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The New 3-tier Typhoon Classification

The Observatory implements a 3-tier typhoon classification in 2009. The typhoon category is sub-divided into three levels, namely typhoon, severe typhoon and supertyphoon. Under the new classification, there will be six classes of tropical cyclones according to their maximum sustained wind speeds near the centre:

Tropical cyclone classes Maximum sustained winds near the centre (km/h)
Tropical Depression 41-62
Tropical Storm 63-87
Severe Tropical Storm 88-117
Typhoon 118-149
Severe Typhoon 150-184
Super Typhoon 185 or above

The new typhoon classification aims to prompt people to be extra vigilant on the approach of more intense typhoons. It has taken into account the intensity and frequency of occurrence of typhoons in the Asia Pacific region and is basically the same as that used in the Mainland to make it easier for people to understand tropical cyclone information from the Observatory and the Mainland meteorological authority.

The new typhoon classification provides more information on the tropical cyclone intensity but does not affect the Observatory's tropical cyclone warning system and the associated contingency measures. This is because the issue of tropical cyclone warning signal depends on the general wind conditions over Hong Kong, and this in turn depends on both the intensity of the tropical cyclone and its distance from Hong Kong. Therefore, the Tropical Cyclone Signal No.10 remains the highest warning signal and activates the highest level of alert and preparedness in Hong Kong. Irrespective of whether it is a typhoon, severe typhoon or super typhoon, the full set of precautionary measures should be taken by the public and all organizations when the Signal No.10 is issued. The table below shows the highest signal to be issued when the various classes of tropical cyclone make a direct hit to Hong Kong :

Tropical cyclone classes Highest tropical cyclone signal in
case of a direct hit at Hong Kong
Tropical Depression No.3
Tropical Storm No.8
Severe Tropical Storm No.9
Typhoon No.10
Severe Typhoon No.10
Super Typhoon No.10

In the 59 years from 1950 to 2008, 12 tropical cyclones passed close to Hong Kong and necessitated the issue of Signal No.10. Four of them belonged to the supertyphoon category (Wanda in 1962; Ruby in 1964; Rose in 1971 and Hope in 1979) while two belonged to the severe typhoon category (Gloria in 1957 and in Ellen 1983). Super Typhoon Wanda, Rose and Hope caused 130, 110 (including 88 deaths in a Hong Kong Macau Ferry) and 12 deaths respectively.