The Weather of October 2017

   With cooler air from the north only reaching the south China coastal areas later in the month, there were fine and unseasonably hot days in the first half of October 2017. The Very Hot Weather Warning was issued on a couple of occasions, the first time such warnings were required in the month of October. Overall, the monthly mean temperature was 26.3 degrees, 0.8 degree 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 percent 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 soon turned sunny and very hot. Temperature at the Hong Kong Observatory rose to a maximum of 33.5 degrees, the highest of the month, on the afternoon of 3 October. Despite the recurrence of cloudy and showery interludes associated with a strengthening of the northeast monsoon on 4 October and again on 9 October, the weather remained mostly fine and hot till 13 October, when the arrival of a northerly airstream brought cooler temperatures to Hong Kong over the next couple of days.

   Meanwhile, clouds and rain associated with an approaching tropical cyclone, Khanun, were also moving in towards the territory. The passage of Khanun over the northern part of the South China Sea, combined with the influence of the northeast monsoon, brought a day of stormy weather on 15 October when the Tropical Cyclone Gale or Storm Signal No.8 was issued for the fifth time this year, a joint record with the years 1964 and 1999. Despite Khanun moving away and weakening, windy and rainy weather continued to affect Hong Kong till 17 October.

   With winds moderating, the weather turned sunny on 18 October and a spell of fine weather then prevailed for the rest of the month as a continental airstream dominated over the south China coastal areas. 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 that caused temperature at the Hong Kong Observatory falling twice to around 20 degrees during the period. The lowest temperature of the month, 19.0 degrees, was recorded on the morning of 31 October.