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  • The Weather of September 2017

  • 4 October 2017

    Hong Kong’s weather was unseasonably hot in September 2017. The monthly mean temperature was 29.0 degrees, 1.3 degrees above the normal figure of 27.7 degrees and one of the hottest September since record began in 1884. The month was also drier than usual with a total rainfall of 192.4 millimetres, about 59 percent of the normal figure of 327.6 millimetres. The accumulated rainfall this year up to September was 2441.3 millimetres, a surplus of 9 percent compared to the normal figure of 2233.1 millimetres for the same period.

    Under the influence of a continental airstream, the weather in Hong Kong was hazy with sunny periods and evening thunderstorms on the first day of the month. Meanwhile, the tropical depression over the northeastern part of the South China Sea was named Mawar and intensified into a tropical storm. While lingering over the northeastern part of the South China Sea, Mawar further intensified into a severe tropical storm the next day. Mawar drifted towards the coast of eastern Guangdong slowly on 3 September and made landfall near Shanwei and weakened into a tropical storm on that night. Mawar moved across inland Guangdong on the morning of 4 September and weakened into an area of low pressure progressively during that day. Affected by the rainbands associated with Mawar, there were squally heavy showers and thunderstorms over the territory on 2-4 September. Local winds strengthened while Mawar skirting more than 100 kilometres to the northeast of Hong Kong on the morning of 4 September. Temperatures at the Hong Kong Observatory also fell to a minimum of 25.3 degrees in that morning, the lowest of the month.

    With a southerly airstream prevailing over the coast of Guangdong and the northern part of the South China Sea, local weather was a mixture of sunshine and showers on the ensuing six days. Under the influence of the anticyclone aloft, the weather in Hong Kong turned generally fine and hot apart from isolated showers on 11 September. While it was sunny and hazy on the morning of 12 September, convective activities triggered by high temperatures brought thundery showers to the territory in the afternoon. Affected by the northeast monsoon, it was mainly fine with isolated showers on 13-14 September.

    Under the dominance of the anticyclone aloft, local weather was sunny and very hot with some haze on the ensuing five days. With the anticyclone aloft weakening gradually, there were some showers and thunderstorms on 20-21 September. The showery activities over the northern part of the South China Sea and the coast of Guangdong brought heavy and thundery morning showers to Hong Kong on the next two days.

    Meanwhile, an area of low pressure over the central part of the South China Sea intensified into a tropical depression on the night of 23 September. The tropical depression moved west-northwest across the northern part of South China Sea on 24 September and made landfall over Hainan Island that night. Affected by the outer rainbands of the tropical depression, there were a few squally showers and thunderstorms on 24 September. Dominated by the anticyclone aloft, it remained generally fine and hot on 25-29 September. With abundant sunshine, temperatures at the Observatory soared to a maximum of 34.1 degrees on the afternoon of 28 September, the highest of the month. Affected by a fresh northeast monsoon, local weather became mainly cloudy with showers and a few thunderstorms in the morning on 30 September.

    Six 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 1.5. Monthly meteorological figures and departures from normal for September are tabulated in Table 2.

Warnings and Signals issued in September 2017

Table 1.1   Tropical Cyclone Warning Signals
Name of
Tropical Cyclone
Signal
Number
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
MAWAR 1 2 / 9 0220 3 / 9 2240
3 3 / 9 2240 4 / 9 1020
1 4 / 9 1020 4 / 9 1410
no name 1 23 / 9 2310 24 / 9 1920


Table 1.2   Strong Monsoon Signal
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
14 / 9 0315 14 / 9 1015
24 / 9 2145 25 / 9 0540


Table 1.3   Rainstorm Warning Signals
Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Amber 23 / 9 0800 23 / 9 0850


Table 1.4   Thunderstorm Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
1 / 9 1510 1 / 9 2000
2 / 9 1715 2 / 9 1915
2 / 9 2040 2 / 9 2245
3 / 9 0640 3 / 9 0730
3 / 9 1545 3 / 9 2345
4 / 9 0225 4 / 9 0730
5 / 9 0355 5 / 9 0530
7 / 9 1240 7 / 9 1520
8 / 9 1020 8 / 9 1200
9 / 9 0925 9 / 9 1030
9 / 9 1215 9 / 9 2145
10 / 9 0415 10 / 9 0715
10 / 9 0815 10 / 9 0915
10 / 9 1325 10 / 9 1500
12 / 9 1440 12 / 9 1930
13 / 9 1830 13 / 9 1930
20 / 9 1310 20 / 9 1415
21 / 9 0920 21 / 9 1130
21 / 9 1425 21 / 9 1530
22 / 9 0605 22 / 9 0900
23 / 9 0430 23 / 9 0900
24 / 9 0455 24 / 9 0630
24 / 9 0920 24 / 9 1205
30 / 9 0445 30 / 9 1130
30 / 9 2230 30 / 9 2335


Table 1.5   Very Hot Weather Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
12 / 9 1115 12 / 9 1645
13 / 9 1310 14 / 9 1620
15 / 9 1245 17 / 9 1930
18 / 9 1320 18 / 9 1800
26 / 9 0645 29 / 9 1900


Table 2   Figures and Departures from Normal - September 2017
Meteorological Element Figure of the Month Departure from Normal*
  Mean Daily Maximum Air Temperature 31.9 degrees C 1.8 degree above normal
  Mean Air Temperature 29.0 degrees C 1.3 degree above normal
  Mean Daily Minimum Air
Temperature
27.2 degrees C 1.4 degree above normal
  Mean Dew Point Temperature 25.0 degrees C 1.6 degree above normal
  Mean Relative Humidity 80 % 2 % above normal
  Mean Cloud Amount 65 % 1 % below normal
  Total Rainfall 192.4 mm 135.2 mm below normal
  Number of hours of Reduced VisibilityΔ 26 hours 55.2 hours below normal§
  Total Bright Sunshine Duration 187.1 hours 14.8 hours above normal
  Mean Daily Global Solar Radiation 15.82 Megajoule / square
metre
1.21 Megajoule above normal
  Total Evaporation 103.8 mm 22.1 mm below 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 September 2017


The percentile map of mean temperature of September 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 September 2017