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  • The Weather of October 2017

  • 2 November 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.

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

Details of issuance and cancellation of various warnings/signals in the month are summarized in Tables 1.1 to 1.6. Monthly meteorological figures and departures from normal for October are tabulated in Table 2.

Warnings and Signals issued in October 2017

Table 1.1   Tropical Cyclone Warning Signals
Name of
Tropical Cyclone
Signal
Number
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
KHANUN 1 14 / 10 1040 14 / 10 1910
3 14 / 10 1910 15 / 10 0840
8 NE 15 / 10 0840 15 / 10 1920
3 15 / 10 1920 15 / 10 2240
1 15 / 10 2240 16 / 10 0220


Table 1.2   Strong Monsoon Signal
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
8 / 10 2245 10 / 10 0940
14 / 10 0420 14 / 10 0845
16 / 10 1530 17 / 10 0310
17 / 10 0615 17 / 10 1245


Table 1.3   Rainstorm Warning Signals
Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Amber 17 / 10 0310 17 / 10 0420


Table 1.4   Thunderstorm Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
1 / 10 0925 1 / 10 1300
1 / 10 1530 1 / 10 1700
2 / 10 0050 2 / 10 0215
2 / 10 1535 2 / 10 1615
4 / 10 1035 4 / 10 1500


Table 1.5   Fire Danger Warnings
Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Yellow 5 / 10 0830 5 / 10 1845
Yellow 7 / 10 0830 7 / 10 1800
Yellow 8 / 10 0600 8 / 10 1800
Yellow 14 / 10 0600 14 / 10 1700
Yellow 21 / 10 0600 22 / 10 0600
Red 22 / 10 0600 23 / 10 1915
Red 24 / 10 1100 24 / 10 1930
Red 27 / 10 0945 31 / 10 2100


Table 1.6   Very Hot Weather Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
3 / 10 1220 3 / 10 1900
12 / 10 1335 12 / 10 1800


Table 2   Figures and Departures from Normal - October 2017
Meteorological Element Figure of the Month Departure from Normal*
  Mean Daily Maximum Air Temperature 29.0 degrees C 1.2 degree above normal
  Mean Air Temperature 26.3 degrees C 0.8 degree above normal
  Mean Daily Minimum Air
Temperature
24.4 degrees C 0.7 degree above normal
  Mean Dew Point Temperature 20.4 degrees C 0.2 degree above normal
  Mean Relative Humidity 71 % 2 % below normal
  Mean Cloud Amount 57 % 1 % below normal
  Total Rainfall 99.6 mm 1.3 mm below normal
  Number of hours of Reduced VisibilityΔ 0 hours 137.1 hours below normal§
  Total Bright Sunshine Duration 228.6 hours 34.7 hours above normal
  Mean Daily Global Solar Radiation 16.35 Megajoule / square
metre
2.30 Megajoule above normal
  Total Evaporation 124.1 mm 0.2 mm above normal


  Remarks : All measurements were made at the Hong Kong Observatory except sunshine, solar radiation and evaporation which were recorded at King's Park Meteorological Station and visibility which was observed at the Hong Kong International Airport.

  Δ

The visibility readings at the Hong Kong International Airport are based on hourly observations by professional meteorological observers in 2004 and before, and average readings over the 10-minute period before the clock hour of the visibility meter near the middle of the south runway from 2005 onwards. The change of the data source in 2005 is an improvement of the visibility assessment using instrumented observations following the international trend.
Before 10 October 2007, the number of hours of reduced visibility at the Hong Kong International Airport in 2005 and thereafter displayed in this web page was based on hourly visibility observations by professional meteorological observers. Since 10 October 2007, the data have been revised using the average visibility readings over the 10-minute period before the clock hour, as recorded by the visibility meter near the middle of the south runway.


  *   Departure from 1981 - 2010 climatological normal, except for number of hours of reduced visibility

  §   Departure from mean value between 1997 and 2016

  &   Data incomplete
daily values of selected meteorological elements for HK and the percentile map of mean temperature for October 2017


The percentile map of mean temperature of October 2017
  Remarks : Extremely high: above 95th percentile
Above normal: between 75th and 95th percentile
Normal: between 25th and 75th percentile
Below normal: between 5th and 25th percentile
Extremely low: below 5th percentile
Percentile and 5-day running average values are
computed based on the data from 1981 to 2010


Extract of Meteorological Observations in Hong Kong for October 2017