The Weather of December 2013

    Under the influence of cold spells brought by the winter monsoon during the second half of the month, the weather of December 2013 was significantly colder than usual.  The monthly mean temperature of 16.1 degrees was 1.8 degrees below the normal figure of 17.9 degrees, the lowest for December since 1975.  The active winter monsoon also maintained generally dry conditions for most parts of the month, and yet the month turned out to be much wetter than usual in terms of rainfall due to the rainy episode between 14 and 17 December.  The total rainfall of the month was 88.3 millimetres, more than three times the normal figure of 26.8 millimetres and the tenth highest for December on record.  The annual total rainfall of 2013 was 2847.3 millimetres, about 19 percent above the normal figure of 2398.5 millimetres.  

    Dominated by the northeast monsoon, fine and very dry weather persisted over Hong Kong for the first seven days of the month.  Affected by cloud bands over the northern part of the South China Sea and the coast of southeastern China, local weather turned cloudier with rain patches on 8 December.  Ahead of the arrival of a replenishment of the northeast monsoon, temperatures reached a warm 24.9 degrees on 9 December, the highest of the month.  The northeast monsoon brought a dry continental airstream to Hong Kong and the weather was mainly fine with haze on 9 and 10 December. 

    Clouds returned to the south China coastal areas on 11 December and brought a few light rain patches to the territory on 12 and 13 December.  With the approach of an upper level disturbance, the rain became heavier and more frequent in Hong Kong over the next four days.  Coupled with the arrival of an intense winter monsoon, temperatures fell significantly and it was gloomy, rainy and cold on 16 and 17 December.  As the rainbands associated with the upper level disturbance moved away on 18 December, local weather turned mainly fine and dry.  Temperatures at the Hong Kong Observatory fell to a minimum of 9.2 degrees that morning, the lowest of the month, and the weather remained cold in the next two days. 

    After a cool and cloudier day on 21 December, further replenishment of the winter monsoon kept the weather generally cold, fine and very dry till the end of the month.  Under clear sky and light wind conditions, nocturnal cooling caused temperatures to drop below 3 degrees at Ta Kwu Ling, where frost was reported on the mornings of 28 - 30 December.

Plants coated with frost in Ta Kwu Ling on the morning of 29 December 2013.
(Photo courtesy of Mr Irwin Wong of Weather diary)

    There was no tropical cyclone over the South China Sea and the western North Pacific in the month.

    Details of issuance and cancellation of various warnings/signals in the month are summarized in Tables 1.1 to 1.4.  Monthly meteorological figures and departures from normal for December are tabulated in Table 2.