A tropical cyclone with sustained winds of 63–117 km/h — the stage between tropical depression and a hurricane/typhoon/severe cyclonic storm.
phenomenaWhat is a Tropical Storm?
A tropical storm is the second formal stage in the life cycle of a tropical cyclone. It develops from a tropical depression when its maximum sustained 10-minute winds reach 63 km/h (34 knots) but remain below the hurricane/typhoon/severe cyclonic storm threshold of 118 km/h (64 knots).
The naming conventions differ by basin:
| Basin | Name at this stage |
|---|---|
| North Indian Ocean (Bay of Bengal, Arabian Sea) | “Cyclonic Storm” |
| Atlantic, Eastern Pacific | Tropical Storm |
| Western Pacific (Japan, Philippines) | Tropical Storm |
| South Pacific, South Indian Ocean | Tropical Storm |
So when IMD declares a “Cyclonic Storm” in the Bay of Bengal, that is meteorologically the same as a “tropical storm” elsewhere.
At this stage, the system receives a name from a pre-assigned regional list. For the Bay of Bengal and Arabian Sea, RSMC New Delhi maintains the naming list, with contributions from the 13 WMO/ESCAP Panel countries.
Tropical storm classification across basins
The standardized IMD scale used for the North Indian Ocean:
| IMD Category | Sustained Wind (km/h) | Equivalent |
|---|---|---|
| Low Pressure Area | < 31 | Pre-cyclone |
| Depression | 31–50 | Tropical depression |
| Deep Depression | 51–62 | Strong depression |
| Cyclonic Storm | 63–88 | Tropical storm |
| Severe Cyclonic Storm | 89–117 | Severe tropical storm |
| Very Severe Cyclonic Storm | 118–166 | Cat 1-2 hurricane |
| Extremely Severe Cyclonic Storm | 167–221 | Cat 3-4 |
| Super Cyclonic Storm | ≥ 222 | Cat 5 |
Note that the IMD uses “Cyclonic Storm” for what other regions call “Tropical Storm” — and “Severe Cyclonic Storm” for what’s elsewhere a “Severe Tropical Storm.”
What happens at the tropical storm stage
The transition from depression to tropical storm involves several developments:
- Better organization — convection becomes more circular and symmetric around the center.
- Closed low-level circulation — winds blow cyclonically around a clear center.
- Banding features — visible spiral cloud bands form.
- Warm core — the storm’s center is warmer than the surrounding atmosphere (latent heat release).
- Pressure drops — central pressure typically falls to 1000-985 hPa.
- Surface winds increase — reaching 63 km/h sustained, with gusts higher.
At this stage, the storm is organized enough to be named but not yet powerful enough to be classified as a severe storm or cyclone.
Tropical storms in South Asia
The Bay of Bengal sees on average 4-6 named tropical storms per year, and the Arabian Sea sees 1-2. Of these, roughly half intensify into severe cyclonic storms or higher, and the remainder weaken or dissipate at the tropical storm stage.
Recent Bay of Bengal “Cyclonic Storms” (tropical storms):
- 2024 Cyclone Dana (October) — Made landfall as cyclonic storm in Odisha, weakened from severe stage.
- 2024 Cyclone Asna (August) — Arabian Sea cyclonic storm, rare August event.
- 2023 Cyclone Hamoon (October) — Bangladesh impact.
- 2023 Cyclone Tej (October) — Arabian Sea, near Yemen.
- 2022 Cyclone Sitrang (October) — Bangladesh impact.
Even though these are “weaker” than severe cyclones, they regularly cause significant damage:
- Flooding from 100-300 mm rainfall over 24 hours
- Coastal inundation from 1-3 metre storm surge
- Wind damage to fragile housing, trees, power infrastructure
- Crop losses to standing rice and standing crops in coastal districts
Why tropical storms still need respect
A common misconception: “It’s only a tropical storm, not a hurricane — so it’s not dangerous.” Several reasons this is wrong:
- Rainfall, not wind, kills most people in tropical systems. A slow-moving tropical storm can dump 500+ mm rain over 3 days.
- Storm surge can still be 1-3 metres even with sub-hurricane winds.
- Inland flooding from heavy rain extends far beyond the coastal landfall zone.
- Landslides in hilly terrain triggered by sustained rainfall.
- Sub-hurricane wind damage to power lines, billboards, fragile structures.
Naming conventions in the Bay of Bengal
RSMC New Delhi (IMD) maintains the cyclone naming list for the North Indian Ocean (NIO). Names contributed by:
- Bangladesh, India, Iran, Maldives, Myanmar, Oman, Pakistan, Qatar, Saudi Arabia, Sri Lanka, Thailand, UAE, Yemen
Each country contributes 13 names, used in rotation. The list is “active” — once exhausted, a new list is published.
Recent names from rotation: 2023: Mocha, Biparjoy, Tej, Hamoon, Midhili, Michaung 2024: Remal, Asna, Dana, Fengal, Yagi (in Pacific) 2025-26: Shakhti, Montha, Senyar, Ditwah…
Frequently asked questions
What’s the difference between a tropical depression and tropical storm? Wind speed. A tropical depression has sustained winds below 63 km/h (34 kt). A tropical storm has sustained winds 63–117 km/h (34–64 kt). A storm becomes a hurricane/typhoon/very severe cyclonic storm when winds exceed 118 km/h.
Why is IMD’s terminology different? The IMD uses “Cyclonic Storm” rather than “Tropical Storm” — a regional naming preference. Internationally, both refer to the same wind-speed range (63-117 km/h sustained). The IMD’s classification adds intermediate “Severe Cyclonic Storm” between this and full cyclone.
Is a tropical storm “less dangerous” than a hurricane? The wind is less dangerous, but rainfall, flooding and storm surge can be equally or more dangerous. Slow-moving tropical storms over land (like 2017 Hurricane Harvey, which stalled and dumped 1,500 mm of rain on Houston) demonstrate this clearly.
Does a tropical storm have an eye? Usually no, or only a poorly-defined eye. The eye structure typically forms when winds reach hurricane/typhoon strength.
Where can I track tropical storms approaching India or Bangladesh? IMD’s RSMC New Delhi website provides 3-hourly updates on Bay of Bengal and Arabian Sea systems. Mausam Online shows precipitation forecasts and severe-weather warnings on every city page. See Chennai, Visakhapatnam, Kolkata, Bhubaneswar, Dhaka.