December was cooler and wetter than usual. The monthly mean temperature of 17.3 degrees was 0.5 degrees below normal, and the total rainfall of 50.2 millimetres in the month was about 46% above the normal figure of 34.5 millimetres.

The annual rainfall for 2009 was 2,182.3 millimetres, about 8% below the normal figure of 2,382.7 millimetres.

A dry northeast monsoon brought generally fine weather to Hong Kong for the first six days of the month. Affected by a fresh to strong easterly airstream over the coastal areas of Guangdong, it became windy and cloudy with rain on December 7 and 8.

The weather remained generally cloudy with a few rain patches for the next two days. A ridge of high pressure established over southeastern China brought sunny periods to the territory from December 10 to 14.

As a cold front developed over southern China on December 15 and crossed the coastal areas of Guangdong in the evening, local weather turned cloudy with a few rain patches. The winter monsoon behind the cold front brought cold weather to Hong Kong for the next six days.

With the weakening of the winter monsoon, temperatures started to rise on December 22. It was mainly fine and mild for the following three days. However, the light wind condition brought low visibility to parts of the territory on December 24 and 25. Under the influence of an easterly airstream over the south China coastal areas, the weather turned cloudy with mist on December 26.

A strong cold front formed over southern China on December 27 and moved southwards quickly. With the passage of the cold front over the territory that afternoon, local weather turned cold and rainy. The temperature at the Hong Kong Observatory fell from about 18 degrees to a minimum of 9.4 degrees on the morning of December 28, the lowest of the month.

The weather remained cloudy with rain and mist for the next two days. A fresh to strong easterly airstream brought generally cloudy conditions with a few rain patches to the territory for the last day of the month.

One tropical cyclone occurred over the western North Pacific and the South China Sea in the month.