The Weather of August 2009

    August 2009 was hotter and drier than usual. The mean temperature was 29.4 degrees, 1.0 degrees above the normal of 28.4 degrees. There were 14 very hot days, making it the hottest August since 1963. The mean minimum temperature of 27.7 degrees was the highest for August since record began. The total rainfall of 334.1 millimetres in the month was about 25 percent below the normal figure of 444.6 millimetres. The accumulated rainfall since 1 January was 1541.0 millimetres, about 18 percent below the normal figure of 1873.7 millimetres for the same period.

    Under the influence of a ridge of high pressure, local weather was sunny for the first day of the month. Affected by the subsidence air mass associated with a tropical depression over the northern part of the South China Sea, it was very hot on 2 and 3 August. Temperatures recorded at the Hong Kong Observatory rose to a maximum of 34.9 degrees on 3 August, the highest of the month.

    The tropical depression over the northern part of the South China Sea intensified into a tropical storm and was named Goni on 3 August. The outer rainbands of Goni brought squally showers and thunderstorms to Hong Kong in that afternoon. Goni further intensified into a severe tropical storm while edging towards the coast of Guangdong on 4 August. Local winds strengthened gradually and there were some squally showers on that day. Gale force winds with maximum gusts up to 85 kilometres per hour were recorded at Cheung Chau in that evening. Goni made landfall over the coast of western Guangdong and weakened gradually the next day. However, the outer-rainbands of Goni continued to bring showers to Hong Kong on 5 and 6 August.

    Meanwhile, Morakot gained typhoon intensity over the western North Pacific and tracked westward in the generally direction of Taiwan on 5 and 6 August. Under the influence of the subsidence air mass associated with Morakot, the weather in Hong Kong became fine and very hot for the ensuing three days. A trough of low pressure developed over the south China coastal areas on 10 August and brought thundery showers to the territory from 10 to 14 August. Affected by a ridge of high pressure, local weather turned mainly fine for the next two days.

    Another trough of low pressure developed over the western coast of Guangdong and brought mainly cloudy weather with a few showers to the territory on 17 and 18 August. A weak ridge of high pressure brought generally fine and very hot conditions apart from a few isolated showers and thunderstorms to Hong Kong for the rest of the month.

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

    Very hot weather warnings were issued for 18 days in the month to remind the public to mitigate the health impact of very hot weather. Following the usual practice, advance information on hot weather and daily temperature forecasts were included in the Observatory's 7-day forecasts. Special mentions of high temperatures, when expected in the following few days, were made in the weather outlook to alert the public.

    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.