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Due to the lack of passage of tropical cyclones and the prevalence of continental air masses part of the month, September 2012 in Hong Kong was drier than usual. The monthly total rainfall was 213.0 millimetres, about 35 percent below the normal figure of 327.6 millimetres. The accumulated rainfall since 1 January was 1758.4 millimetres, a deficit of 21 percent comparing to the normal figure of 2233.1 millimetres for the same period. The month was also warmer and sunnier than usual. The mean temperature of the month was 28.0 degrees, 0.3 degrees above the normal figure of 27.7 degrees. The monthly total duration of bright sunshine of 187.4 hours was about 9 percent above normal.
Under the combined effect of a fresh easterly airstream and the broad trough of low pressure over the northern part of the South China Sea, there were sunny periods interspersed with showers in Hong Kong for the first seven days in the month. Winds turned light on 8 September and there were heat showers mainly in the New Territories in the afternoon.
With the dominance of the anticyclone aloft over southern China, local weather became mainly fine from 9 to 12 September. The abundant sunshine brought very hot weather on 11 and 12 September. The temperatures at the Observatory rose to a maximum of 33.6 degrees on 11 September, the highest of the month.
A cold front developed over southern China on 13 September and moved across the coastal areas of Guangdong that evening. Local winds became moderate to fresh northerly that night. The northeast monsoon behind the cold front maintained dry and slightly cooler weather in the territory from 14 to 18 September. As the winds turned easterly and the air moistened up, the weather in Hong Kong was cloudy with a few rain patches for the ensuing three days.
A trough of low pressure developed over Guangdong and brought warmer and unstable weather with isolated thundery showers to the territory on 22 and 23 September. With the arrival of a surge of cooler easterlies, local weather deteriorated further with heavy thundery showers on 24 September. The rain was particularly heavy that night, with over 100 millimetres of rainfall recorded in Sai Kung and the urban areas. With the prevalence of the cooler easterlies over the Pearl River Estuary, the rain over the territory eased off gradually the next day. It was mainly cloudy with isolated showers on 26 September.
Affected by a dry continental airstream, local weather became generally fine and dry for the rest of the month.
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