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The Weather of September 2008

    September 2008 was hotter and drier than usual. The monthly mean temperature of 29.0 degrees equaled the record set in 1969. The total rainfall of the month was 159.2 millimetres, 128.3 millimetres below normal. The accumulated rainfall since 1 January was 2858.3 millimetres, about 32 percent above the normal figure of 2161.2 millimetres for the same period.

    The weather was mainly fine and hot on the first day of the month. Affected by a trough of low pressure over the south China coastal areas, it was cloudy with showers from 2 to 4 September. A moist easterly airstream brought showery weather to Hong Kong in the next two days. A weak ridge of high pressure over southern China brought mainly fine weather to the territory from 7 to 9 September.

    Under the influence of a dry continental airstream over southern China, local weather remained sunny and dry from 10 to 17 September. A trough of low pressure developed over the northern part of the South China Sea and brought showery conditions to Hong Kong on 18 and 19 September.

    Typhoon Hagupit formed over the western North Pacific on 19 September. It moved first west-southwestwards and then northwestwards in the next two days. Affected by the hot subsiding air ahead of Hagupit, local weather became sunny and very hot from 20 to 22 September. The daily mean air temperature of 31.2 degrees recorded at the Hong Kong Observatory on 22 September was the highest in September on record.

    Hagupit entered the South China Sea in the evening of 22 September and approached the south China coast at a high speed of some 30 kilometres per hour on 23 September. It made landfall over the western coast of Guangdong the next morning. Under the influence of Hagupit, local weather was windy and rainy on 23 and 24 September. Local winds generally reached gale to storm force and a total of 77.8 millimetres of rainfall was recorded at the Hong Kong Observatory in these two days.

    Local weather stayed cloudy with some isolated showers and thunderstorms on 25 and 26 September. Under the influence of a fresh easterly airstream over the coast of Guangdong, the weather remained showery on 27 September. A dry northerly airstream brought mainly fine and dry conditions to the territory on the last three days of the month.

    Five tropical cyclones occurred in the western North Pacific and the South China Sea 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 September are tabulated in Table 2.


 

Warnings and Signals issued in September 2008


Table 1.1   Tropical Cyclone Warning Signals

Name of
Tropical Cyclone		 Signal
Number		 Beginning Time Ending Time
Day/Month HKT Day/Month HKT
HAGUPIT 1 22 / 9 1840 23 / 9 1025
3 23 / 9 1025 23 / 9 1800
8 NE 23 / 9 1800 24 / 9 0040
8 SE 24 / 9 0040 24 / 9 0630
3 24 / 9 0630 24 / 9 1250


Table 1.2   Strong Monsoon Signal

Beginning Time Ending Time
Day/Month HKT Day/Month HKT
24 / 9 2100 25 / 9 0615


Table 1.3   Rainstorm Warning Signals

Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Amber 24 / 9 1105 24 / 9 1620


Table 1.4   Thunderstorm Warning

Beginning Time Ending Time
Day/Month HKT Day/Month HKT
1 / 9 2215 2 / 9 0700
2 / 9 1110 2 / 9 1400
2 / 9 1445 2 / 9 1645
3 / 9 0205 3 / 9 1000
4 / 9 0105 4 / 9 0215
5 / 9 0420 5 / 9 0600
5 / 9 1045 5 / 9 1445
6 / 9 1110 6 / 9 1515
7 / 9 1445 7 / 9 1545
7 / 9 2100 8 / 9 0600
11 / 9 1500 11 / 9 1600
11 / 9 1650 11 / 9 1800
12 / 9 1730 12 / 9 1830
17 / 9 1330 17 / 9 1430
17 / 9 1800 17 / 9 1900
18 / 9 0705 18 / 9 1200
18 / 9 1950 18 / 9 2100
19 / 9 0215 19 / 9 1435
19 / 9 2350 20 / 9 0700
23 / 9 1415 23 / 9 1615
24 / 9 1010 24 / 9 1800
25 / 9 1450 25 / 9 1600
25 / 9 2240 26 / 9 0030
26 / 9 1505 26 / 9 1630
26 / 9 2345 27 / 9 0220


Table 1.5   Fire Danger Warnings

Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Red 13 / 9 0600 16 / 9 0600
Yellow 28 / 9 0600 29 / 9 0600
Red 29 / 9 1130 30 / 9 0600


Table 1.6   Very Hot Weather Warning

Beginning Time Ending Time
Day/Month HKT Day/Month HKT
13 / 9 1300 14 / 9 1800
15 / 9 1325 16 / 9 1755
17 / 9 1145 18 / 9 0845
22 / 9 1525 23 / 9 0645


Table 2   Figures and Departures from Normal - September 2008

Meteorological Element Figure of the Month Departure from Normal*
Mean Daily Maximum Air Temperature 32.0 degrees C 1.8 degrees above normal
Mean Air Temperature 29.0 degrees C 1.4 degrees above normal
Mean Daily Minimum Air Temperature 26.8 degrees C 1.2 degrees above normal
Mean Dew Point Temperature 23.8 degrees C 0.4 degree above normal
Mean Relative Humidity 75 % 4 % below normal
Mean Cloud Amount 66 % 1 % above normal
Total Rainfall 159.2 mm 128.3 mm below normal
Number of hours of Reduced VisibilityΔ 186 hours 76.5 hours above normal§
Total Bright Sunshine Duration 213.9 hours 42.1 hours above normal
Mean Daily Global Solar Radiation 16.75 Megajoule / square metre 1.61 Megajoule above normal
Total Evaporation 146.1 mm 11.9 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 1971 - 2000 climatological normal, except for number of hours of reduced visibility

  §   Departure from mean value between 1997 and 2007
daily values of selected meteorological elements for HK for September 2008

Extract of Meteorological Observations in Hong Kong for September 2008