An abnormally hot May

     May 2020 was generally fine and hot in the first half of the month, while unsettled weather and occasional heavy showers set in in the latter half. Overall, the month was much hotter than usual. The monthly mean minimum temperature was 25.9 degrees, 1.8 degrees above the normal figure and the joint second highest on record for May. The monthly mean temperature of 27.7 degrees and the mean maximum temperature of 30.4 degrees were 1.8 degrees and 2.0 degrees above their corresponding normal figures respectively and both were the fifth highest on record for May. Moreover, the spring from March to May this year was exceptionally warm. The mean minimum temperature of 21.9 degrees, the mean temperature of 23.7 degrees and the mean maximum temperature of 26.4 degrees were the fourth, fifth and sixth highest on record for the same period respectively. May 2020 was also wetter than usual. The monthly rainfall was 352.5 millimetres, about 16 per cent above the normal figure of 304.7 millimetres. The accumulated rainfall recorded in the first five months of the year was 566.2 millimetres, about 12 per cent below the normal figure of 640.8 millimetres for the same period.
      
     Under the dominance of the anticyclone aloft, except for a few isolated showers from May 8 to 10, the weather of Hong Kong was generally fine and hot on the first 10 days of the month. On May 11, it was fine and very hot during the day with the maximum temperature recorded at the Hong Kong Observatory soaring to 33.5 degrees, the highest of the month. A thundery rainband associated with a trough of low pressure moved across the coastal areas of Guangdong that night and brought heavy showers and severe squally thunderstorms to Hong Kong. More than 30 millimetres of rainfall were recorded over some parts of the New Territories. With the trough of low pressure moving southward and getting away from Hong Kong, local weather improved gradually on May 12 with sunny intervals in the afternoon.
      
     Under the influence of a fresh to strong easterly airstream, the weather of Hong Kong was a mixture of sunshine and showers on May 13 and 14. With the gradual replacement of the easterly airstream by a southerly airstream, the weather of Hong Kong became generally fine and very hot from May 15 to 17. Affected by a trough of low pressure lingering over the coastal areas of Guangdong, local weather was mainly cloudy with occasional heavy showers and squally thunderstorms on May 18. About 50 millimetres of rainfall were recorded over Hong Kong Island and Kowloon. As the trough of low pressure moved away from the south China coast, showery activities on the next day receded with sunny periods during the day. With the setting in of a strong easterly airstream, the weather on May 20 turned mainly cloudy with a few showers and isolated thunderstorms.
      
     Under the influence of a trough of low pressure over southern China, local weather was unsettled with occasional heavy showers and squally thunderstorms on May 21 and 22. More than 80 millimetres of rainfall were recorded over most parts of the territory on these two days and the rainfall even exceeded 120 millimetres over some parts of Hong Kong Island and Tseung Kwan O. As the trough of low pressure moved away from the coastal areas, the showers over Hong Kong gradually eased off on the next two days. There were sunny intervals on May 24.
      
     Heavy showers and squally thunderstorms returned to the territory on May 25 and 26 when another trough of low pressure moved across the south China coastal areas. More than 40 millimetres of rainfall were recorded over most parts of the territory on these two days. The showery activities in Hong Kong became less active with sunny intervals on the next three days. Affected by an active southwest monsoon, local weather deteriorated again with heavy showers and thunderstorms on May 30. More than 100 millimetres of rainfall were recorded over most parts of the territory and the rainfall even exceeded 150 millimetres over Wong Tai Sin and Tseung Kwan O on that day. Apart from isolated thunderstorms in the morning, the shower activities receded with sunny intervals on the last day of the month.
      
     One tropical cyclone occurred over the South China Sea and the western North Pacific in May 2020.

     Details of the 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.