02 June 2021

Mainly attributing to the stronger than usual subtropical ridge over southern China, May 2021 was the hottest May in Hong Kong on record. The monthly mean temperature of 29.0 degrees and monthly mean minimum temperature of 27.0 degrees were 2.7 degrees and 2.5 degrees above their corresponding normals (or 3.1 degree and 2.9 degrees above their corresponding 1981-2010 normals) and both were the highest on record for May. The mean maximum temperature of 32.1 degrees was 3.3 degrees above normal (or 3.7 degrees above the 1981-2010 normals) and the second highest on record for May. Together with the exceptionally warm weather in March and April 2021, Hong Kong experienced the warmest spring on record from March to May 2021. The mean temperature of 25.0 degrees, mean minimum temperature of 23.2 degrees and mean maximum temperature of 28.0 degrees for March to May 2021 were all the highest on record for the same period. There were in total 14 hot nights in the month including 6 consecutive hot nights that started from 16 May, both breaking the records for May. The month was also much drier than usual with a total rainfall of only 65.0 millimetres, about 22 percent of the normal figure of 290.6 millimetres (or 21 percent of the 1981-2010 normal of 304.7 millimetres). The accumulated rainfall recorded in the first five months of the year was 163.1 millimetres, a deficit of 72 percent when compared to the normal of 590.9 millimetres (or 75 percent below the 1981-2010 normal of 640.8 millimetres) and the second lowest on record for the same period.

After a generally fine day on 1 May, an upper-air disturbance brought a few showers and isolated thunderstorms to Hong Kong the next day. With the strengthening of an easterly airstream, it was mainly cloudy with a few showers on 3 May. Affected by a southerly airstream, the weather became hot with sunny periods during the day on 4 May. Thundery rainband associated with a trough of low pressure moved across the coastal areas of Guangdong that night and brought heavy showers and thunderstorms to Hong Kong. More than 30 millimetres of rainfall were recorded over many places of the territory. Under the rain, the temperature at the Observatory dropped to a minimum of 23.1 degrees, the lowest of the month.

With the departure of the trough of low pressure, weather improved during the day on 5 May with sunny periods. An easterly airstream picked up that night and local weather turned cloudy with a few rain patches. With the moderation of the easterly airstream and thinning out of the cloud band over the coast of Guangdong, the weather of Hong Kong became generally fine during the day on 6 May. Dominated by the anticyclone aloft, local weather remained generally fine and hot on 7-10 May. Under the influence of a southerly airstream, it was hot with sunny periods apart from some showers on 11 – 14 May. There were also isolated thunderstorms on 13 – 14 May. Showers were particularly heavy on the morning of 14 May. More than 10 millimetres of rainfall were recorded over many places in the western part of the territory, and rainfall even exceeded 70 millimetres over Tuen Mun District.

Under the influence of the subtropical ridge, it was sunny and very hot in Hong Kong on 15 – 16 May. Affected by a southerly airstream and later by the anticyclone aloft, apart from a few showers, the mainly fine and very hot weather persisted during 17 – 23 May. The daily minimum temperature of 28.8 degrees on 19 May was the highest on record for Buddha's Birthday. The daily minimum temperature of 29.5 degrees on 21 May was the highest on record for May. With plenty of sunshine, the temperature at the Observatory soared to a maximum of 36.1 degrees on the afternoon of 23 May, the highest of the month and also the highest maximum temperature for May on record. The daily mean temperature of 31.4 degrees on that day was also the highest on record for May. Moreover, a waterspout was reported near Lamma Island on the morning of 20 May.

Affected by a trough of low pressure, the weather of Hong Kong became unsettled with more showers and thunderstorms on 24 – 25 May. Showers were heavier in some areas of the territory on the morning of 25 May. More than 70 millimetres of rainfall were recorded over the northern part of the New Territories and Sai Kung. Under the influence of the anticyclone aloft over the northern part of the South China Sea, apart from isolated thunderstorms and a few showers on 26 May, local weather turned generally fine and very hot again on 26 – 28 May. Affected by a trough of low pressure over the coastal area of southern China, local weather turned cloudier with a few showers towards the end of the month. It was more showery with a few thunderstorms on 31 May. More than 10 millimetres of rainfall were recorded over many places on that day.

Two tropical cyclones occurred over the South China Sea and the western North Pacific in May 2021.

Details of issuance and cancellation of various warnings/signals in the month are summarized in Tables 1.1 to 1.4.  Monthly meteorological figures and departures from normal for May are tabulated in Table 2.