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  • The Weather of January 2018

  • 2 February 2018

    Despite some mild interludes, January 2018 was dominated by the northeast monsoon with two episodes of cold weather during the first half and towards the end of the month. Overall, the month was slightly cooler than usual with a mean temperature of 16.1 degrees, 0.2 degree below the normal figure of 16.3 degrees. It also had more rain than usual with 62.2 millimetres of rainfall recorded in the month, more than double the normal figure of 24.7 millimetres.

    Under the influence of an easterly airstream, the weather in Hong Kong was mainly cloudy with sunny periods on the first two days of the month. After a generally fine and mild day on 3 January, a broad band of clouds moved in from the northern part of the South China Sea, bringing cloudy weather and light rain patches to the territory over the next couple of days. The weather turned windy and cooler and the rain became heavier on 6 – 7 January as a strong easterly airstream affected the south China coastal areas.

    A cold front over southern China moved across the coastal areas of Guangdong on 8 January, bringing gloomy conditions and more rain to Hong Kong. Under the strengthening northerly winds, the weather became much colder and temperature at the Hong Kong Observatory fell to around 8 degrees on the morning of 9 January. As a continental air mass spread towards the south China coast, the clouds thinned out and the weather turned brighter and drier on 10 January. With fine and sunny weather settling in, dry conditions persisted for another couple of days as daytime relative humidity fell below 40 percent. Meanwhile, another northerly replenishment of the winter monsoon helped to maintain the cold weather in Hong Kong till 14 January.

    With fine weather persisting and the northeast monsoon going into a lull, temperature rose significantly, culminating in a warm and hazy day on 17 January. Temperature at the Hong Kong Observatory climbed to the month’s highest of 25.5 degrees that day. Apart from a cloudy interlude on 19 January with rain patches in the morning, generally fine and mild weather persisted for another four days with the occurrence of mist on 21 January and haze on 22 January under light wind conditions.

    Even though the weather remained mostly fine on 24 - 25 January, conditions became windy and appreciably cooler as easterly winds strengthened along the south China coast. The weather turned increasingly cloudy and light rain patches affected the territory on 26 -27 January. Following the passage of a cold front on 28 January, northerly winds set in and an intense winter monsoon brought a spell of cold weather with overcast skies and rain patches that persisted till the end of the month. Rain was particularly heavy during the day on 31 January and temperature at the Hong Kong Observatory dropped to the month’s lowest of 7.8 degrees before midnight.

     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 January are tabulated in Table 2.
 

Warnings and Signals issued in January 2018

Table 1.1   Strong Monsoon Signal
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
6 / 1 0200 7 / 1 1145
8 / 1 1200 9 / 1 1340
24 / 1 0745 25 / 1 1600
31 / 1 1600 1 / 2 0745


Table 1.2   Fire Danger Warnings
Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Yellow 1 / 1 0600 1 / 1 1800
Red 10 / 1 0600 13 / 1 0600
Yellow 13 / 1 0600 13 / 1 1940
Yellow 14 / 1 0600 14 / 1 1915
Yellow 21 / 1 1145 21 / 1 1900


Table 1.3   Cold Weather Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
8 / 1 0600 14 / 1 1100
28 / 1 1200 8 / 2 1115


Table 2   Figures and Departures from Normal - January 2018
Meteorological Element Figure of the Month Departure from Normal*
Mean Daily Maximum Air Temperature 18.5 degrees C 0.1 degree below normal
Mean Air Temperature 16.1 degrees C 0.2 degree below normal
Mean Daily Minimum Air Temperature 14.1 degrees C 0.4 degree below normal
Mean Dew Point Temperature 11.7 degrees C 0.3 degree above normal
Mean Relative Humidity 77 % 3 % above normal
Mean Cloud Amount 69 % 8 % above normal
Total Rainfall 62.2 mm 37.5 mm above normal
Number of hours of Reduced VisibilityΔ 112 hours 120.2 hours below normal§
Total Bright Sunshine Duration 136.1 hours 6.9 hours below normal
Mean Daily Global Solar Radiation 10.38 Megajoule / square metre 0.21 Megajoule above normal
Total Evaporation 62.5 mm 8.8 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.
Before 10 October 2007, the number of hours of reduced visibility at the Hong Kong International Airport in 2005 and thereafter displayed in this web page was based on hourly visibility observations by professional meteorological observers. Since 10 October 2007, the data have been revised using the average visibility readings over the 10-minute period before the clock hour, as recorded by the visibility meter near the middle of the south runway.


  *   Departure from 1981 - 2010 climatological normal, except for number of hours of reduced visibility

  §   Departure from mean value between 1997 and 2017

daily values of selected meteorological elements for HK for January 2018

The percentile map of mean temperature of January 2018

  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