The signals do not describe how strong the storm is. They describe the wind expected near sea level in Hong Kong.
The No. 1 has no wind threshold. It means a tropical cyclone has come within 800 km of Hong Kong and may affect it. The No. 3 corresponds to sustained winds of 41 to 62 km/h near sea level. The city stops at the No. 8.
The No. 8 threshold is a sustained 63 to 117 km/h — Beaufort force 8 through 11. The No. 10 begins at 118 km/h, hurricane force. Between the No. 3 and the No. 8, the ceiling almost doubles.
Why a network, not one station
Wind across Hong Kong is far from uniform: Cheung Chau can be in a gale while inland districts see no more than a stiff breeze. So the Observatory does not read one station. It asks how many in a reference network of anemometers record, or are expected to record, gale force.
The condition for a No. 8 is gale or storm force wind near sea level in Hong Kong — a sustained 63 to 117 km/h — either already blowing or expected. That second half carries the weight: the signal can go up before the wind arrives.
Sustained wind and gusts
All of the above are ten-minute mean sustained winds. Gusts typically run 30 to 50 per cent higher, so a 130 km/h gust under a No. 8 does not mean a No. 10 was warranted.
Nor does a signal come down the moment a storm weakens. Before replacing a No. 8, the Observatory judges whether the wind is settling for good. The trailing side of a circulation often lasts longer than the centre's passage, and offshore areas are the last to ease.
Whether a signal goes up, and when, is the Hong Kong Observatory's decision, not the output of a threshold. The signal probabilities this site computes are its own estimate, appear only inside the map with their caveats, and never override an official bulletin.
