October was much wetter and cloudier than usual in Hong Kong, the Hong Kong Observatory said today.

The mean amount of cloud in the month was 79%, much higher than normal and the second highest on record for October.

The duration of bright sunshine in the month was only 138.9 hours, 30% lower than the normal figure and the fourth lowest on record for October.

Mainly due to the record-breaking rainfall associated with tropical cyclone Koinu on October 8 and 9, the month was also much wetter than usual.

The monthly total rainfall was 546mm, more than four times the normal figure and the fifth highest on record for October.

The accumulated rainfall this year up to October was 2,770.3mm, about 17% more than the normal figure of 2,363.1mm for the same period.

Despite the below normal duration of sunshine, the month was warmer than usual. The mean temperature of 26.4 degrees Celsius was 0.7 degrees above the norm and one of the fourth highest for October on record. There were three very hot days in the early part of the month, the highest on record for October.

Under the influence of an anticyclone aloft, severe typhoon Koinu over the western North Pacific moved northwestwards towards the Luzon Strait and turned westwards on October 4. Koinu moved slowly across the northern part of the South China Sea and edged towards the Pearl River Estuary over the next two days.

Koinu weakened into a typhoon on October 8 and skirted past about 70km to the south of Hong Kong that night.

Under the influence of Koinu’s outer subsiding air, the maximum temperature soared to 34.6 degrees Celsius on October 4, the highest of the month and the highest monthly absolute maximum temperature on record for October. Moreover, the daily mean temperature on that day reached 30.8 degrees, the highest on record for October.

With Koinu moving towards the vicinity of the Pearl River Estuary, Hong Kong’s weather deteriorated significantly on October 8 and the Gale or Storm Signal No. 8 was issued on that afternoon. The Increasing Gale or Storm Signal No. 9 was also issued that night.

With Koinu weakening and departing from Hong Kong, local winds moderated later on October 9. Koinu brought squally heavy showers to the city on October 8 and 9, and the Black Rainstorm Warning was issued October 9.

The rainfall recorded on October 9 reached 369.7mm, more than three times October’s monthly total normal figure of 120.3mm and the highest daily rainfall on record for October. Moreover, the 24-hour rainfall from 3pm on October 8 to 3pm next day reached 439.8mm, breaking the record for October.

The city’s weather became slightly cooler in the latter part of the month. Temperatures fell to a minimum of 22 degrees Celsius on October 21 under rain patches, the lowest of the month.

 
Three tropical cyclones occurred over the South China Sea and the western North Pacific in the month.