With stronger than normal northeast monsoon affecting southern China in the month, the weather of Hong Kong was much cooler than usual in May 2022. The monthly mean temperature was 25.0 degrees, 1.3 degrees below the normal figure of 26.3 degrees. Mainly attributing to the heavy rain episode on 11 – 13 May, the month was also wetter than usual with a total rainfall of 436.5 millimetres, about 50 percent above the normal of 290.6 millimetres. The accumulated rainfall recorded in the first five months of the year was 705.3 millimetres, about 19 percent above the normal figure of 590.9 millimetres for the same period.

Under the influence of the northeast monsoon and an upper-air disturbance, the weather of Hong Kong was cloudy with showers and rather cool on the first two days of the month. More than 60 millimetres of rainfall were recorded over many places of the territory on these two days. The temperature at the Observatory dropped to a minimum of 16.4 degrees on the morning of 2 May, the lowest record in May since 1917. The daily mean temperature of 18.5 degrees on that day was also the lowest on record for May. Affected by a dry continental airstream and the subsequent easterly airstream, local weather was generally fine on 3 – 6 May. It was also dry with the relative humidity staying around 50 percent on the afternoons of 3 – 4 May.

Affected by an upper-air disturbance, it was mainly cloudy with some showers in Hong Kong on 7 – 10 May. Meanwhile, an active trough of low pressure over the northern part of southern China edged south on 10 May and lingered along the coast of Guangdong in the next few days. The trough of low pressure brought heavy showers and squally thunderstorms to the Pearl River Estuary areas on 11 – 13 May. Locally, more than 300 millimetres of rainfall were generally recorded over the territory and rainfall even exceeded 400 millimetres over parts of Hong Kong Island on these three days. The heavy rain on the morning of 13 May also necessitated the issuance of the first Red Rainstorm Warning Signal of the year.

With the trough of low pressure shifting to the northern part of the South China Sea and the onset of an easterly airstream over the coastal areas, the showery weather abated gradually on 14 – 15 May. Affected by the northeast monsoon, the weather became slightly cooler with a few showers the next day. Dominated by the dry northeast monsoon, local weather turned mainly fine on 17 – 18 May. It was also very dry on 18 May with the relative humidity over most parts of the territory falling to about 30 percent around noon.

With the moderation of the northeast monsoon, apart from a few showers at first on 19 May, it was generally fine and hot during the day on 19 – 21 May. Under the influence of a broad trough of low pressure over the northern part of the South China Sea and the onset of a fresh to strong easterly airstream, local weather turned mainly cloudy with a few showers on 22 – 25 May. While the weather of Hong Kong was a mixture of sunshine and showers on 26 May, local weather became showery with a few thunderstorms on 27 May as affected by a trough of low pressure along the coast of Guangdong. The showers were more persistent over the New Territories with more than 50 millimetres of rainfall recorded over Yuen Long, North District and Tai Po on that day. With the anticyclone aloft over the western North Pacific extending westwards, the showers eased off gradually with sunny periods during the day on 28 May. Apart from a few showers, the weather was generally fine and hot on 29 – 30 May. The temperature at the Observatory rose to a maximum of 32.7 degrees on the afternoon of 30 May, the highest of the month. Under the influence of a southerly airstream, there were sunny intervals, a few showers and thunderstorms on the last day of the month.

Warnings and Signals issued in May 2022

Table 1.1   Strong Monsoon Signal

Beginning Time		Ending Time
Day/Month	HKT	Day/Month	HKT
1 / 5	1745	1 / 5	2130

Table 1.2   Rainstorm Warning Signals

Colour	Beginning Time		Ending Time
	Day/Month	HKT	Day/Month	HKT
Amber	12 / 5	0525	12 / 5	0640
Amber	12 / 5	1210	12 / 5	2245
Amber	13 / 5	0355	13 / 5	0425
Red	13 / 5	0425	13 / 5	0705
Amber	13 / 5	0705	13 / 5	0740
Amber	13 / 5	1905	13 / 5	2230

Table 1.3   Landslip Warning

Beginning Time		Ending Time
Day/Month	HKT	Day/Month	HKT
12 / 5	2115	13 / 5	1515

Table 1.4   Thunderstorm Warning

Beginning Time		Ending Time
Day/Month	HKT	Day/Month	HKT
10 / 5	1755	10 / 5	1930
10 / 5	2300	13 / 5	0045
13 / 5	0210	13 / 5	0900
13 / 5	1500	14 / 5	0100
14 / 5	0340	14 / 5	0500
15 / 5	0020	15 / 5	0630
15 / 5	2100	15 / 5	2230
26 / 5	1215	26 / 5	1330
27 / 5	0550	27 / 5	1700
29 / 5	1245	29 / 5	1400
30 / 5	0830	30 / 5	1030
31 / 5	1015	31 / 5	1330
31 / 5	1945	31 / 5	2100

Table 1.5   Fire Danger Warnings

Colour	Beginning Time		Ending Time
	Day/Month	HKT	Day/Month	HKT
Red	18 / 5	0745	18 / 5	2315

Table 1.6   Very Hot Weather Warning

Beginning Time		Ending Time
Day/Month	HKT	Day/Month	HKT
30 / 5	1115	30 / 5	1745

Table 1.7   Special Announcement on Flooding
in the Northern New Territories

Beginning Time		Ending Time
Day/Month	HKT	Day/Month	HKT
12 / 5	0307	12 / 5	0640
12 / 5	2130	13 / 5	0100

Table 2   Figures and Departures from Normal - May 2022

Meteorological Element	Figure of the Month	Departure from Normal*
Mean Daily Maximum Air Temperature	27.7 degrees C	1.1 degrees below normal
Mean Air Temperature	25.0 degrees C	1.3 degrees below normal
Mean Daily Minimum Air Temperature	23.3 degrees C	1.2 degrees below normal
Mean Dew Point Temperature	21.3 degrees C	1.7 degrees below normal
Mean Relative Humidity	81 %	2 % below normal
Mean Cloud Amount	82 %	6 % above normal
Total Rainfall	436.5 mm	145.9 mm above normal
Number of hours of Reduced VisibilityΔ	1 hour	38.5 hours below normal§
Total Bright Sunshine Duration	124.6 hours	14.2 hours below normal
Mean Daily Global Solar Radiation	14.27 Megajoule / square metre	0.19 Megajoule below normal
Total Evaporation	82.7 mm	27.1 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 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