track and intensity 5-7 days out.
- Mobile alerts via Sachet app (India), the BMD Weather App and the toll-free 1090 line (Bangladesh), PMD (Pakistan).
Frequently asked questions
What’s the difference between a tropical depression and a low-pressure area? A low-pressure area is unorganized — just a region of relatively low surface pressure with no clear circulation. A tropical depression has organized convection and a defined cyclonic circulation, though weak.
Do tropical depressions get names? No. Names are assigned only when a system reaches “Cyclonic Storm” stage (sustained winds ≥ 62 km/h). Depressions and deep depressions are referred to by an IMD identifier (e.g., “BOB/01/2024”).
Are tropical depressions dangerous? Yes — primarily for flooding and inland impacts. Most casualties from tropical cyclones in South Asia come from storm surge, not from cyclone-grade winds. Don’t dismiss a “weak” depression — its rain footprint can be devastating.
How many tropical depressions hit India per year? The Bay of Bengal averages 7.8 depressions or stronger systems a year, out of 11.2 for the whole North Indian Ocean (IMD, 1965-2021). Many cross over Odisha, Bengal, Bangladesh. The Arabian Sea sees 2.3 annually.
Where can I check for depression alerts? IMD’s RSMC New Delhi website is the official source for the North Indian Ocean. Mausam Online displays precipitation forecasts and severe-weather codes on every city page. See Bhubaneswar, Kolkata, Visakhapatnam, Dhaka, Chittagong.