The Weather of December 2017

3 January 2018

    In contrast to November, the weather in Hong Kong was sunny and bright in December 2017 and, apart from three days on 13, 17 and 23 December, the territory enjoyed more than two hours of sunshine each day during the month. The monthly total sunshine duration amounted to 209.6 hours, 37.4 hours and 22 percent above the December normal of 172.2 hours. The monthly mean temperature was 17.8 degrees, 0.1 degree below the December normal of 17.9 degrees. Only traces of rainfall were recorded in the month, with most of them occurring during a surge of strong easterly monsoon on 12 – 14 December. The only rain-free December was way back in 1909, and December 2017 was one of ten Decembers with traces of rainfall since records began in 1884. Nevertheless, the total rainfall of 2572.1 millimetres accumulated in 2017 still exceeded the annual normal of 2398.5 millimetres by about 7 percent.

    Under the prevalence of dry continental air brought by the northeast monsoon, the weather in Hong Kong was generally fine for the first eleven days of the month, with conditions becoming cooler and drier on 8 – 10 December as a replenishment of the northeast monsoon reached the south China coastal areas.

    A fresh to strong easterly airstream brought windy and cloudier weather with light rain patches to the territory on 12 – 14 December. The easterly winds were soon replaced by a strong northerly airstream as a cold front formed over Guangdong on 15 December and moved across the coastal areas during the night. Affected by an intense winter monsoon behind the cold front, local weather became cold, windy and dry over the next six days. Temperatures at the Hong Kong Observatory on 18 December dropped to a minimum of 9.8 degrees, the lowest of the month, and daytime relative humidity fell below 40 percent on 19 – 21 December. Apart from a cloudy day on 17 December, the weather in Hong Kong remained mostly fine with plenty of sunshine during the period.

    The monsoon winds started to subside on 22 December and moister maritime air returned to the south China coast, with cloud bands from the northern part of the South China Sea covering the territory the next day. A replenishment of dry continental air on 24 December soon cleared away the clouds, and generally fine conditions then persisted in Hong Kong till the end 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 2017

Table 1.1 Strong Monsoon Signal

Beginning Time		Ending Time
Day/Month	HKT	Day/Month	HKT
8 / 12	0705	8 / 12	1100
13 / 12	0315	14 / 12	0900
16 / 12	1500	17 / 12	0945

Table 1.2 Fire Danger Warnings

Colour	Beginning Time		Ending Time
Colour	Day/Month	HKT	Day/Month	HKT
Yellow	3 / 12	0600	3 / 12	1800
Red	8 / 12	0600	11 / 12	2130
Red	17 / 12	0600	21 / 12	2040
Yellow	24 / 12	0600	24 / 12	1130
Red	24 / 12	1130	25 / 12	2100
Yellow	26 / 12	0600	26 / 12	1930
Yellow	30 / 12	1030	30 / 12	1940
Yellow	31 / 12	0600	31 / 12	1940

Table 1.3 Cold Weather Warning

Beginning Time		Ending Time
Day/Month	HKT	Day/Month	HKT
16 / 12	1620	21 / 12	1100

Table 2 Figures and Departures from Normal - December 2017

Meteorological Element	Figure of the Month	Departure from Normal*
Mean Daily Maximum Air Temperature	20.1 degrees C	0.1 degree below normal
Mean Air Temperature	17.8 degrees C	0.1 degree below normal
Mean Daily Minimum Air Temperature	16.0 degrees C	0.1 degree above normal
Mean Dew Point Temperature	11.2 degrees C	0.7 degree below normal
Mean Relative Humidity	66 %	3 % below normal
Mean Cloud Amount	54 %	2 % above normal
Total Rainfall	Trace	26.8 mm below normal
Number of hours of Reduced VisibilityΔ	106 hours	112.1 hours below normal§
Total Bright Sunshine Duration	209.6 hours	37.4 hours above normal
Mean Daily Global Solar Radiation	13.43 Megajoule / square metre	2.54 Megajoule above normal
Total Evaporation	94.9 mm	11.2 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 1981 - 2010 climatological normal, except for number of hours of reduced visibility
§ Departure from mean value between 1997 and 2016
Trace means rainfall less than 0.05 mm

daily values of selected meteorological elements for HK for December 2017

The percentile map of mean temperature of December 2017

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

Rainfall Map For December 2017 (isohyets are in millimetres)

Extract of Meteorological Observations in Hong Kong for December 2017