2 September 2021

August 2021 was characterized by cloudier than usual weather with localized heavy rain over parts of the New Territories. The mean amount of cloud in the month was 77 percent, 7 percent above the normal of 70 percent. As for monthly rainfall, while over 600 millimetres of rainfall were recorded in parts of the North District of the New Territories, the monthly rainfall recorded at the Observatory was 350.5 millimetres, about 23 percent below the normal figure of 453.2 millimetres (or 19 percent below the 1981-2010 normal of 432.2 millimetres). The accumulated rainfall recorded in the first eight months of the year was 1521.1 millimetres, about 21 percent below the normal figure of 1921.5 millimetres (or 20 percent below the 1981-2010 normal of 1905.5 millimetres) for the same period. The monthly mean temperature of 28.8 degrees was near the normal figure of 28.7 degrees (or 0.2 degrees above the 1981-2010 normal). Mainly attributing to the exceptionally hot weather in July 2021, the summer of this year from June to August was much hotter than usual. The mean temperature of 29.1 degrees was the sixth highest on record for the same period.

Under the influence of a southwesterly airstream, the weather of Hong Kong was a mixture of sunny periods, showers and thunderstorms on the first two days of the month. The showers were rather heavy in some areas on 1 August with more than 70 millimetres of rainfall recorded over Lamma Island. Meanwhile, an area of low pressure over the northern part of the South China Sea intensified into a tropical depression on the night of 2 August. It moved generally eastwards slowly across the northern part of the South China Sea in the next two days. The tropical depression developed into a tropical storm and was named Lupit on the morning of 4 August. Lupit then moved northeastwards and skirted past the coastal areas of Fujian on 5 – 6 August. It continued to track northeastwards towards Japan in the next two days and evolved into an extratropical cyclone over the seas north of Honshu of Japan on 9 August.

Affected by the rainbands associated with Lupit, there were occasional heavy showers and squally thunderstorms in Hong Kong on 3 – 5 August. More than 100 millimetres of rainfall were recorded over most parts of the territory, and rainfall even exceeded 140 millimetres over the central part of Hong Kong Island, the northern part of New Territories and Lantau Island on these three days. With the departure of Lupit, a southwesterly airstream continued to bring unstable weather to the coastal areas of Guangdong on 6 – 10 August. Locally, it was mainly cloudy with occasional showers and thunderstorms on these few days. The showers were particularly heavy in some areas of the New Territories with daily rainfall over 100 millimetres on 6, 7 and 9 August. The weather became less showery with sunny periods on 11 – 13 August.

Under the influence of an upper-air disturbance, there were some showers and thunderstorms in Hong Kong on 14 – 15 August. The showers were heavy in the North District of the New Territories on 14 August with more than 70 millimetres of rainfall recorded. Affected by a southwesterly airstream, local weather was a mixture of sunshine and showers on 16 August. Dominated by an anticyclone aloft, apart from isolated morning showers, the weather became generally fine and very hot on the afternoon of 17 August.

Against the background of a southerly flow over the South China coast, it was hot with sunny periods in Hong Kong on 18 – 19 August. Under light wind conditions, the day heating also triggered localized heavy showers and thunderstorms on these two days. More than 90 millimetres of rainfall were recorded over San Tin and Ngau Tam Mei on 19 August. Dominated by an anticyclone aloft, apart from a few showers, it was mainly fine and very hot for most of the time in the following week. With plenty of sunshine, the temperature at the Observatory soared to 34.4 degrees on 25 August, the highest of the month. Under the influence of an upper-air disturbance, there were occasional showers and thunderstorms on 27 – 29 August. The temperature at the Observatory dropped to a minimum of 23.4 degrees during the thunderstorms on the morning of 27 August, the lowest of the month. While there were sunny periods on 30 August, the upper-air disturbance again brought some showers and a few thunderstorms to Hong Kong on the last day of the month.

Four tropical cyclones occurred over the South China Sea and the western North Pacific in August 2021.

Details of issuance and cancellation of various warnings/signals in the month are summarized in Tables 1.1 to 1.5.  Monthly meteorological figures and departures from normal for August are tabulated in Table 2.