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.
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
| Name of Tropical Cyclone |
Signal Number |
Beginning Time | Ending Time | ||
|---|---|---|---|---|---|
| Day/Month | HKT | Day/Month | HKT | ||
| RAI | 1 | 20 / 12 | 1120 | 21 / 12 | 1220 |
| Beginning Time | Ending Time | ||
|---|---|---|---|
| Day/Month | HKT | Day/Month | HKT |
| 30 / 11 | 0900 | 2 / 12 | 0145 |
| 8 / 12 | 0045 | 8 / 12 | 0845 |
| 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 |
| Beginning Time | Ending Time | ||
|---|---|---|---|
| Day/Month | HKT | Day/Month | HKT |
| 26 / 12 | 1100 | 28 / 12 | 1145 |
| 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. |
| § Departure from mean value between 1997 and 2020 |
|
| 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 |
