An August with outbreaks of heavy rain
The month of August was much wetter than usual, mainly due to the heavy rain associated with an active southwest monsoon and upper-air disturbances in early August, and a number of tropical cyclones in mid-August. The monthly rainfall recorded at the Hong Kong Observatory was 939.2 millimetres, more than double the August normal of 453.2 millimetres, and the third highest on record for August. Although the first half of the year was much drier than usual, with outbreaks of heavy rain in July and August, the accumulated rainfall in the first eight months of the year was 1 985.3 millimetres, approximating the normal value of 1 921.5 millimetres for the same period.The month was also much cloudier than usual. The mean amount of cloud in the month was 81 per cent, 11 per cent above the normal of 70 per cent, one of the fourth highest on record for August. While the monthly mean temperature of 29.0 degrees was only 0.3 degrees above the normal, together with higher than normal temperatures in June and July, the summer of this year from June to August was hotter than normal. The mean minimum temperature of 27.2 degrees, the mean maximum temperature of 31.9 degrees and the mean temperature of 29.1 degrees were respectively one of the sixth, one of the seventh and one of the ninth highest on record for the same period.
Under the influence of a southwesterly airstream, the weather of Hong Kong was very hot with sunny periods on the first day of the month. There were also heavy showers and thunderstorms in some areas in the afternoon with more than 70 millimetres of rainfall recorded over the North District. An active southwest monsoon and upper-air disturbances brought heavy showers and squally thunderstorms to the coast of Guangdong from August 2 to 5. The outbreaks of heavy showers necessitated the issuance of Black Rainstorm Warnings three times in just four days. The Hong Kong Observatory Headquarters registered a record-breaking daily rainfall of 368.9 millimetres on August 5, the highest daily rainfall in August since records began in 1884. This daily rainfall amount was already more than 80 per cent of the August normal rainfall. The accumulated rainfall recorded at the Observatory during these four days was nearly 600 millimetres, with rainfall even exceeding 800 millimetres over Tseung Kwan O and Tsz Wan Shan. In the midst of the incessant downpour, the temperature at the Observatory dropped to the month's lowest of 24.0 degrees on the morning of August 5.
While showery conditions persisted on August 6, with the establishment of an anticyclone aloft, apart from a few showers, it was generally fine with prolonged heat from August 7 to 12. The maximum temperature at the Observatory rose to 34.4 degrees on the afternoon of August 10, the highest of the month. Meanwhile, Typhoon Podul over the western North Pacific moved west-northwestwards towards Taiwan on August 12. It further intensified into a severe typhoon before making landfall over the southeastern part of Taiwan the next day. Under the influence of its outer subsiding air, the weather in Hong Kong was mainly fine apart from isolated showers on August 13. It was also extremely hot in the afternoon. Podul rapidly weakened into a tropical depression as it moved into inland Guangdong on August 14. Under the influence of its outer band of intense thundery showers, it was overcast with heavy showers and severe squally thunderstorms in Hong Kong. More than 100 millimetres of rainfall were recorded over many places, and rainfall even exceeded 200 millimetres over Lantau Island, Tuen Mun and parts of Hong Kong Island. The incessant downpour necessitated the issuance of the Black Rainstorm Warning for the fifth time this year, breaking the previous record of three times set in 2000 and 2006 since the commencement of the rainstorm system operation in 1992. While there were still a few showers on August 15, with the dissipation of Podul and the establishment of a ridge of high pressure over southeastern China, local weather turned mainly fine on August 16.
Meanwhile, the broad area of low pressure over the central part of the South China Sea intensified into a tropical depression on August 16. It moved west-northwestwards towards the seas south of Hainan Island on August 17 and skirted across Beibu Wan the next day. Under the combined effect of the tropical depression and the ridge of high pressure over southeastern China, it was windy with squally showers and thunderstorms on August 17. The heavy showery weather continued on August 18 with more than 100 millimetres of rainfall recorded over many places in Hong Kong.
The weather remained unsettled on August 19 under the influence of an active southerly airstream. More than 30 millimetres of rainfall were recorded over most parts of the territory, and rainfall even exceeded 100 millimetres over parts of Kowloon and the New Territories. With the strengthening of an anticyclone aloft, local weather was generally fine and very hot from August 20 to 22.
The area of low pressure over the seas west of Luzon intensified into a tropical depression on the night of August 22 and was later named Kajiki. Kajiki moved westwards across the central part of the South China Sea and intensified progressively into a severe tropical storm the next day. It rapidly intensified into a severe typhoon over the seas south of Hainan Island on August 24 and made landfall over the northern part of Vietnam the next night. Locally, it was mainly fine on August 23 apart from a few showers and squally thunderstorms at night. There were sunny periods during the following two days. Under the influence of a ridge of high pressure over southeastern China, apart from isolated showers and thunderstorms over the western part of the New Territories in the afternoon, local weather was mainly fine and very hot on August 26 and 27. Affected by upper-air disturbances, it was mainly cloudy with a few showers and squally thunderstorms locally on August 28. Moreover, Nongfa formed as a tropical depression over the central part of the South China Sea that afternoon and moved northwestwards towards the seas south of Hainan Island. The Standby Signal, No. 1 was issued that night, for the eighth time this year, breaking the highest record between January and August since 1946. Nongfa intensified into a tropical storm and tracked across the seas south of Hainan Island on August 30. It made landfall over central and northern Vietnam that afternoon and then weakened into a tropical depression at night. Locally, it was very hot with sunny periods on August 29. It was also windy under the combined effect of a ridge of high pressure over southeastern China and Nongfa. An easterly airstream affected the coast of Guangdong on August 30 and the weather of Hong Kong was mainly cloudy with a few showers. The weather turned generally fine and very hot on the last day of the month.
Eleven tropical cyclones occurred over the South China Sea and the western North Pacific in August 2025.
Details of issuance and cancellation of various warnings/signals in the month are summarised in Table 1. Monthly meteorological figures and departures from normal for August are tabulated in Table 2.
Source: AI-found images
