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The Weather of September 2014

    Under the dominance of the subtropical ridge over southern China, September 2014 was the hottest September on record.  The monthly mean minimum temperature of 27.0 degrees and mean temperature of 29.0 degrees were respectively the highest and one of the highest for September since record began in 1884.  The month was also drier than usual with a monthly total rainfall amount of 140.6 millimetres, only about 43 percent of the September normal of 327.6 millimetres.  The accumulated rainfall since 1 January was 2452.7 millimetres, about 10 percent above the normal of 2233.1 millimetres for the same period.

    The weather in Hong Kong was sunny and very hot apart from a few isolated showers for the first three days of the month.  With an easterly airstream setting in, it became cloudier with a few showers and thunderstorms on 4 - 5 September.  As the easterly airstream weakened, the weather turned mainly fine and very hot in Hong Kong on 6 September.

    An area of low pressure over the northern part of the South China Sea intensified into a tropical depression on 7 September and brought cloudy and showery weather with isolated squally thunderstorms to the territory that day.  The tropical depression moved gradually away from Hong Kong and made landfall near Zhanjiang the next day.  Under the influence of its rainbands, showers continued to affect Hong Kong.

    Generally fine and very hot conditions returned on 9 - 11 September.  Under light wind conditions, temperatures at the Hong Kong Observatory rose to a maximum of 34.1 degrees on 11 September, the highest of the month.  High temperatures also triggered isolated showers during the day.  As another area of low pressure approached the coast of western Guangdong, local weather turned mainly cloudy with showers on 12 September.  Showers then eased off over the next two days as generally fine weather returned.

    Meanwhile, tropical cyclone Kalmaegi intensified into a typhoon east of the Philippines on 14 September.  It moved across Luzon and tracked generally west-northwestwards across the South China Sea on 15 September.  Local weather was very hot with sunny periods at first.  It became cloudy to overcast as winds strengthened gradually with a few squally showers and thunderstorms later that day.  Kalmaegi passed to the south-southwest of Hong Kong and brought gale force winds to the territory with heavy squally showers during the night.  Under the combined effect of Kalmaegi and a ridge of high pressure along the southeastern coast of China, local weather remained showery and rather windy over the next couple of days.

    With the establishment of a continental anticyclone over China, the weather in Hong Kong became generally fine and hot on 18 September.  Dominated by a dry continental airstream, the weather remained dry with sunny periods for the next five days.  Under light winds, hazy conditions affected the territory on 24 and 25 September.  Apart from some isolated showers on 26 September, generally fine and hot weather prevailed till the end of the month.  High temperatures on 30 September also triggered some thundery showers that evening.
 

    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.

 

Table 1.1   Tropical Cyclone Warning Signals

Name of
Tropical Cyclone		 Signal
Number		 Beginning Time Ending Time
Day/Month HKT Day/Month HKT
no name 1 7 / 9 0940 8 / 9 0910
KALMAEGI 1 14 / 9 2335 15 / 9 1240
3 15 / 9 1240 15 / 9 2230
8 SE 15 / 9 2230 16 / 9 1040
3 16 / 9 1040 16 / 9 2040
1 16 / 9 2040 17 / 9 0210


Table 1.2   Strong Monsoon Signal

Beginning Time Ending Time
Day/Month HKT Day/Month HKT
17 / 9 0211 17 / 9 1815


Table 1.3   Thunderstorm Warning

Beginning Time Ending Time
Day/Month HKT Day/Month HKT
4 / 9 1250 4 / 9 1630
5 / 9 0045 5 / 9 0145
5 / 9 0330 5 / 9 1030
5 / 9 1355 5 / 9 1500
6 / 9 1630 6 / 9 1745
7 / 9 1555 7 / 9 2030
8 / 9 1210 8 / 9 1315
12 / 9 0420 12 / 9 0530
12 / 9 1055 12 / 9 1400
12 / 9 1555 12 / 9 1800
13 / 9 0145 13 / 9 0445
14 / 9 1625 14 / 9 1750
15 / 9 1735 15 / 9 2200
16 / 9 0045 16 / 9 0145
16 / 9 0940 16 / 9 1345
16 / 9 1955 16 / 9 2200
17 / 9 0355 17 / 9 0500
19 / 9 1420 19 / 9 1700
30 / 9 1400 30 / 9 1500
30 / 9 1800 30 / 9 2000


Table 1.4   Fire Danger Warnings

Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Yellow 9 / 9 0600 9 / 9 2130
Yellow 20 / 9 0700 21 / 9 2345
Yellow 27 / 9 1200 28 / 9 2000


Table 1.5   Very Hot Weather Warning

Beginning Time Ending Time
Day/Month HKT Day/Month HKT
1 / 9 1125 2 / 9 1850
3 / 9 0645 3 / 9 1900
6 / 9 0645 6 / 9 1900
10 / 9 1145 11 / 9 1845
14 / 9 1230 15 / 9 1455
19 / 9 0645 19 / 9 1845
30 / 9 1245 30 / 9 1830


Table 2   Figures and Departures from Normal - September 2014

Meteorological Element Figure of the Month Departure from Normal*
Mean Daily Maximum Air Temperature 32.0 degrees C 1.9 degrees above normal
Mean Air Temperature 29.0 degrees C 1.3 degrees above normal
Mean Daily Minimum Air Temperature 27.0 degrees C 1.2 degrees above normal
Mean Dew Point Temperature 24.5 degrees C 1.1 degrees above normal
Mean Relative Humidity 77 % 1 % below normal
Mean Cloud Amount 57 % 9 % below normal
Total Rainfall 140.6 mm 187.0 mm below normal
Number of hours of Reduced VisibilityΔ 34 hours 56.6 hours below normal§
Total Bright Sunshine Duration 203.0 hours 30.7 hours above normal
Mean Daily Global Solar Radiation 16.49 Megajoule / square metre 1.88 Megajoule above normal
Total Evaporation 129.7^ mm 3.8 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 2013

  ^  Total for 29 days

daily values of selected meteorological elements for HK for September 2014

The percentile map of mean temperature of September 2014

  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



Rainfall Map For September 2014 (isohyets are in millimetres)

Extract of Meteorological Observations in Hong Kong for September 2014