4 September 2023

Mainly attributing to the warmer than normal sea surface temperature over the northern part of the South China Sea and a stronger than usual southwesterly flow in the lower atmosphere over the south China coast, August 2023 was much hotter than usual in Hong Kong. The monthly mean temperature of 29.7 degrees and monthly mean minimum temperature of 27.8 degrees were respectively 1.0 degree and 1.1 degrees above their normal and both were the highest on record for August. Moreover, the monthly mean maximum temperature of 32.4 degrees was 1.1 degrees above normal and one of the second highest on record for August. Together with the exceptionally hot weather in June and July, Hong Kong experienced the hottest summer on record from June to August 2023 with a record-breaking high mean temperature of 29.7 degrees. The mean maximum temperature of 32.4 degrees and mean minimum temperature of 27.6 degrees were both the second highest on record for the same period. There were 15 hot nights in August 2023, one of the highest on record for August. The month was also much drier than usual with a total rainfall of 140.7 millimetres, about 31 percent of the normal figure of 453.2 millimetres and the ninth lowest on record for August. The accumulated rainfall up to August this year was 1157.2 millimetres, a deficit of about 40 percent compared with the normal of 1921.5 millimetres for the same period.

Under the influence of a broad trough of low pressure, there were some showers and sunny periods on the first day of the month. Showers and thunderstorms triggered by high temperatures also affected parts of the territory around noon on that day. The showers were heavier in some places with more than 30 millimetres of rainfall recorded over Tsing Yi, Tuen Mun and Yuen Long. Under light wind conditions, it was mainly fine and very hot on 2 August. Meanwhile, tropical cyclone Khanun moved west-northwestwards towards the East China Sea and lingered over there on 3 – 4 August. Under the influence of the outer subsiding air of Khanun, it was mainly fine and extremely hot on the afternoon of 3 August. The maximum temperature at the Observatory soared to 35.1 degrees that afternoon, the highest of the month.

With the prevalence of a southwesterly airstream and the subsequent establishment of the anticyclone aloft, the weather was very hot with sunny periods apart from a few showers during the day on 4 – 9 August. There were also a few thunderstorms on 5 and 9 August. Affected by a trough of low pressure, apart from sunny intervals on 12 and 14 August, it was mainly cloudy with occasional showers and squally thunderstorms in Hong Kong on 10 – 14 August. More than 50 millimetres of rainfall were recorded generally over the territory and rainfall even exceeded 100 millimetres over Tseung Kwan O, parts of Hong Kong Island and Lantau Island on these five days. Under the rain, temperatures at the Observatory dropped to a minimum of 25.7 degrees on 11 August, the lowest of the month but the highest monthly absolute minimum temperature on record for August.

Under the influence of an anticyclone aloft, apart from a few showers and isolated thunderstorms, the weather of Hong Kong improved with more sunshine on 15 – 17 August. It was also very hot during the day on 16 August. Affected by a trough of low pressure over the south China coast, local weather became mainly cloudy with showers and squally thunderstorms on 18 – 19 August. More than 40 millimetres of rainfall were recorded over many places and rainfall even exceeded 100 millimetres over the northern part of the New Territories on these two days. With the weakening of the trough of low pressure, the showers abated on 20 August. Under the influence of a southerly airstream, the weather of Hong Kong was a mixture of sunshine, showers and isolated thunderstorms on 21 – 28 August. It was also very hot during the day on 22 – 23 and 28 August.

Affected by a trough of low pressure along the coast of Guangdong and a weak northeast monsoon over southern China, the weather of Hong Kong turned mainly cloudy and more showery with thunderstorms on 29 August. The showers were particularly heavy in some places with more than 90 millimetres of rainfall recorded over the northern part of the New Territories. Meanwhile, over the western North Pacific, tropical cyclone Saola tracked northwestwards across the Luzon Strait on 29 – 30 August. It continued to edge towards the coast of eastern Guangdong on 31 August. Locally, it was hot and dry with sunny periods during the day on the last two days of the month. With Saola edging closer to the coast of eastern Guangdong, local winds strengthened gradually and there were a few showers later on 31 August.

Seven tropical cyclones occurred over the South China Sea and the western North Pacific in August 2023.

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 August are tabulated in Table 2.

Warnings and Signals issued in August 2023

Table 1.1   Tropical Cyclone Warning Signals

Name of Tropical Cyclone	Signal Number	Beginning Time		Ending Time
		Day/Month	HKT	Day/Month	HKT
SAOLA	1	30 / 8	1740	31 / 8	1540
	3	31 / 8	1540	1 / 9	0240

Table 1.2   Rainstorm Warning Signals

Colour	Beginning Time		Ending Time
	Day/Month	HKT	Day/Month	HKT
Amber	10 / 8	1310	10 / 8	1545
Amber	11 / 8	0130	11 / 8	0330
Amber	13 / 8	1005	13 / 8	1115
Amber	18 / 8	1030	18 / 8	1300
Amber	19 / 8	1155	19 / 8	1340

Table 1.3   Thunderstorm Warning

Beginning Time		Ending Time
Day/Month	HKT	Day/Month	HKT
1 / 8	1048	1 / 8	1330
4 / 8	0730	4 / 8	1000
5 / 8	0500	5 / 8	1030
6 / 8	0905	6 / 8	1200
9 / 8	1222	9 / 8	1900
10 / 8	0530	10 / 8	0730
10 / 8	0917	10 / 8	1615
10 / 8	2200	11 / 8	0610
11 / 8	0930	11 / 8	1130
11 / 8	1545	11 / 8	1645
12 / 8	0040	12 / 8	0500
12 / 8	1435	12 / 8	1630
13 / 8	0840	13 / 8	1300
13 / 8	1357	13 / 8	1500
13 / 8	2235	14 / 8	0415
14 / 8	0840	14 / 8	1430
15 / 8	1140	15 / 8	1510
17 / 8	0155	17 / 8	0315
17 / 8	1215	17 / 8	1530
18 / 8	0840	18 / 8	1330
18 / 8	1745	18 / 8	1840
19 / 8	0212	19 / 8	1430
20 / 8	0640	20 / 8	0830
20 / 8	2121	20 / 8	2400
21 / 8	0956	21 / 8	1500
22 / 8	0345	22 / 8	0500
22 / 8	1303	22 / 8	1430
24 / 8	0117	24 / 8	0930
24 / 8	1130	24 / 8	1330
24 / 8	1342	24 / 8	1600
25 / 8	1052	25 / 8	1700
26 / 8	0945	26 / 8	1230
26 / 8	1436	26 / 8	1530
27 / 8	0515	27 / 8	0700
27 / 8	1255	27 / 8	1530
28 / 8	0115	28 / 8	0245
28 / 8	0348	28 / 8	1130
28 / 8	1405	28 / 8	1530
28 / 8	2317	29 / 8	0200
29 / 8	1055	29 / 8	1800

Table 1.4   Very Hot Weather Warning

Beginning Time		Ending Time
Day/Month	HKT	Day/Month	HKT
2 / 8	0645	3 / 8	1900
4 / 8	1015	4 / 8	1830
5 / 8	1045	5 / 8	1900
6 / 8	1120	6 / 8	1750
7 / 8	1430	7 / 8	1800
8 / 8	0730	9 / 8	1830
16 / 8	0645	16 / 8	1830
17 / 8	0745	17 / 8	1300
22 / 8	1500	22 / 8	1730
23 / 8	1100	23 / 8	1745
25 / 8	1000	25 / 8	1415
26 / 8	1155	26 / 8	1615
28 / 8	1440	28 / 8	1730
29 / 8	1130	29 / 8	1415
30 / 8	0645	30 / 8	1620

Table 1.5   Special Announcement on Flooding
in the Northern New Territories

Beginning Time		Ending Time
Day/Month	HKT	Day/Month	HKT
19 / 8	1255	19 / 8	1525
29 / 8	1500	29 / 8	1945

Table 2   Figures and Departures from Normal - August 2023

Meteorological Element	Figure of the Month	Departure from Normal*
Mean Daily Maximum Air Temperature	32.4 degrees C	1.1 degrees above normal
Mean Air Temperature	29.7 degrees C	1.0 degree above normal
Mean Daily Minimum Air Temperature	27.8 degrees C	1.1 degrees above normal
Mean Dew Point Temperature	25.6 degrees C	0.5 degrees above normal
Mean Relative Humidity	79 %	2 % below normal
Mean Cloud Amount	81 %	11 % above normal
Total Rainfall	140.7 mm	312.5 mm below normal
Number of hours of Reduced VisibilityΔ	0 hours	39.4 hours below normal§
Total Bright Sunshine Duration	166.4& hours	15.7 hours below normal
Mean Daily Global Solar Radiation	16.02& Megajoule / square metre	0.29 Megajoule above normal
Total Evaporation	108.5 mm	21.2 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 1991 - 2020 climatological normal, except for number of hours of reduced visibility
§   Departure from mean value between 1997 and 2022
&   Data incomplete

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