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Tropical Cyclones necessitating the issuing of the Hurricane Signal No.10

Typhoons necessitating the issuing of the Hurricane Signal No.10

It is not very often that one sees the issuing of the ultimate tropical cyclone warning signal of No. 10.

When this signal is issued, it means that winds of hurricane force (i.e.118 km/h or more) are expected to affect the territory. Since hurricane force winds are largely confined to a relatively narrow strip around the eye wall of a tropical cyclone, it would normally require a storm of typhoon or above status to pass fairly close to (say, within 100 km of) Hong Kong. This is commonly referred to as a "direct hit". Both Vicente in 2012 and Mangkhut in 2018 passed about 100 km of Hong Kong, and both were the farthest tropical cyclones that had necessitated the issuance of the No. 10 Signal since 1946.

Considering Hong Kong is such a small target along the long coastline of eastern Asia, one finds it not surprising that only a dozen or so of typhoons have managed to score a direct hit in the post Second World War years, even with an average production rate of about 15 typhoons per year (normal for 1961-2020) over the South China Sea and the western North Pacific. From 1946 onwards, only 17 typhoons necessitated the issuance of the No. 10 Signal, meaning that only one out of about 70 typhoons in the region necessitated the issuance of the No. 10 Signal in Hong Kong.

The early 1960s was a busy time for the No. 10 signal. It averaged once a year between 1960 and 1964, with 1964 being the special year when the signal was issued twice. Since those hectic days, the frequency was reduced to once in every three or four years. It was relatively quiet from 1981 to 2010 with only two No. 10 Signals issued in 1983 and 1999. There were four No.10 Signals issued after 2010, respectively in 2012, 2017, 2018 and 2023.

Of all the typhoons since 1946 for which the No. 10 had to be issued, Alice in 1961 and Shirley in 1968 crossed the Hong Kong Observatory. York in 1999 had the No.10 in force the longest - 11 hours. In terms of ferocity, Wanda sets the record. As the eye of Wanda passed over the territory, the barometer plunged to the all-time low of 953.2 hPa on 1 September 1962. Hourly mean winds of 133km/h and gusts of 259 km/h recorded in the harbour as well as gusts of 284 km/h recorded at Tate's Cairn during the passage of Wanda are records yet to be broken.

The closest approach of Wanda coincided with the high tide and caused severe flooding in low-lying areas. Water level in the Tolo Harbour was as high as 5.4 metres above chart datum as compared with the normal high tide of 2.2 metres. Damage due to the tidal surge was particularly severe at Sha Tin and Tai Po. Although Hong Kong was near the period of neap tides during the passage of Mangkhut on 16 September 2018, the extensive circulation and fierce winds of Mangkhut triggered record-breaking storm surge over many places and caused severe flooding in many coastal low-lying areas in Hong Kong.