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The Weather of January 2021

2 February 2021

January 2021 was characterized by colder weather during the first half of the month and relatively milder weather in the latter part. Overall, the month was colder than usual with a mean temperature of 16.2 degrees, 0.3 degrees below the normal figure of 16.5 degrees (or 0.1 degrees below the 1981-2010 normal). With dry winter monsoon dominating over southern China for most of the time in the month, January 2021 was also much sunnier and drier than usual. The monthly total sunshine duration amounted to 217.3 hours, 49 percent above the normal of 145.8 hours (or 52 percent above the 1981-2010 normal). Only traces of rainfall was recorded in the month, making it one of the eight Januaries with traces of rainfall since records began in 1884.

The intense cold surge that affected the south China coastal areas in late December 2020 continued its dominance and brought cold and very dry weather to Hong Kong on the first two days of January 2021. Frost was reported in places over the northern part of the New Territories on 2 January. With the moderation of the winter monsoon, local temperatures rose gradually with sunny periods in the next four days.

With the strengthening of the winter monsoon, the weather of Hong Kong became appreciably colder on the night of 7 January and the temperature at the Observatory further dropped to a minimum of 7.7 degrees on the morning of 8 January, the lowest of the month. The temperature at Tai Mo Shan even fell to -2.5 degrees on that morning. It was also mainly cloudy and very dry during the day on that day. Under the influence of the winter monsoon and its replenishment, cold and very dry weather persisted on 9 – 14 January. While it was mainly cloudy on 9 – 11 January, the weather became generally fine on 12 January and remained so in the following six days. During the cold episode, icing was reported on high ground of Tai Mo Shan on 8 – 9 and 12 – 13 January. With significant radiation cooling under clear sky, the minimum temperature recorded at Ta Kwu Ling was -0.9 degrees on 13 January, the lowest since records began at that station. Besides, there were frost reports in parts of the New Territories on that morning.

Affected by the cloud bands over the northern part of the South China Sea, the weather became cloudier on the afternoon of 19 January and the next two days. With the cloud bands thinning out, the weather in Hong Kong turned fine and rather warm during the day on 22 - 23 January. With plenty of sunshine, the temperature at the Observatory rose to a maximum of 24.5 degrees on the afternoon of 22 January, the highest of the month. Under light wind conditions, visibility was relatively low in some areas on 21 – 23 January.

Following the arrival of a surge of northeast monsoon, local weather became cloudier and cooler on 24 January. Under the influence of the dry northeast monsoon, it was generally fine and mild during the day on 25 – 28 January. With the strengthening of the monsoon, the weather of Hong Kong became cooler and remained fine on the last three days of the month.

There was no tropical cyclone over the South China Sea and the western North Pacific in January 2021.


Frost at Tai Po on 2 January 2021
Frost at Tai Po on 2 January 2021

Frost at Ta Kwu Ling on 13 January 2021
Frost at Ta Kwu Ling on 13 January 2021

Icing at Tai Mo Shan on 13 January 2021
Icing at Tai Mo Shan on 13 January 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 January are tabulated in Table 2.
 

Warnings and Signals issued in January 2021

Table 1.1   Strong Monsoon Signal
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
8 / 1 0210 8 / 1 1200
11 / 1 0845 12 / 1 1130
16 / 1 2315 17 / 1 0745


Table 1.2   Fire Danger Warnings
Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Red 30 / 12 0600 2 / 1 2000
Yellow 3 / 1 0600 3 / 1 2000
Yellow 7 / 1 0600 8 / 1 0600
Red 8 / 1 0600 14 / 1 2300
Yellow 15 / 1 0800 15 / 1 2030
Yellow 16 / 1 0930 16 / 1 1900
Red 17 / 1 0600 18 / 1 2230
Yellow 25 / 1 0600 25 / 1 1900
Red 28 / 1 0600 29 / 1 0600
Yellow 29 / 1 0600 29 / 1 2045
Yellow 30 / 1 0600 31 / 1 1945


Table 1.3   Frost Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
31 / 12 1630 1 / 1 0945
1 / 1 1630 2 / 1 0945
2 / 1 1630 3 / 1 0745
8 / 1 1630 9 / 1 0945
12 / 1 1630 13 / 1 1000
13 / 1 1630 14 / 1 0900


Table 1.4   Cold Weather Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
29 / 12 1620 3 / 1 0930
7 / 1 1620 15 / 1 0845
17 / 1 1620 19 / 1 0945


Table 2   Figures and Departures from Normal - January 2021
Meteorological Element Figure of the Month Departure from Normal (1991-2020) Departure from Normal (1981-2010)
Mean Daily Maximum Air Temperature 19.3 degrees C 0.6 degree above normal 0.7 degree above normal
Mean Air Temperature 16.2 degrees C 0.3 degree below normal 0.1 degree below normal
Mean Daily Minimum Air Temperature 13.8 degrees C 0.8 degree below normal 0.7 degree below normal
Mean Dew Point Temperature 8.5 degrees C 3.2 degrees below normal 2.9 degrees below normal
Mean Relative Humidity 62 % 12 % below normal 12 % below normal
Mean Cloud Amount 47 % 15 % below normal 14 % below normal
Total Rainfall Trace 33.2 mm below normal 24.7 mm below normal
Number of hours of Reduced VisibilityΔ 78 hours 135.6 hours below normal§ 135.6 hours below normal§
Total Bright Sunshine Duration 217.3 hours 71.5 hours above normal 74.3 hours above normal
Mean Daily Global Solar Radiation 14.43 Megajoule / square metre 3.88 Megajoule above normal 4.26 Megajoule above normal
Total Evaporation 87.9 mm 18.8 mm above normal 16.6 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 HK for January 2021

The percentile map of mean temperature of January 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


Extract of Meteorological Observations in Hong Kong for January 2021