The Weather of December 2015

    With a relatively humid air mass affecting the territory for most part of the month, the weather of December 2015 was gloomier and wetter than usual. The total duration of sunshine recorded in the month was 75.9 hours, only about 44 percent of the normal figure of 172.2 hours. Two rainy episodes on 5 and 9 December mostly contributed to the monthly rainfall of 64.3 millimetres, more than double the normal figure of 26.8 millimetres. However, the annual rainfall of 1874.5 millimetres was still about 22 percent below the normal of 2398.5 millimetres. December 2015 was also warmer than usual with a monthly mean temperature of 18.6 degrees, 0.7 degrees above the normal figure of 17.9 degrees.

    December 2015 began with mainly fine and warm weather for the first two days of the month. A cold front formed over the northern part of Guangdong and moved across the coast of Guangdong on the night of 2 December. Affected by the northeast monsoon behind the cold front, local weather turned cloudy with light rain patches over the next couple of days. The northeast monsoon strengthened further and brought windy, rainy and even cooler weather on 5 December. Under the prevailing northeast monsoon, the weather remained cool with a few rain patches over the next three days.

    Under the influence of an upper-air disturbance, it was overcast and rainy in Hong Kong on 9 December with more than 40 millimetres of rainfall generally recorded over the territory. As the upper-air disturbance moved eastwards away from Guangdong, local weather improved gradually with sunny periods over the next couple of days. Affected by a fresh easterly airstream, the clouds returned with light rain patches on 12 December and persisted for the next two days.

    The strengthening of the winter monsoon over the south China coastal areas brought significantly cooler weather to Hong Kong on 15 December. Under the influence of a continental air mass brought by a strengthening northerly airstream, temperatures plunged further and local weather turned fine on 16 December with cold mornings over the next two days. Temperatures at the Observatory fell to a minimum of 11.3 degrees on the morning of 18 December, the lowest of the month.

    With the moderation of the winter monsoon, local temperature recovered gradually and the weather turned increasingly mild and humid with light rain patches during the period leading up to an unseasonably warm Christmas Eve. Mist and fog patches were reported in Hong Kong on 22 – 24 December, with visibility at Waglan Island once dropping below 500 metres on the morning of 24 December.

    Meanwhile, a cold front formed over the northern part of southern China and moved across the coastal areas of Guangdong in the early hours of Christmas Day. Local weather became appreciably cooler with a few rain patches that day. The prevailing northeast monsoon then maintained generally cloudy and cool weather with light rain patches over the next three days, with more sunny periods appearing towards the end of the month.