Why do the CMA, Japan’s JMA, and the U.S. JTWC report different wind speeds for the same typhoon?
Key SummaryFor the same typhoon, why does the U.S. JTWC report a wind speed of 70 m/s (super typhoon), while the CMA reports 62 m/s and the Japan Meteorological Agency reports only 50 m/s? Which agency is the most accurate? This article explains the global weather agencies’ wind-speed averaging codes.
It’s not that anyone is calculating incorrectly—the “stopwatch” is different!
When you switch between agencies in the TyphoonSays multi-agency forecast panel, you will often find that the “maximum sustained wind speed” reported for the same typhoon at the same time varies by 10%–25%. This is mainly not due to measurement error, but to differences in the averaging periods used for wind speed:
| Meteorological agency | Wind-speed averaging period | Physical meaning | How the value compares for the same typhoon |
|---|---|---|---|
| Joint Typhoon Warning Center (JTWC) / NHC | 1-minute average wind speed | The highest average of sustained gusts over a 1-minute period | Highest value (close to the short-term extreme) |
| China Meteorological Administration (CMA) / Hong Kong Observatory | 2-minute average wind speed | China’s national standard, balancing sustained intensity and destructive potential | Intermediate value (about 0.88–0.92 times the 1-minute wind speed) |
| Japan Meteorological Agency (JMA) / World Meteorological Organization (WMO) | 10-minute average wind speed | The average after smoothing out wind-speed fluctuations over 10 minutes | Lowest value (about 0.80–0.85 times the 1-minute wind speed) |
With no anemometers at sea, how do agencies determine the wind speed at a typhoon’s center?
Across the vast northwestern Pacific Ocean, there are almost no fixed weather stations, and the U.S. military stopped routine reconnaissance aircraft penetration of western North Pacific typhoons in 1987. Today, more than 95% of the three major agencies’ assessments rely on the Dvorak Technique:
- Scoring satellite imagery (the CI number): Forecaster and AI algorithms use infrared satellite imagery from geostationary meteorological satellites, such as FY-4 and Himawari-9, to measure the warmth of the typhoon’s eye and the coldness of the eyewall cloud tops (the colder the cloud tops, the stronger the convection). This produces an intensity index ranging from 0.0 to 8.0 (the T number/CI value).
- Converting through empirical formulas: Each agency uses its own calibrated empirical conversion tables to translate the same CI value into wind speed and atmospheric pressure. For example, for a top-tier T8.0 typhoon such as HAIYAN in 2013, the JTWC converted it to 170 knots (87 m/s), the CMA assessed it at 72–78 m/s, and the JMA corresponded to 125 knots (65 m/s).
Which agency’s information is most useful to the general public?
- If a typhoon is about to affect or make landfall along China’s coast: follow the China Meteorological Administration (CMA) and your local provincial or municipal meteorological bureau! Once a typhoon enters the 400-km coastal waters of China, the country has the world’s densest coastal Doppler weather radar network, along with island automated weather stations, buoy stations, and typhoon-hunting drones. The precision of these nearshore observations far exceeds satellite-only estimates made by foreign agencies.