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The Weather of June 2021

5 July 2021

June 2021 was marked by the long awaited rainy weather brought by tropical cyclone Koguma, troughs of low pressure and the active southwesterly airstream. The monthly rainfall of June 2021 was 628.0 millimetres, about 28 percent above the normal of 491.5 millimetres (or 38 percent above the 1981-2010 normal of 456.1 millimetres). The abundant rainfall in the month alleviated the very dry condition of Hong Kong in the last few months. The accumulated rainfall recorded in the first half of the year was 791.1 millimetres, a deficit of 27 percent compared to the normal of 1082.5 millimetres (or 28 percent below the 1981-2010 normal of 1096.9 millimetres). The month was also warmer than usual with a mean temperature of 28.8 degrees, 0.5 degrees above the normal figure of 28.3 degrees (or 0.9 degrees above the 1981-2010 normals). Mainly attributing to the well above normal temperatures in the previous four months, the first half of this year from January to June 2021 was exceptionally warm. The mean maximum temperature of 26.3 degrees, mean temperature of 23.3 degrees and mean minimum temperature of 21.3 degrees were all the highest on record for the same period.

Under the influence of a trough of low pressure, the local weather was unstable with heavy showers and squally thunderstorms on the first day of the month. More than 100 millimetres of rainfall were recorded over many places of Lantau Island and Hong Kong Island in that afternoon, and rainfall even exceeded 150 millimetres over southern part of Lantau Island, Cheung Chau, Lamma Island and Stanley. With the trough of low pressure moving to the north, local weather improved with sunny intervals and less showers the next day. Affected by an anticyclone aloft, it was generally fine and very hot on 3 June. With plenty of sunshine, the temperature at the Observatory soared to a maximum of 34.0 degrees in the afternoon, the highest of the month.

A trough of low pressure moved across the coast of Guangdong and brought some showers and a few thunderstorms to Hong Kong on 4 June. With the trough of low pressure departing from Hong Kong and the band of clouds over the coast of Guangdong thinning out gradually, local weather improved with sunny periods on the next two days. Under light wind conditions, there was also some haze on the morning of 6 June. With the setting in of an easterly airstream, apart from a few showers, local weather gradually became mainly fine on 7 – 8 June. It was also very hot during the day on 8 June.

Affected by a broad trough of low pressure and subsequently an area of low pressure over the South China Sea, there were sunny intervals and a few heavy showers and thunderstorms in Hong Kong on 9 – 11 June. The area of low pressure over the South China Sea intensified into a tropical depression on the afternoon of 11 June. It moved across the southwestern part of Hainan Island and entered Beibu Wan on 12 June. The tropical depression further intensified into a tropical storm over Beibu Wan and was named Koguma later that day. It made landfall over the northern part of Vietnam and then weakened into an area of low pressure inland on 13 June. The thundery showers associated with the outer rain bands of Koguma continued to affect Hong Kong on 12 – 13 June. During 9 – 12 June, more than 50 millimetres of rainfall were generally recorded over the territory and rainfall even exceeded 100 millimetres over Tai Po, parts of Kowloon and Hong Kong Island.

Under the influence of a southwesterly airstream, local weather was a mixture of sunny periods and a few showers on 14 – 20 June. It was also very hot during the day on 16 – 20 June. The daily minimum temperature of 29.5 degrees on 19 June was one of the highest on record for June. A trough of low pressure over southern China edged towards the coastal areas gradually on 21 June. Locally, there were sunny intervals with a few showers and squally thunderstorms on that day. With the trough of low pressure lingering over the coast of Guangdong, it was mainly cloudy with occasional heavy showers and thunderstorms in Hong Kong on 22 – 26 June. More than 100 millimetres of rainfall were generally recorded over the territory and rainfall even exceeded 200 millimetres over Sai Kung, many parts of Kowloon and Hong Kong Island on these five days.

With an active southwesterly airstream affecting the coast of Guangdong, the weather of Hong Kong remained unsettled with occasional showers and squally thunderstorms on 27 – 29 June. The rain was particularly heavy and persistent on the morning of 28 June and the incessant downpour necessitated the issuance of the first Black Rainstorm Warning this year. More than 150 millimetres of rainfall were recorded over Kowloon peninsula, Hong Kong Island, Tsing Yi, Tseung Kwan O, Cheung Chau and Lantau Island. Under the torrential rain, the temperature at the Observatory dropped to a minimum of 24.0 degrees, the lowest of the month. With the showers eased off gradually, it was very hot with sunny intervals on the last day of the month.

Three tropical cyclones occurred over the South China Sea and the western North Pacific in June 2021.


Total rainfall on 28 June 2021
Total rainfall on 28 June 2021

Details of issuance and cancellation of various warnings/signals in the month are summarized in Tables 1.1 to 1.5.  Monthly meteorological figures and departures from normal for June are tabulated in Table 2.


 

Warnings and Signals issued in June 2021

Table 1.1   Tropical Cyclone Warning Signals
Name of
Tropical Cyclone
Signal
Number
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
KOGUMA 1 11 / 6 1615 12 / 6 1410


Table 1.2   Rainstorm Warning Signals
Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Amber 1 / 6 0810 1 / 6 0935
Amber 1 / 6 1530 1 / 6 1715
Red 1 / 6 1715 1 / 6 1815
Amber 1 / 6 1815 1 / 6 2110
Amber 11 / 6 2025 11 / 6 2245
Amber 22 / 6 0650 22 / 6 0830
Amber 23 / 6 1410 23 / 6 1530
Amber 24 / 6 0700 24 / 6 0820
Amber 24 / 6 1330 24 / 6 1445
Amber 26 / 6 0600 26 / 6 0825
Amber 28 / 6 0505 28 / 6 0555
Red 28 / 6 0555 28 / 6 0820
Black 28 / 6 0820 28 / 6 1105
Red 28 / 6 1105 28 / 6 1145
Amber 28 / 6 1145 28 / 6 1230


Table 1.3   Landslip Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
28 / 6 1110 28 / 6 1730


Table 1.4   Thunderstorm Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
31 / 5 1802 1 / 6 0010
1 / 6 0110 1 / 6 1130
1 / 6 1215 1 / 6 2300
4 / 6 0805 4 / 6 1400
9 / 6 0349 9 / 6 1345
11 / 6 1525 11 / 6 1645
11 / 6 1835 12 / 6 1130
12 / 6 1655 12 / 6 1830
13 / 6 0000 13 / 6 0200
14 / 6 1232 14 / 6 1330
15 / 6 0435 15 / 6 0830
15 / 6 1035 15 / 6 1230
21 / 6 0840 21 / 6 1200
21 / 6 1325 21 / 6 1550
21 / 6 2240 21 / 6 2355
22 / 6 0245 22 / 6 0500
22 / 6 0620 22 / 6 1030
22 / 6 1915 22 / 6 2100
23 / 6 1215 23 / 6 1600
24 / 6 0625 24 / 6 1530
26 / 6 0530 26 / 6 1030
26 / 6 1150 26 / 6 1435
26 / 6 1913 26 / 6 2100
27 / 6 1043 27 / 6 1400
28 / 6 0425 28 / 6 1430
29 / 6 0945 29 / 6 1530


Table 1.5   Very Hot Weather Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
3 / 6 0745 3 / 6 1845
8 / 6 1215 8 / 6 1800
16 / 6 1200 21 / 6 1510
30 / 6 1315 30 / 6 1620


Table 2   Figures and Departures from Normal - June 2021
Meteorological Element Figure of the Month Departure from Normal(1991-2020) Departure from Normal (1981-2010)
Mean Daily Maximum Air Temperature 31.2 degrees C 0.5 degree above normal 1.0 degree above normal
Mean Air Temperature 28.8 degrees C 0.5 degree above normal 0.9 degree above normal
Mean Daily Minimum Air Temperature 26.9 degrees C 0.4 degree above normal 0.7 degree above normal
Mean Dew Point Temperature 25.4 degrees C 0.5 degree above normal 0.8 degree above normal
Mean Relative Humidity 82 % normal normal
Mean Cloud Amount 83 % 6 % above normal 6 % above normal
Total Rainfall 628.0 mm 136.5 mm above normal 171.9 mm above normal
Number of hours of Reduced VisibilityΔ 1 hour 13.8 hours below normal§ 13.8 hours below normal§
Total Bright Sunshine Duration 122.2 hours 22.1 hours below normal 23.9 hours below normal
Mean Daily Global Solar Radiation 14.81 Megajoule / square metre 0.2 Megajoule above normal 0.62 Megajoule above normal
Total Evaporation 99.6 mm 14.2 mm below normal 17.5 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.

  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 June 2021

The percentile map of mean temperature of June 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 June 2021