clacke@musicman It's the Hong Kong Observatory's own system, introduced in the early 20th century with several revisions until the 1970s.
There is an international rating, but I'm not familiar with how it works.
The HKO typhoon warning rating is directly tied to institutional responses:
T1 is triggered by the distance to a typhoon from Hong Kong and is a warning only, no particular operational response.
T3, T8 and T10 are tied to wind speeds and cause certain institutions and businesses to shut down, go into mobilization, etc.
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