2 June 2020

May 2020 was characterized by generally fine and hot weather during the first part of the month and unsettled weather with outbreaks of heavy showers in the latter part. 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 one of the second highest on record for May. The monthly mean temperature of 27.7 degrees and mean maximum temperature of 30.4 degrees were respectively 1.8 degrees and 2.0 degrees above their corresponding normal figures and both were the fifth highest on record for May. Moreover, the spring of this year from March to May was exceptionally warm. The mean minimum temperature of 21.9 degrees, mean temperature of 23.7 degrees and mean maximum temperature of 26.4 degrees were respectively the fourth, fifth and sixth highest on record for the same period. May 2020 was also wetter than usual. The monthly rainfall was 352.5 millimetres, about 16 percent 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 percent below the normal figure of 640.8 millimetres for the same period.

Under the dominance of the anticyclone aloft, apart from some isolated showers on 8-10 May, the weather of Hong Kong was generally fine and hot on the first ten days of the month. On 11 May, it was fine and very hot during the day with the maximum temperature at the Observatory soaring to 33.5 degrees, the highest of the month. Thundery rainband associated with a trough of low pressure moved across the coastal areas of Guangdong at night and brought heavy showers and severe squally thunderstorms to Hong Kong. More than 30 millimetres of rainfall were recorded over parts of the New Territories. With the trough of low pressure departing to the south on 12 May, local weather improved gradually 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 13 – 14 May. With the gradual replacement of easterly airstream by a southerly airstream, the weather of Hong Kong became generally fine and very hot on 15 – 17 May. Affected by a trough of low pressure lingering over the coastal areas of Guangdong, local weather became mainly cloudy with occasional heavy showers and squally thunderstorms on 18 May. About 50 millimetres of rainfall were recorded over Hong Kong Island and Kowloon. The showery activities receded with sunny periods during the day the next day when the trough of low pressure moved away from the south China coast. With the setting in of a strong easterly airstream, the weather turned mainly cloudy with a few showers and isolated thunderstorms on 20 May.

Under the influence of a trough of low pressure over southern China, local weather was unsettled with outbreaks of heavy showers and squally thunderstorms on 21 – 22 May. More than 80 millimetres of rainfall were recorded over most parts of the territory and rainfall even exceeded 120 millimetres over parts of Hong Kong Island and Tseung Kwan O during these two days. 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 with sunny intervals on 24 May.

Heavy showers and squally thunderstorms returned to the territory on 25 – 26 May 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 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 issuance and cancellation of various warnings/signals in the month are summarized in Tables 1.1 to 1.6.  Monthly meteorological figures and departures from normal for May are tabulated in Table 2.