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Typhoons necessitating the issuing of the Hurricane Signal No.10
It is not very often that one sees the issuance of the highest tropical cyclone warning signal, Hurricane Signal 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". While Ragasa in 2025 passed about 120 km south of Hong Kong, No. 10 Signal was issued under the influence of its extensive hurricane force winds, making Ragasa the farthest tropical cyclone 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 small number 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 19 typhoons necessitated the issuance of the No. 10 Signal, meaning that only one out of about 60 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. The signal was issued on average once a year between 1960 and 1964, and even twice in 1964. Since those hectic days, the frequency was reduced to once in every three or four years. The three decades from the 1980s to the 2000s were relatively calm, with only two No. 10 Signals issued in 1983 and 1999. Since the 2010s, the occurrence has become relatively more frequent again. There were six No.10 Signals issued since 2010, respectively in 2012, 2017, 2018, 2023 and twice in 2025. In 2025, Wipha and Ragasa necessitated the issuance of the No. 10 signal by the Observatory, again since 1964 that the highest tropical cyclone warning signal was issued twice in a single year.
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.
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