Tropical Storm
A tropical storm is a rapid rotating storm originating over tropical oceans. It has a low pressure centre and clouds spiralling towards the eyewall surrounding the ‘eye’. Its diameter is typically around 200 to 500 km, but can reach 1000 km. The related hazards are very violent winds, torrential rain, high waves, storm surges and in some cases tornadoes, causing direct effects such as flash floods, flooding, coastal inundation, and indirect effects such as landslides and mudslides. The winds blow anti-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere (WMO, 2020).
The intensity of tropical storms is based on the wind speed. A tropical storm is a tropical cyclone with the maximum sustained winds of 34 knots (17.5 m/s, 63 km/h) to 47 knots (24.2 m/s, 87 km/h) near the centre. When reaching this intensity, they are named in the interests of public safety (WMO, 2021).
Primary reference(s)
WMO, 2020. Tropical Cyclone Operational Plans. World Meteorological Organization (WMO).
WMO, 2021. Tropical Cyclone. World Meteorological Organization (WMO).
Additional scientific description
A tropical cyclone originates over tropical oceans from where it draws the energy to develop. In addition to sufficient energy from the ocean, a favourable environment is associated with enough moisture in the atmosphere, low to moderate windshear (difference between winds at low and upper atmospheric levels), and enough Coriolis force (a force associated with the rotation).
They can develop in the North Atlantic, Indian Ocean and Pacific Ocean. Depending on the basin, the terminology for this weather phenomenon differs: hurricane in the North Atlantic, typhoon in the western North Pacific, and tropical cyclone in the Indian Ocean and South Pacific Ocean.
Metrics and numeric limits
Depending on the maximum sustained wind speed, tropical cyclones are designated as follows:
- Tropical depression: when the maximum sustained wind speed is less than 63 km/h k or 34 knots
- Tropical storm: when the maximum sustained wind speed is more than 63 km/h or 34 knots. It is then also given a name.
- Hurricane, typhoon, tropical cyclone, cyclonic storm: when the maximum sustained wind speed exceeds 117 km/h or 63 knots.
Key relevant UN convention / multilateral treaty
Not applicable.
Examples of drivers, outcomes and risk management
.
References
WHO, 2020. Tropical cyclones. World Health Organization (WHO). Accessed 7 October 2020.
WMO, 2021. Storm Surge. World Meteorological Organization (WMO).
WMO, 2021. Tropical Cyclone. World Meteorological Organization (WMO).
The Operational Plans of the five WMO Tropical Cyclone Programme regional bodies, and their associated glossaries are available here.
World Meteorological Organization (WMO) Regional Association I - Tropical Cyclone Operational Plan for the South-West Indian Ocean, Tropical Cyclone Programme, report No. TCP-12, WMO-NO 1178
Typhoon Committee Operational Manual, Tropical Cyclone Programme, report No. TCP-23, WMO/TD No.196.
Panel on Tropical cyclones Operational Plan for the Bay of Bengal and the Arabian Sea, report No. TCP-21, WMO/TD-No. 84
WMO Regional Association IV – Hurricane Operational Plan for North America, Central America and the Caribbean. Tropical Cyclone Programme. WMO-No. 1163.
WMO Regional Association V - Tropical Cyclone Operational Plan for the South Pacific and South-East Indian Ocean, Tropical Cyclone Programme, report No. TCP-24, WMO-NO 1181.
Rappaport, E., 2014. Fatalities in the United States from Atlantic Tropical Cyclones: New Data and Interpretation. Accessed 13 May 2021.