The Weather of December 2015
5 January 2016
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.
One tropical cyclone occurred 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 1.3. Monthly meteorological figures and departures from normal for December are tabulated in Table 2.
Warnings and Signals issued in December 2015
| Beginning Time | Ending Time | ||
|---|---|---|---|
| Day/Month | HKT | Day/Month | HKT |
| 5 / 12 | 1020 | 6 / 12 | 1335 |
| 9 / 12 | 0115 | 9 / 12 | 2315 |
| 16 / 12 | 0140 | 17 / 12 | 1145 |
| 25 / 12 | 0725 | 25 / 12 | 1245 |
| Colour | Beginning Time | Ending Time | ||
|---|---|---|---|---|
| Day/Month | HKT | Day/Month | HKT | |
| Yellow | 6 / 12 | 1100 | 6 / 12 | 1800 |
| Red | 16 / 12 | 0600 | 19 / 12 | 0600 |
| Yellow | 19 / 12 | 0600 | 20 / 12 | 1130 |
| Yellow | 26 / 12 | 0600 | 26 / 12 | 2315 |
| Beginning Time | Ending Time | ||
|---|---|---|---|
| Day/Month | HKT | Day/Month | HKT |
| 16 / 12 | 1620 | 19 / 12 | 1045 |
| Meteorological Element | Figure of the Month | Departure from Normal* |
|---|---|---|
| Mean Daily Maximum Air Temperature | 20.3 degrees C | 0.1 degree above normal |
| Mean Air Temperature | 18.6 degrees C | 0.7 degree above normal |
| Mean Daily Minimum Air Temperature | 17.1 degrees C | 1.2 degrees above normal |
| Mean Dew Point Temperature | 14.0 degrees C | 2.1 degrees above normal |
| Mean Relative Humidity | 76 % | 7 % above normal |
| Mean Cloud Amount | 77 % | 25 % above normal |
| Total Rainfall | 64.3 mm | 37.5 mm above normal |
| Number of hours of Reduced VisibilityΔ | 101 hours | 130.8 hours below normal§ |
| Total Bright Sunshine Duration | 75.9 hours | 96.3 hours below normal |
| Mean Daily Global Solar Radiation | 7.89 Megajoule / square metre | 3.00 Megajoule below normal |
| Total Evaporation | 68.9 mm | 14.8 mm below normal |
| Remarks : | All measurements were made at the Hong Kong Observatory except sunshine,
solar radiation and evaporation which were recorded at King's Park
Meteorological Station and visibility which was observed at the Hong
Kong International Airport. |
| Δ | The visibility readings at the Hong Kong International Airport are based on hourly observations by professional meteorological observers in 2004 and before, and average readings over the 10-minute period before the clock hour of the visibility meter near the middle of the south runway from 2005 onwards. The change of the data source in 2005 is an improvement of the visibility assessment using instrumented observations following the international trend.
|
* Departure from 1981 - 2010 climatological normal, except for number of hours of reduced visibility |
|
§ Departure from mean value between 1997 and 2014 |
|
| Remarks : | Extremely high: above 95th percentile Above normal: between 75th and 95th percentile Normal: between 25th and 75th percentile Below normal: between 5th and 25th percentile Extremely low: below 5th percentile Percentile and 5-day running average values are computed based on the data from 1981 to 2010 |
