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The Weather of December 2007

    December 2007 was warm and hazy. The mean temperature of 19.3 degrees was 1.7 degrees above normal, the 4th highest on record for December. The number of hours of reduced visibility observed at the Hong Kong Observatory reached 305 hours (41 % of the time) in December 2007. The total rainfall in the month was 15.7 millimetres, about 58% of the normal figure of 27.3 millimeters. The accumulated rainfall since the beginning of the year was 1706.9 millimetres, about 23 percent below the normal figure of 2214.3 millimetres for the same period.

    A ridge of high pressure over southern China brought generally fine weather to Hong Kong on the first two days of the month. Local winds strengthened from the north on 3 December after a cold front moved across the south China coast. Under the influence of a continental airstream, it was mainly fine and dry on 4 December.

    The weather became cloudy on 5 December as a cloud band covered the coast of Guangdong. Clouds thinned out quickly the next day and there were sunny periods. With the northeast monsoon prevailing over southern China, it was mainly fine but hazy between 7 and 9 December.

    The northeast monsoon moderated on 10 December and the weather was sunny and warm in the next two days. Haze returned on 13 December with visibility at the Hong Kong International Airport dropping to below 2000 metres. The weather remained generally fine but hazy in the ensuing two days.

    The weather turned cloudy on 16 December when a broad cloud band affected the coast of Guangdong. It was cloudy with a few light rain patches in the following six days.

    A replenishment of the northeast monsoon reached the south China coast on 23 December, bringing cooler weather in the ensuing three days. Generally fine weather persisted from 26 to 28 December.

    Temperature dropped progressively on 29 and 30 December upon the arrival of the cold air associated with an intense winter monsoon. The weather was cold and very dry on 31 December with temperatures falling to 12.0 degrees, the lowest in the month.

    There was no tropical cyclone 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 1.3.  Monthly meteorological figures and departures from normal for December are tabulated in Table 2.


 

Warnings and Signals issued in December 2007


Table 1.1   Fire Danger Warnings

Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Red 26 / 11 0600 1 / 12 0600
Yellow 1 / 12 0600 3 / 12 0600
Red 3 / 12 1100 5 / 12 0600
Red 7 / 12 1130 7 / 12 2315
Yellow 9 / 12 0600 9 / 12 1800
Yellow 16 / 12 0600 16 / 12 2300
Yellow 25 / 12 0000 27 / 12 0600
Yellow 29 / 12 0600 30 / 12 0600
Red 30 / 12 0600 4 / 1 0600


Table 1.2   Frost Warning

Beginning Time Ending Time
Day/Month HKT Day/Month HKT
31 / 12 1620 1 / 1 0745


Table 1.3   Cold Weather Warning

Beginning Time Ending Time
Day/Month HKT Day/Month HKT
30 / 12 1620 4 / 1 0945


Table 2   Figures and Departures from Normal - December 2007

Meteorological Element Figure of the Month Departure from Normal*
Mean Daily Maximum Air Temperature 21.7 degrees C 1.2 degrees above normal
Mean Air Temperature 19.3 degrees C 1.7 degrees above normal
Mean Daily Minimum Air Temperature 17.6 degrees C 2.2 degrees above normal
Mean Dew Point Temperature 14.0 degrees C 2.8 degrees above normal
Mean Relative Humidity 72 % 4 % above normal
Mean Cloud Amount 57 % 8 % above normal
Total Rainfall 15.7 mm 11.6 mm below normal
Number of hours of Reduced VisibilityΔ 420 hours 189.6 hours above normal§
Total Bright Sunshine Duration 182.8 hours 1.3 hours above normal
Mean Daily Global Solar Radiation 11.85 Megajoule / square metre 0.18 Megajoule below normal
Total Evaporation 85.8 mm 25.7 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 December 2007

Extract of Meteorological Observations in Hong Kong for December 2007