How they form and why they spin
Typhoons form over tropical seas warmer than about 26–27°C. Water vapour evaporating from the warm ocean releases latent heat as it rises and condenses into cloud; that heat warms the air further, strengthening the updraft and drawing in surrounding air. The Coriolis effect of Earth's rotation turns the inflow into a vortex that spins anticlockwise in the northern hemisphere, which is why typhoons rarely form within about 5 degrees of the equator, where the effect is almost zero. A mature typhoon has a calm, sometimes clear eye at its centre, surrounded by an eyewall where the strongest winds and heaviest rain occur. About 25 typhoons form in the north-western Pacific each year, roughly three of which affect Korea, mostly from July to September.
Intensity scales and names
Under the international scale, storms are tropical storms (17–24 m/s), severe tropical storms (25–32 m/s) or typhoons (33 m/s and above). The Korea Meteorological Administration calls every storm of 17 m/s or more a typhoon and grades its strength as below; the "super strong" category was added in 2020 to reflect a rising share of intense storms.
| Strength (KMA) | Max sustained wind | Typical impact |
|---|---|---|
| Moderate | 25 to under 33 m/s | Signs and branches damaged |
| Strong | 33 to under 44 m/s | Roofs may be torn off |
| Very strong | 44 to under 54 m/s | People and large stones can be blown away |
| Super strong | 54 m/s and above | Buildings may collapse (added 2020) |
Since 2000 the Typhoon Committee of 14 Asia-Pacific members has used a rotating list of 140 names, ten from each member. Names of particularly destructive storms are retired and replaced, as happened to Maemi after 2003.
The dangerous semicircle and notable storms
In the northern hemisphere the right side of a typhoon's track is the "dangerous semicircle", because the storm's own winds add to the steering winds that move it; the left side is weaker. Heavy rain and storm surge often cause more damage than wind. Typhoon Rusa in 2002 dumped 870.5 mm on Gangneung in a single day, still South Korea's daily rainfall record, leaving 246 dead or missing and about 5 trillion won in damage; Maemi in 2003 brought violent winds and surge to the south coast and more than 4 trillion won in damage; Hinnamnor in 2022 flooded Pohang, where people died in an underground car park. As oceans warm, the total number of typhoons may not rise, but the share of the most intense storms and their rainfall are projected to increase (greenhouse effect). In El Niño years typhoons tend to form farther east and have longer to strengthen.
FAQ
- What is the difference between a typhoon and a hurricane?
- They are the same kind of tropical cyclone. It is called a typhoon in the north-western Pacific, a hurricane in the Atlantic and north-eastern Pacific, and a cyclone in the Indian Ocean and South Pacific; thresholds differ slightly.
- What should you do when a typhoon approaches?
- Follow official warnings, stay away from rivers, coasts and underground spaces, secure windows firmly in their frames, and move cars out of flood-prone underground car parks in advance.
Sources reviewed: 2026-10-26
References
- 기상청 국가태풍센터. (2024). 태풍의 정의와 강도·크기 분류. 기상청.
- World Meteorological Organization. (2023). Typhoon Committee Operational Manual: Meteorological Component. WMO/TD-No. 196.
- Emanuel, K. (2005). Increasing destructiveness of tropical cyclones over the past 30 years. Nature, 436(7051), 686–688.
- Knutson, T., et al. (2020). Tropical cyclones and climate change assessment: Part II. Projected response to anthropogenic warming. Bulletin of the American Meteorological Society, 101(3), E303–E322.