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.
|Beginning Time||Ending Time|
|27 / 12||2320||28 / 12||0440|
|Colour||Beginning Time||Ending Time|
|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|
|Beginning Time||Ending Time|
|28 / 12||2000||2 / 1||1620|
|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.
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.
* Departure from 1981 - 2010 climatological normal, except for number of hours of reduced visibility
§ Departure from mean value between 1997 and 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