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  • The Weather of December 2018

  • 3 January 2019

   With the northeast monsoon over southern China weaker than normal for most of the time in the month, December 2018 was much warmer than usual in Hong Kong. The monthly mean temperature was 19.2 degrees, 1.3 degrees above the normal of 17.9 degrees and among the sixth highest on record for December. The mean minimum temperature was 17.6 degrees, 1.7 degrees above normal of 15.9 degrees and among the fourth highest on record for December. The month was also drier than usual with a total rainfall of 11.9 millimetres, only about 44 percent of the normal of 26.8 millimetres. The annual total rainfall of 2162.9 millimetres in 2018 was about 10 percent below the annual normal of 2398.5 millimetres.

   Under the influence of the northeast monsoon, the weather of Hong Kong was generally fine in the first two days of the month. Affected by a relatively warm and humid maritime airstream, local weather became exceptionally warm in the next two days with the maximum temperature at the Hong Kong Observatory soaring to 27.1 degrees on 4 December, the highest of the month. The daily mean temperature of 24.8 degrees on that day was also the highest on record for December.

   With a fresh to strong easterly airstream setting in, it was mainly cloudy with a few rain patches in Hong Kong on 5 - 6 December. A cold front moved across the coastal areas of Guangdong on the morning of 7 December and the northeast monsoon behind the cold front brought cooler weather together with a few rain patches to the territory in the next three days. It was rather cool on 11 - 14 December under the influence of a replenishment of the northeast monsoon. With the northeast monsoon moderating, the weather improved gradually with plenty of sunshine on the afternoon of 15 December.

   Another replenishment of the northeast monsoon reached the South China coastal areas on 16 December, bringing mainly cloudy weather with a few light rain patches to Hong Kong. Under the influence of a dry northeast monsoon, it was generally fine and dry with cool mornings in Hong Kong in the next two days. As the northeast monsoon affecting the south China coastal areas was gradually replaced by a relatively warm and humid maritime airstream, local temperatures rose up gradually on 19 - 20 December. With abundant sunshine during the day, it was rather warm in the next two days and the daily mean temperature on 22 December reached 22.2 degrees, making it the warmest Winter Solstice on record.

   With the northeast monsoon reaching the coastal areas of Guangdong, it was rainy and cooler on 23 December. While the rain eased off gradually, the weather remained cloudy and cool in the next two days. As the clouds covering southern China thinned out, local weather turned generally fine and mild on 26 - 27 December. Meanwhile, the winter monsoon affecting southern China strengthened and spread south gradually. Under the influence of the intense winter monsoon, local weather became windy with temperatures falling progressively towards the end of the month. With the chilly northerlies prevailing and cloudy skies, it was cold on the last two days of the month with the temperature at the Hong Kong Observatory dropping to a minimum of 10.3 degrees on 30 December, the lowest of the month.


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

Warnings and Signals issued in December 2018

Table 1.1   Strong Monsoon Signal
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
27 / 12 2320 28 / 12 0440


Table 1.2   Fire Danger Warnings
Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Yellow 2 / 12 0945 2 / 12 1800
Yellow 16 / 12 0600 16 / 12 1610
Red 17 / 12 0600 18 / 12 2315
Yellow 26 / 12 0600 26 / 12 1800
Yellow 29 / 12 0600 29 / 12 2120
Yellow 30 / 12 0600 30 / 12 2130


Table 1.3   Cold Weather Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
28 / 12 2000 2 / 1 1620


Table 2   Figures and Departures from Normal - December 2018
Meteorological Element Figure of the Month Departure from Normal*
Mean Daily Maximum Air Temperature 21.3 degrees C 1.1 degrees above normal
Mean Air Temperature 19.2 degrees C 1.3 degrees above normal
Mean Daily Minimum Air Temperature 17.6 degrees C 1.7 degrees above normal
Mean Dew Point Temperature 14.8 degrees C 2.9 degrees above normal
Mean Relative Humidity 76 % 7 % above normal
Mean Cloud Amount 75 % 23 % above normal
Total Rainfall 11.9 mm 14.9 mm below normal
Number of hours of Reduced VisibilityΔ 52 hours 160.8 hours below normal§
Total Bright Sunshine Duration 122.0 hours 50.2 hours below normal
Mean Daily Global Solar Radiation 10.03 Megajoule / square metre 0.86 Megajoule below normal
Total Evaporation 70.0 mm 13.7 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.


  *   Departure from 1981 - 2010 climatological normal, except for number of hours of reduced visibility

  §   Departure from mean value between 1997 and 2017

daily values of selected meteorological elements for HK for December 2018

The percentile map of mean temperature of December 2018

  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


Extract of Meteorological Observations in Hong Kong for December 2018