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The Weather of December 2011

     Attributed to the frequent replenishment of the winter monsoon, December 2011 was colder than usual with a monthly mean temperature of 16.9 degrees, 0.9 degrees below the normal figure of 17.8 degrees.  There were six cold days (daily minimum temperature at 12.0 degrees or below) in the month, about two days more than normal.  The prevalence of continental airstream also brought drier than usual weather to the territory in December 2011.  The total rainfall recorded in the month was 2.8 millimetres, less than a tenth of the monthly normal.  The annual rainfall for 2011 was 1487.2 millimetres, a deficit of about 38 percent compared to the annual normal of 2382.7 millimetres.

    A cold front crossed the coast of Guangdong on the morning of the first day in the month and the associated intense winter monsoon brought significantly cooler weather with sunny periods to Hong Kong on that day.  Dominated by the winter monsoon, it remained rather cool, fine and dry for the ensuing three days.  Affected by a fresh easterly airstream, the weather became cloudy with rain patches on 5 and 6 December.  With the moderation of the easterly airstream, there were sunny intervals on 7 December.  

    An intense winter monsoon reached the south China coast on the morning of 8 December and brought significantly cooler weather to the territory on 8 and 9 December.  Affected by the intense winter monsoon, it was cold, fine and very dry for the next three days.  The temperatures at the Hong Kong Observatory fell to a minimum of 9.6 degrees on 11 December, the lowest of the month.  While the winter monsoon moderated gradually on 13 December, daytime temperatures rose to above 20 degrees generally from 13 to 15 December.  

    With the arrival of a replenishment of the winter monsoon at southern China, it remained generally fine and dry from 16 to 19 December.  Affected by a broad band of clouds, the weather became mainly cloudy for the ensuing two days.  

    Another replenishment of the winter monsoon reached southern China on the morning of 22 December and brought fine and dry weather to the territory on 22 and 23 December.  With the strengthening of the winter monsoon, local weather was cold and very dry on the next two days.   With the persistence of the winter monsoon, it remained cool and dry with localized haze for the rest of the month.

     One tropical cyclone 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 1.3.  Monthly meteorological figures and departures from normal for December are tabulated in Table 2.
 

Warnings and Signals issued in December 2011


Table 1.1 Strong Monsoon Signal

Beginning Time Ending Time
Day/Month HKT Day/Month HKT
1 / 12 1145 2 / 12 1145
8 / 12 2045 9 / 12 1145
10 / 12 0700 10 / 12 1600
11 / 12 0400 11 / 12 1035


Table 1.2 Fire Danger Warnings

Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Red 2 / 12 0900 4 / 12 0600
Yellow 4 / 12 0600 4 / 12 1945
Red 9 / 12 0600 13 / 12 1800
Red 15 / 12 0600 18 / 12 1800
Red 22 / 12 0600 27 / 12 0600
Yellow 27 / 12 0600 27 / 12 1800


Table 1.3 Cold Weather Warning

Beginning Time Ending Time
Day/Month HKT Day/Month HKT
3 / 12 0120 3 / 12 1015
9 / 12 1620 12 / 12 1045
23 / 12 1620 26 / 12 0745


Table 2 Figures and Departures from Normal - December 2011

Meteorological Element Figure of the Month Departure from Normal*
Mean Daily Maximum Air Temperature 19.4 degrees C 0.9 degree below normal
Mean Air Temperature 16.9 degrees C 0.9 degree below normal
Mean Daily Minimum Air Temperature 14.6 degrees C 1.1 degrees below normal
Mean Dew Point Temperature 9.9 degrees C 1.7 degrees below normal
Mean Relative Humidity 65 % 4 % below normal
Mean Cloud Amount 50 % 1 % below normal
Total Rainfall 2.8 mm 31.7 mm below normal
Total Bright Sunshine Duration 193.9 hours 20.6 hours above normal
Mean Daily Global Solar Radiation 12.61 Megajoule / square metre 1.48 Megajoule above normal
Total Evaporation 101.6 mm 7.1 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

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 2010

daily values of selected meteorological elements for HK for December 2011 

Extract of Meteorological Observations in Hong Kong for December 2011