September 2009 was hotter and wetter than usual.  The mean temperature was 28.8 degrees, 1.2 degrees above the normal of 27.6 degrees.  There were 10 Very Hot Days with daily maximum temperatures of 33.0 degrees or above in the month, breaking the record for September in 1963 and 1969.  The mean minimum temperature of 26.9 degrees was also the highest for September since record began.  The total rainfall of 486.3 millimetres in the month was about 69 percent above the normal figure of 287.5 millimetres.  The accumulated rainfall since 1 January was 2027.3 millimetres, about 6 percent below the normal figure of 2161.2 millimetres for the same period.

    Under the influence of a ridge of high pressure over southern China, the weather in Hong Kong was fine and very hot for the first eight days of the month.  Affected by an easterly airstream, the weather became cloudy with showers on 9 September.  Meanwhile, an area of low pressure over the northern part of the South China Sea intensified into a tropical depression and tracked west-northwestwards.  It further intensified into a tropical storm named Mujigae on 10 September.  Locally, winds strengthened gradually and there were some scattered squally showers on that day.  Affected by the outer rainbands associated with Mujigae, the weather remained cloudy with occasional rain on 11 September.  The rain eased off while Mujigae made landfall over northern Vietnam and weakened on 12 September.

    A tropical depression over the western North Pacific entered the northern part of the South China Sea on 13 September.  It intensified into a tropical storm that evening and was named Koppu.  The outer rainbands of Koppu brought a few thundery showers to Hong Kong in that evening.  Koppu intensified into a typhoon on 14 September and the weather became cloudy with squally showers.  Local winds also strengthened gradually towards the evening.  Gale force southeasterly winds with maximum gusts up to 151 kilometres per hour were recorded at the Cheung Chau Beach in the late evening of 14 September.  Koppu made landfall over the western coast of Guangdong in the morning of 15 September and weakened into a tropical storm in the afternoon.  The rainbands associated with Koppu brought heavy squally showers to the territory on that day.  With Koppu moving further inland and dissipating, the showers eased off gradually on 16 September.

    A ridge of high pressure brought generally fine weather to Hong Kong from 17 to 20 September.  A cold front formed over the northern part of southern China in the morning of 21 September and crossed the coastal areas of Guangdong in that evening, bringing a few thundery showers to Hong Kong.  The northeast monsoon behind the cold front brought generally fine weather condition from 22 to 27 September.  In the mean time, Tropical Storm Ketsana entered the South China Sea on 26 September and intensified into a severe tropical storm on the next day.  It gained typhoon intensity while crossing the northern part of the South China Sea on 28 September.  Under the combined effect of Ketsana and the northeast monsoon, local weather was windy with rain on 28 and 29 September.  Ketsana made landfall over the coast of central Vietnam in the afternoon of 29 September and weakened gradually afterwards.  The weather remained rainy for the last day of the month.

    Nine tropical cyclones occurred in the western North Pacific and the South China Sea in the month.  
    
    Details of issuance and cancellation of various warnings/signals in the month are summarized in Tables 1.1 to 1.8.   Monthly meteorological figures and departures from normal for September are tabulated in Table 2.

The visibility readings at the Hong Kong International Airport are based on hourly observations by professional meteorological observers in 2004 and before, and average readings over the 10-minute period before the clock hour of the visibility meter near the middle of the south runway from 2005 onwards. The change of the data source in 2005 is an improvement of the visibility assessment using instrumented observations following the international trend.
Before 10 October 2007, the number of hours of reduced visibility at the Hong Kong International Airport in 2005 and thereafter displayed in this web page was based on hourly visibility observations by professional meteorological observers. Since 10 October 2007, the data have been revised using the average visibility readings over the 10-minute period before the clock hour, as recorded by the visibility meter near the middle of the south runway.