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

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

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