Tropical Cyclone Classification
Tropical Cyclone Classification
CHAN Chik-cheung
How many categories are there in the classification of tropical cyclones?
Tropical cyclones of different intensity are given different names. Tropical cyclones are classified in accordance with the World Meteorological Organization's recommendation by their maximum sustained wind speeds near the centre. In Hong Kong, the classification is defined in terms of wind speeds averaged over 10 minutes. With effect from 2009, a new classification is used and consists of 6 categories as follows:
| Tropical Cyclone Classification | Maximum sustained winds near the centre (km/h) |
|---|---|
| Tropical Depression (TD) | 41-62 |
| Tropical Storm (TS) | 63-87 |
| Severe Tropical Storm (STS) | 88-117 |
| Typhoon (T) | 118-149 |
| Severe Typhoon (ST)* | 150-184 |
| Super Typhoon (SuperT)* | 185 or above |
*New categories starting 2009
| Tropical Cyclone Classification | Maximum sustained winds near the centre (km/h) |
|---|---|
| Tropical Depression (TD) | 41-62 |
| Tropical Storm (TS) | 63-87 |
| Severe Tropical Storm (STS) | 88-117 |
| Typhoon (T) | 118 or above |
The former classification consists of only 4 categories
What is the difference between the new and old classifications?
The first three categories, namely TD, TS and STS, remain the same as before. The old typhoon category will be further divided into three levels, viz T, ST and SuperT.
With the new typhoon classification, what will be the frequency of occurrence of different categories of typhoons?
For all typhoons requiring the Signal No.8 or above from 1950 to 2008, 75% were typhoons, 15% severe typhoons and 10% super typhoons.
How often will Hong Kong have a direct hit of a super typhoon or severe typhoon?
In the 53 years from 1956 to 2008, 12 tropical cyclones necessitated the issue of Signal No.10 in Hong Kong. At a certain point in their lifetime, four of them belonged to the super typhoon category (Wanda 1962; Ruby 1964; Rose 1971 and Hope 1979) while two belonged to the severe typhoon category ( Gloria 1957 and Ellen 1983).
Why is typhoon classification introduced?
Typhoon classification is introduced to facilitate the public in recognizing more intense typhoons, prompting people to be extra vigilant with the approach of more intense tropical cyclones.
How does the new classification affect the Tropical Cyclone Warning System in Hong Kong?
The Tropical Cyclone Warning System in Hong Kong will remain unchanged. The Observatory does not anticipate that the introduction of typhoon classification will affect the existing contingency plans for tropical cyclones.