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The Weather of December 2021

4 January 2022

With tropical cyclone Rai affecting the northern part of the South China Sea and skirting past to the south of Hong Kong, the Observatory issued the Standby Signal No. 1 on 20 December 2021. It was the second time in Hong Kong necessitating the issuance of tropical cyclone warning signal in December since 1946. December 2021 was warmer than usual with a mean temperature of 18.9 degrees, 0.7 degrees above the normal figure of 18.2 degrees (or 1.0 degree above the 1981-2010 normals). Moreover, with eleven out of the twelve months warmer than usual, 2021 was the warmest year on record in Hong Kong. The annual mean temperature of 24.6 degrees, annual mean maximum temperature of 27.5 degrees and annual mean minimum temperature of 22.6 degrees were all the highest since records began in 1884. December 2021 was also drier than usual with a total rainfall of 19.5 millimetres, about 32 percent below the normal of 28.8 millimetres (or 27 percent below the 1981-2010 normal of 26.8 millimetres). The annual total rainfall of 2307.1 millimetres in 2021 was about 5 percent below the annual normal of 2431.2 millimetres (or 4 percent below the 1981-2010 normal of 2398.5 millimetres).

Dominated by a dry northeast monsoon and the subsequent replenishment, the weather of Hong Kong was generally fine in the first two weeks of the month. The weather was also very dry on 1 – 4 December. With the moderation of the northeast monsoon and a cloud band covering the coast of Guangdong and the northern part of the South China Sea, local weather became mainly cloudy with a few light rain patches on the night of 14 December and the following day. Apart from a few rain patches in the morning, it was rather warm with sunny periods during the day on 16 December. The temperatures at the Observatory rose to a maximum of 25.8 degrees that day, the highest of the month. Under the influence of an intense northeast monsoon, the weather of Hong Kong became appreciably cooler and dry on 17 December and the next two days.

Meanwhile, after wreaking havoc in the Philippines, Super Typhoon Rai weakened into a severe typhoon and continued to move westwards entering the southern part of the South China Sea on 17 December. It re-intensified into a super typhoon on 18 December, making it the first super typhoon occurred in the South China Sea in December since 1961. Rai gradually turned to move north towards the northern part of the South China Sea on 19 December and weakened progressively. With Rai weakening into a severe tropical storm and tracking northeastwards towards the coast of Guangdong, the Observatory issued the Standby Signal No. 1 on the morning of 20 December 2021. Breaking the record of Irma in 1974, Rai became the storm which necessitated the issuance of the latest tropical cyclone warning signal in a year since 1946. Rai continued to move northeastwards across the northern part of the South China Sea and progressively weakened into an area of low pressure over the northern part of the South China Sea on 21 December.

Affected by the rain-bearing cloud band associated with Rai, it was rainy and cool in Hong Kong on 20 – 21 December. More than 20 millimeters of rainfall were recorded over Lantau Island and the northwestern part of the New Territories on these two days. Local weather remained mainly cloudy with a few rain patches on 22 – 24 December.

An intense winter monsoon reached the coast of Guangdong on the morning of 25 December. Locally, the weather was mainly cloudy with sunny periods during the day and became cool that night. Under the influence of the winter monsoon and an upper air disturbance, it was cloudy and cold with a few rain patches on 26 – 28 December. The temperatures at the Observatory plummeted to a minimum of 9.9 degrees on the morning of 27 December, the lowest of the month. With the prevalence of the dry northeast monsoon and thinning out of clouds covering the coast of Guangdong, the weather of Hong Kong turned generally fine and dry with cool mornings for the rest of the month.

Two tropical cyclones occurred over the South China Sea and the western North Pacific in December 2021.

Details of issuance and cancellation of various warnings/signals in the month are summarized in Tables 1.1 to 1.4.  Monthly meteorological figures and departures from normal for December are tabulated in Table 2.

 

Warnings and Signals issued in December 2021

Table 1.1   Tropical Cyclone Warning Signals
Name of
Tropical Cyclone
Signal
Number
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
RAI 1 20 / 12 1120 21 / 12 1220


Table 1.2   Strong Monsoon Signal
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
30 / 11 0900 2 / 12 0145
8 / 12 0045 8 / 12 0845


Table 1.3   Fire Danger Warnings
Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Red 30 / 11 0600 6 / 12 2145
Yellow 12 / 12 0600 12 / 12 1800
Yellow 18 / 12 0600 19 / 12 0745
Red 19 / 12 0745 19 / 12 2345


Table 1.4   Cold Weather Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
26 / 12 1100 28 / 12 1145


Table 2   Figures and Departures from Normal - December 2021
Meteorological Element Figure of the Month Departure from Normal(1991-2020) Departure from Normal (1981-2010)
Mean Daily Maximum Air Temperature 21.4 degrees C 1.0 degree above normal 1.2 degrees above normal
Mean Air Temperature 18.9 degrees C 0.7 degrees above normal 1.0 degree above normal
Mean Daily Minimum Air Temperature 16.8 degrees C 0.6 degrees above normal 0.9 degrees above normal
Mean Dew Point Temperature 12.6 degrees C 0.2 degrees above normal 0.7 degrees above normal
Mean Relative Humidity 68 % 2 % below normal 1 % below normal
Mean Cloud Amount 57 % normal 5 % above normal
Total Rainfall 19.5 mm 9.3 mm below normal 7.3 mm below normal
Number of hours of Reduced VisibilityΔ 66 hours 127.6 hours below normal§ 127.6 hours below normal§
Total Bright Sunshine Duration 172.1 hours 10.5 hours above normal 0.1 hours below normal
Mean Daily Global Solar Radiation 12.19 Megajoule / square metre 1.28 Megajoule above normal 1.3 Megajoule above normal
Total Evaporation 91.8 mm 10.9 mm above normal 8.1 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

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 mean value between 1997 and 2020

Daily values of selected meteorological elements for Hong Kong for December 2021

The percentile map of mean temperature of December 2021

  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 1991 to 2020