A wet and cooler May


     With a stronger than normal northeast monsoon affecting southern China, 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 attributed to the heavy rain episode from May 11 to 13, the month was wetter than usual with a total rainfall of 436.5 millimetres, about 50 per cent above the normal figure of 290.6mm. The accumulated rainfall recorded in the first five months of the year was 705.3mm, about 19 per cent above the normal figure of 590.9mm for the same period.


     Under the influence of the northeast monsoon and an upper-air disturbance, the weather was cloudy with showers and rather cool on the first two days of the month, with more than 60mm of rainfall recorded over many places of the territory. The temperature recorded at the Hong Kong Observatory (HKO) dropped to a minimum of 16.4 degrees in the morning of May 2, 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 from May 3 to 6. It was also dry, with the relative humidity staying around 50 per cent in the afternoons from May 3 to 4.


     Affected by an upper-air disturbance, it was mainly cloudy with some showers from May 7 to 10. An active trough of low pressure over the northern part of southern China edged south on May 10 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 from May 11 to 13. Locally, more than 300mm of rainfall were generally recorded over the territory and rainfall even exceeded 400mm over parts of Hong Kong Island on these three days. The heavy rain in the morning of May 13 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 from May 14 to 15. 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 from May 17 to 18. It was very dry on May 18 with the relative humidity over most parts of the territory falling to about 30 per cent around noon. 


     With the moderation of the northeast monsoon, apart from a few showers at first on May 19, it was generally fine and hot during the day from May 19 to 21. 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 from May 22 to 25. While the weather was a mixture of sunshine and showers on May 26, it became showery with a few thunderstorms on May 27 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 50mm 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 May 28. Apart from a few showers, the weather was generally fine and hot from May 29 to 30. The temperature recorded at the HKO rose to a maximum of 32.7 degrees on the afternoon of May 30, 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.


     There was no tropical cyclone over the South China Sea and the western North Pacific in May 2022.


     Details of issuance and cancellation of various warnings/signals in May are summarised in Table 1. Monthly meteorological figures and departures from normal for May are tabulated in Table 2.