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Despite several cold episodes in the month, December's mean temperature was 18.1 degrees, 0.3 degrees above normal. This was mainly due to high daytime temperatures and generally fine and dry weather brought by the dry winter monsoon. While there were seven cold days (daily minimum temperature at 12 degrees or below) and five days with frost reported, there were 22 days with maximum temperatures shooting up to 20 degrees or higher in the month. The lowest temperature recorded on December 17 was 5.8 degrees, level with the record low for December registered in 1999; and the highest temperature, recorded on December 6, was 26.8 degrees - the highest for December since 1968.
The total rainfall in December 2010 was 18.4 millimetres, only about 53 percent of normal. The annual rainfall for 2010 was 2371.7 millimetres, close to the normal figure of 2382.7 millimetres.
A ridge of high pressure covering southern China brought generally fine and dry weather to Hong Kong for the first six days of the month. With abundant sunshine, the mercury on December 6 rose to 26.8 degrees, the highest of the month.
Following the passage of a cold front over south China coastal areas and the arrival of the winter monsoon on December 7 morning, the weather became appreciably cooler and drier. It remained cool and very dry on December 8 and 9 with a minimum relative humidity of about 20%. With the moderation of the winter monsoon, local temperatures started to rise on December 10. The weather turned cloudy on December 11 with light rain patches setting in on December 12. A humid easterly airstream brought mist and fog to Hong Kong in the following two days. Locally, visibility in the harbour fell below 1,000 metres due to fog on the morning of December 14.
In northern China, cold air associated with an intense winter monsoon spread southward across the country and reached Guangdong on December 15. Local temperatures fell sharply with rain during the overnight period from December 15 to 16, bringing very cold weather to Hong Kong. The minimum temperature on the morning of December 17 dropped to 5.8 degrees, the lowest of the year. The dry continental airstream cleared the clouds and the weather became fine and dry on December 17. With the moderation of the winter monsoon, temperatures began to rise on December 18 but inland temperatures remained on the low side due to a radiation cooling effect with the mercury at Ta Kwu Ling falling to 0.2 degrees, equalling the record low for December registered in 1999.
A moderate northeast monsoon prevailed from December 19 to 23, bringing mainly fine and dry weather with haze. Daytime temperatures rose above 20 degrees during this period.
The weather turned cloudy with sunny intervals on December 24 and some haze remained. A cold front over southern China moved southward and crossed the coast on December 25, bringing rain and cold weather to Hong Kong. The mercury fell to 9.8 degrees on the morning of December 26 and the sky cleared up under the influence of the dry winter monsoon, which maintained fine and dry weather for the ensuing four days. It remained cold with frost in the morning on December 27 and 28 while temperatures rose in general until December 30. The arrival of a replenishment of the winter monsoon brought cooler and very dry weather to Hong Kong on the last day of the month.
One tropical cyclone occurred over the western North Pacific and the South China Sea in the month.
Details of the issuance and cancellation of various warnings/signals in the month are summarised in Table 1. Monthly meteorological figures and departures from December norms are tabulated in Table 2.
Ends/Tuesday, January 4, 2011
Issued at HKT 18:13
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