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The Weather of September 2007

    September 2007 was drier than usual. The monthly rainfall was 102.3 millimetres, about one third of the normal figure of 299.7 millimetres. The accumulated rainfall since the beginning of the year was 1635.6 millimetres, about 19 percent below the normal figure of 2007.1 millimetres for the same period.

    It was sunny on the first day of the month. A trough of low pressure over the south China coast brought showery weather to the territory on 2 September and the next three days. With the trough of low pressure moved south to the northern part of the south China Sea, there were sunny intervals during the day on 6 September. The weather stayed mainly cloudy with isolated showers in the following four days.

    Under the influence of a continental airstream, the weather turned fine on 11 September and remained so in the ensuing two days. Under light wind conditions, it was hazy from 14 to 17 September with visibility dropping below 3000 metres in some places. With winds strengthening from the north, it was mainly fine and dry on 18 September and the next four days. The relative humidity dropped to about 40 percent over most parts of the territory during the day on 18 and 19 September.

    The combined effect of the northeast monsoon and the tropical storm Francisco over the northern part of the South China Sea brought windy and rainy weather to Hong Kong from 23 to 25 September.

    Under the influence of a ridge of high pressure over southeastern China, it was mainly fine from 26 to 30 September.

    Six 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 1.4.  Monthly meteorological figures and departures from normal for September are tabulated in Table 2.


 

Warnings and Signals issued in September 2007


Table 1.1   Tropical Cyclone Warning Signals

Name of
Tropical Cyclone		 Signal
Number		 Beginning Time Ending Time
Day/Month HKT Day/Month HKT
FRANCISCO 1 23 / 9 0240 24 / 9 1115


Table 1.2   Strong Monsoon Signal

Beginning Time Ending Time
Day/Month HKT Day/Month HKT
25 / 9 0145 25 / 9 0630
30 / 9 2145 1 / 10 0630


Table 1.3   Thunderstorm Warning

Beginning Time Ending Time
Day/Month HKT Day/Month HKT
2 / 9 0630 2 / 9 0830
2 / 9 1000 2 / 9 1500
3 / 9 0415 3 / 9 0545
3 / 9 0915 3 / 9 1415
4 / 9 0500 4 / 9 0830
4 / 9 1230 4 / 9 1330
4 / 9 1645 4 / 9 1845
4 / 9 2015 5 / 9 0030
11 / 9 2245 12 / 9 0045
24 / 9 0225 24 / 9 0330
24 / 9 0820 24 / 9 1015
24 / 9 1315 24 / 9 1615
25 / 9 0815 25 / 9 1015
25 / 9 1135 25 / 9 1545


Table 1.4   Fire Danger Warnings

Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Red 18 / 9 0600 20 / 9 2315
Yellow 22 / 9 0600 23 / 9 0300
Yellow 26 / 9 0600 26 / 9 2215
Yellow 29 / 9 0600 1 / 10 0600


Table 2   Figures and Departures from Normal - September 2007

Meteorological Element Figure of the Month Departure from Normal*
Mean Daily Maximum Air Temperature 30.5 degrees C 0.2 degree above normal
Mean Air Temperature 27.7 degrees C 0.1 degree above normal
Mean Daily Minimum Air Temperature 25.9 degrees C 0.4 degree above normal
Mean Dew Point Temperature 22.6 degrees C 0.7 degree below normal
Mean Relative Humidity 75 % 3 % below normal
Mean Cloud Amount 69 % 6 % above normal
Total Rainfall 102.3 mm 197.4 mm below normal
Number of hours of Reduced VisibilityΔ 238 hours 141.3 hours above normal§
Total Bright Sunshine Duration 175.4 hours 6.3 hours below normal
Mean Daily Global Solar Radiation 15.56 Megajoule / square metre 0.93 Megajoule below normal
Total Evaporation 129.4 mm 20.9 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.


 

The calibration factors of the pyranometer at King's Park in 2006 and 2007 have been re-computed based on the results of a regional instrument comparison in Asia in 2007. The readings of total global solar radiation at King's Park in 2006 and 2007 displayed in this web page before 13 June 2008 have been revised on 13 June 2008 using the new calibration factors.

  *   Departure from 1961-1990 climatological normal, except for number of hours of reduced visibility.

  §   Departure from mean value between 1997 and 2006


daily values of selected meteorological elements for HK for September 2007

Extract of Meteorological Observations in Hong Kong for September 2007