A relatively warm and sunny October

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The weather in Hong Kong was fine and unseasonably hot in the first half of October 2017. The Hong Kong Observatory issued the Very Hot Weather Warning twice in the month, the first time such warnings were issued in October, while cooler air from the north reached the south China coastal areas later in the month. Overall, the monthly mean temperature was 26.3 degrees, 0.8 degrees above the normal figure of 25.5 degrees. The monthly total rainfall was 99.6 millimetres, near the October normal of 100.9 millimetres. The accumulated rainfall this year up to October was 2540.9 millimetres, about 9 per cent higher than the normal figure of 2334.0 millimetres for the same period.

     Under the influence of a fresh easterly airstream, the weather in Hong Kong was mainly cloudy and showery on the first day of the month. Showery conditions persisted till the next morning and with winds moderating, the weather turned sunny and very hot. Temperatures at the Hong Kong Observatory rose to a maximum of 33.5 degrees on the afternoon of October 3, the highest of the month. Despite the recurrence of cloudy and showery interludes associated with a strengthening of the northeast monsoon on October 4 and October 9, the weather remained mostly fine and hot till October 13. The arrival of a northerly airstream brought cooler temperatures to Hong Kong over the next two days.

     Meanwhile, clouds and rain associated with an approaching tropical cyclone, Khanun, were gradually affecting the territory. The passage of Khanun over the northern part of the South China Sea, combined with the influence of the northeast monsoon, brought stormy weather to Hong Kong on October 15. The Observatory issued the No.8 Gale or Storm Signal for the fifth time this year, tying with the records of 1964 and 1999 as the years with the highest number of No.8 Gale or Storm Signal warnings issued. Despite Khanun moving away and weakening, windy and rainy weather continued to affect Hong Kong till October 17.

     With winds moderating, the weather turned sunny on October 18 and a spell of fine weather prevailed for the rest of the month as a continental airstream dominated over the south China coastal areas. Weather conditions got progressively drier with daytime relative humidity dropping below 50 per cent towards the end of the month. Meanwhile, freshening northerly winds brought replenishment of cooler air to the coast of Guangdong, and temperatures at the Observatory fell twice to around 20 degrees during this period. The lowest temperature of the month, 19.0 degrees, was recorded on the morning of October 31.

     Five tropical cyclones occurred over the South China Sea and the western North Pacific in October 2017.

     Details of issuance and cancellation of various warnings/signals in October are summarised in Table 1. Monthly meteorological figures and departures from normal for October are tabulated in Table 2.

Ends/Thursday, November 2, 2017
Issued at HKT 16:50

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Table 1 (Warnings and signals issued in October 2017)

Table 2 (Figures and departures from normal - October 2017)