The weather of Hong Kong was drier than usual in May 2023. The monthly rainfall was 182.8 millimetres, about 37 percent below the normal of 290.6 millimetres in May. The accumulated rainfall recorded in the first five months of the year was 350.4 millimetres, about 41 percent below the normal figure of 590.9 millimetres for the same period. The month was also slightly warmer than usual with the mean temperature of 26.6 degrees, 0.3 degrees above the normal of 26.3 degrees. Together with the above normal temperatures in March and April, the spring of this year from March to May was exceptionally warm. The mean minimum temperature of 22.0 degrees, mean temperature of 23.8 degrees and mean maximum temperature of 26.5 degrees were respectively one of the fourth, sixth and eighth highest on record for the same period.

With the northeast monsoon affecting the southern China and a band of clouds covering the coast of Guangdong, it was mainly cloudy with sunny intervals in Hong Kong on the first two days of the month. There were also a few light rain patches on the morning of 1 May. Under the influence of an anticyclone aloft, apart from isolated showers on the morning of 3 May, the weather of Hong Kong became hot with sunny periods on 3 – 5 May. Affected by a southerly airstream, the weather turned cloudier with isolated showers on 6 May.

A trough of low pressure moved across the south China coastal area and brought showery weather and squally thunderstorms to Hong Kong on 7 – 8 May. The rain was particularly heavy on the afternoon of 7 May and necessitated the issuance of the first Red Rainstorm Warning Signal of the year. More than 50 millimetres of rainfall were recorded generally over the territory and rainfall even exceeded 100 millimetres over parts of Hong Kong Island on these two days. Affected by a fresh to strong easterly airstream, there were sunny intervals and a few rain patches on 9 – 11 May. With a band of clouds covering Guangdong, it was mainly cloudy with a few showers on 12 – 13 May.

Under the influence of a trough of low pressure, the showers affecting Hong Kong became more frequent during the day on 14 May. More than 30 millimetres of rainfall were recorded over most parts of the territory and rainfall even exceeded 50 millimetres over Lantau Island on that day. Under the rain, temperatures at the Observatory dropped to a minimum of 20.2 degrees on that morning, the lowest of the month. With the departure of the trough of low pressure, the weather improved with sunny periods during the day on 15 - 16 May.

With the setting in of a southerly airstream, local weather turned showery with a few thunderstorms on 17 May. The showers were heavier around noon on that day with more than 30 millimetres of rainfall recorded over many places. With the showers petering out, it was hot with sunny periods in Hong Kong on 18 – 21 May. Visibility was also rather low in some areas on 18 – 20 May. With abundant sunshine, it was very hot on the afternoon of 22 May. Affected by a trough of low pressure, local weather became cloudy with showers and squally thunderstorms on 23 – 24 May. The rain was particularly heavy in some areas on the morning of 23 May. More than 30 millimetres of rainfall were recorded over parts of the territory and rainfall even exceeded 70 millimetres over Yuen Long. It remained mainly cloudy with a few showers on 25 May.

With the strengthening of the anticyclone aloft, apart from a few showers, local weather gradually turned mainly fine and hot on 26 - 29 May. Meanwhile, over the western North Pacific, tropical cyclone Mawar moved across the sea areas to the east of Luzon and Taiwan on 28 - 31 May. Under the influence of the outer subsiding air of Mawar, it was mainly fine and extremely hot on the afternoons of last two days of the month. Temperatures over many places rose to 35 degrees or above. The maximum temperature recorded at the Observatory was 34.7 degrees on the afternoon of 31 May, the highest of the month. Moreover, the daily minimum temperature of 29.6 degrees and daily mean temperature of 31.4 degrees on that day were the highest and one of the highest on record for May. Showers and thunderstorms triggered by high temperatures also affected parts of the territory on the afternoons of 30 – 31 May. The showers were heavier in some places on the afternoon of 31 May with more than 30 millimetres of rainfall recorded over North District and Tai Po District.

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

 

Warnings and Signals issued in May 2023

Table 1.1   Rainstorm Warning Signals

Colour	Beginning Time		Ending Time
	Day/Month	HKT	Day/Month	HKT
Amber	7 / 5	1210	7 / 5	1335
Red	7 / 5	1335	7 / 5	1445
Amber	7 / 5	1445	7 / 5	1515
Amber	17 / 5	1110	17 / 5	1300
Amber	23 / 5	0430	23 / 5	0845

Table 1.2   Thunderstorm Warning

Beginning Time		Ending Time
Day/Month	HKT	Day/Month	HKT
7 / 5	1140	7 / 5	1830
8 / 5	0624	8 / 5	1445
17 / 5	1009	17 / 5	1430
23 / 5	0330	23 / 5	1000
23 / 5	1240	23 / 5	1445
24 / 5	0750	24 / 5	0950
24 / 5	1545	24 / 5	1815
30 / 5	1514	30 / 5	1645
31 / 5	1330	31 / 5	1705
31 / 5	1820	31 / 5	1920

Table 1.3   Fire Danger Warnings

Colour	Beginning Time		Ending Time
	Day/Month	HKT	Day/Month	HKT
Yellow	1 / 5	0945	1 / 5	1800

Table 1.4   Very Hot Weather Warning

Beginning Time		Ending Time
Day/Month	HKT	Day/Month	HKT
22 / 5	1145	22 / 5	1730
29 / 5	0645	1 / 6	0530

Table 1.5   Special Announcement on Flooding
in the Northern New Territories

Beginning Time		Ending Time
Day/Month	HKT	Day/Month	HKT
23 / 5	0555	23 / 5	0920

Table 2   Figures and Departures from Normal - May 2023

Meteorological Element	Figure of the Month	Departure from Normal*
Mean Daily Maximum Air Temperature	29.2 degrees C	0.4 degree above normal
Mean Air Temperature	26.6 degrees C	0.3 degree above normal
Mean Daily Minimum Air Temperature	24.9 degrees C	0.4 degree above normal
Mean Dew Point Temperature	23.1 degrees C	0.1 degree above normal
Mean Relative Humidity	81 %	2 % below normal
Mean Cloud Amount	75 %	1 % below normal
Total Rainfall	182.8 mm	107.8 mm below normal
Number of hours of Reduced VisibilityΔ	7 hours	31.0 hours below normal§
Total Bright Sunshine Duration	131.9 hours	6.9 hours below normal
Mean Daily Global Solar Radiation	14.59 Megajoule / square metre	0.13 Megajoule above normal
Total Evaporation	91.9 mm	17.9 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

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