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The Weather of May 2021

02 June 2021

Mainly attributing to the stronger than usual subtropical ridge over southern China, May 2021 was the hottest May in Hong Kong on record. The monthly mean temperature of 29.0 degrees and monthly mean minimum temperature of 27.0 degrees were 2.7 degrees and 2.5 degrees above their corresponding normals (or 3.1 degree and 2.9 degrees above their corresponding 1981-2010 normals) and both were the highest on record for May. The mean maximum temperature of 32.1 degrees was 3.3 degrees above normal (or 3.7 degrees above the 1981-2010 normals) and the second highest on record for May. Together with the exceptionally warm weather in March and April 2021, Hong Kong experienced the warmest spring on record from March to May 2021. The mean temperature of 25.0 degrees, mean minimum temperature of 23.2 degrees and mean maximum temperature of 28.0 degrees for March to May 2021 were all the highest on record for the same period. There were in total 14 hot nights in the month including 6 consecutive hot nights that started from 16 May, both breaking the records for May. The month was also much drier than usual with a total rainfall of only 65.0 millimetres, about 22 percent of the normal figure of 290.6 millimetres (or 21 percent of the 1981-2010 normal of 304.7 millimetres). The accumulated rainfall recorded in the first five months of the year was 163.1 millimetres, a deficit of 72 percent when compared to the normal of 590.9 millimetres (or 75 percent below the 1981-2010 normal of 640.8 millimetres) and the second lowest on record for the same period.

After a generally fine day on 1 May, an upper-air disturbance brought a few showers and isolated thunderstorms to Hong Kong the next day. With the strengthening of an easterly airstream, it was mainly cloudy with a few showers on 3 May. Affected by a southerly airstream, the weather became hot with sunny periods during the day on 4 May. Thundery rainband associated with a trough of low pressure moved across the coastal areas of Guangdong that night and brought heavy showers and thunderstorms to Hong Kong. More than 30 millimetres of rainfall were recorded over many places of the territory. Under the rain, the temperature at the Observatory dropped to a minimum of 23.1 degrees, the lowest of the month.

With the departure of the trough of low pressure, weather improved during the day on 5 May with sunny periods. An easterly airstream picked up that night and local weather turned cloudy with a few rain patches. With the moderation of the easterly airstream and thinning out of the cloud band over the coast of Guangdong, the weather of Hong Kong became generally fine during the day on 6 May. Dominated by the anticyclone aloft, local weather remained generally fine and hot on 7-10 May. Under the influence of a southerly airstream, it was hot with sunny periods apart from some showers on 11 – 14 May. There were also isolated thunderstorms on 13 – 14 May. Showers were particularly heavy on the morning of 14 May. More than 10 millimetres of rainfall were recorded over many places in the western part of the territory, and rainfall even exceeded 70 millimetres over Tuen Mun District.

Under the influence of the subtropical ridge, it was sunny and very hot in Hong Kong on 15 – 16 May. Affected by a southerly airstream and later by the anticyclone aloft, apart from a few showers, the mainly fine and very hot weather persisted during 17 – 23 May. The daily minimum temperature of 28.8 degrees on 19 May was the highest on record for Buddha's Birthday. The daily minimum temperature of 29.5 degrees on 21 May was the highest on record for May. With plenty of sunshine, the temperature at the Observatory soared to a maximum of 36.1 degrees on the afternoon of 23 May, the highest of the month and also the highest maximum temperature for May on record. The daily mean temperature of 31.4 degrees on that day was also the highest on record for May. Moreover, a waterspout was reported near Lamma Island on the morning of 20 May.

Affected by a trough of low pressure, the weather of Hong Kong became unsettled with more showers and thunderstorms on 24 – 25 May. Showers were heavier in some areas of the territory on the morning of 25 May. More than 70 millimetres of rainfall were recorded over the northern part of the New Territories and Sai Kung. Under the influence of the anticyclone aloft over the northern part of the South China Sea, apart from isolated thunderstorms and a few showers on 26 May, local weather turned generally fine and very hot again on 26 – 28 May. Affected by a trough of low pressure over the coastal area of southern China, local weather turned cloudier with a few showers towards the end of the month. It was more showery with a few thunderstorms on 31 May. More than 10 millimetres of rainfall were recorded over many places on that day.

Two tropical cyclones occurred over the South China Sea and the western North Pacific in May 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 May are tabulated in Table 2.



Warnings and Signals issued in May 2021

Table 1.1   Rainstorm Warning Signals
Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Amber 4 / 5 1915 4 / 5 2145


Table 1.2   Thunderstorm Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
2 / 5 1345 2 / 5 1530
4 / 5 1905 4 / 5 2330
13 / 5 0932 13 / 5 1045
14 / 5 0710 14 / 5 1400
18 / 5 0325 18 / 5 0500
18 / 5 0545 18 / 5 0620
18 / 5 0748 18 / 5 0900
19 / 5 1325 19 / 5 1425
24 / 5 0938 24 / 5 1500
25 / 5 0650 25 / 5 1400
26 / 5 0951 26 / 5 1430
29 / 5 1155 29 / 5 1255
30 / 5 1220 30 / 5 1400
31 / 5 0040 31 / 5 0545
31 / 5 1145 31 / 5 1600
31 / 5 1802 1 / 6 0010


Table 1.3   Very Hot Weather Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
12 / 5 0645 12 / 5 1620
14 / 5 1230 17 / 5 1800
18 / 5 1100 20 / 5 1815
21 / 5 0745 24 / 5 1620
26 / 5 0830 29 / 5 1730


Table 1.4   Special Announcement on Flooding
in the Northern New Territories
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
25 / 5 1025 25 / 5 1305


Table 2   Figures and Departures from Normal - May 2021
Meteorological Element Figure of the Month Departure from Normal(1991-2020) Departure from Normal (1981-2010)
Mean Daily Maximum Air Temperature 32.1 degrees C 3.3 degrees above normal 3.7 degrees above normal
Mean Air Temperature 29.0 degrees C 2.7 degrees above normal 3.1 degrees above normal
Mean Daily Minimum Air Temperature 27.0 degrees C 2.5 degrees above normal 2.9 degrees above normal
Mean Dew Point Temperature 24.7 degrees C 1.7 degrees above normal 2.1 degrees above normal
Mean Relative Humidity 78 % 5 % below normal 5 % below normal
Mean Cloud Amount 75 % 1 % below normal 1 % below normal
Total Rainfall 65.0 mm 225.6 mm below normal 239.7 mm below normal
Number of hours of Reduced VisibilityΔ 4 hours 37.0 hours below normal§ 37.0 hours below normal§
Total Bright Sunshine Duration 214.0 hours 75.2 hours above normal 73.6 hours above normal
Mean Daily Global Solar Radiation 19.64 Megajoule / square metre 5.18 Megajoule above normal 5.45 Megajoule above normal
Total Evaporation 141.0 mm 31.2 mm above normal 30.3 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 May 2021

The percentile map of mean temperature of May 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 May 2021