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  • The Weather of April 2018

  • 3 May 2018

    April 2018 was warmer than usual with a mean temperature of 23.6 degrees, 1.0 degree above the normal of 22.6 degrees. The month was also much drier than usual with only 28.1 millimetres of rainfall recorded in the month, about one-sixth of the normal of 174.7 millimetres and the eighth lowest on record for April. The accumulated rainfall recorded in the first four months of the year was 117.5 millimetres, a deficit of 65 percent compared to the normal of 336.1 millimetres for the same period.

    Under the influence of a ridge of high pressure, the weather in Hong Kong was generally fine and warm on the first five days of the month. A cold front moved across the coast of Guangdong on 6 April. Local weather turned cloudy with a few showers and became significantly cooler when winds strengthened from the north in that afternoon. The weather remained generally cloudy on 7 April with temperature at the Hong Kong Observatory dropping to a minimum of 16.1 degrees in the morning, the lowest of the month. Under the influence of the dry northeast monsoon, it was fine and dry in Hong Kong on 8 April.

    With the moderation of the northeast monsoon and the onset of a southerly airstream, the temperatures rose gradually on 9 - 13 April. With plenty of sunshine, the temperature at the Hong Kong Observatory reached a maximum of 30.6 degrees on 13 April, the highest of the month.

    A cold front moved across the coast of Guangdong on the night of 14 April and the associated northeast monsoon brought cooler weather to Hong Kong in next two days. Under the influence of an upper-air disturbance, there were also showers and squally thunderstorms on 15 April with more than 10 millimetres of rainfall recorded generally over the territory.

    With a broad area of cloud band covering the coastal areas of Guangdong, it was mainly cloudy with a mixture of isolated showers and sunny intervals in Hong Kong on 17 – 21 April. Under the influence of a southerly airstream, apart from coastal fog on the morning of 23 April, the weather was hot with sunny periods on 22 - 23 April.

    A trough of low pressure moved across the coast of Guangdong on 24 April. The weather in Hong Kong turned cloudy with showers and a few thunderstorms during the day. More than 20 millimetres of rainfall were recorded over Hong Kong Island, Tuen Mun and Lantau Island. Under the influence of a broad cloud band over the south China coast, local weather remained cloudy with isolated showers on 25 – 27 April.

    With the onset of an anticyclone aloft, the weather gradually became mainly fine and hot towards the end of the month apart from isolated showers.

     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 April are tabulated in Table 2.
 

Warnings and Signals issued in April 2018

Table 1.1   Strong Monsoon Signal
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
6 / 4 1250 7 / 4 1445


Table 1.2   Thunderstorm Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
15 / 4 0910 15 / 4 1300
24 / 4 0915 24 / 4 1300


Table 1.3   Fire Danger Warnings
Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Yellow 1 / 4 0600 2 / 4 2100
Yellow 5 / 4 0600 5 / 4 1930
Red 7 / 4 0930 8 / 4 2045


Table 2   Figures and Departures from Normal - April 2018
Meteorological Element Figure of the Month Departure from Normal*
Mean Daily Maximum Air Temperature 26.9 degrees C 1.9 degrees above normal
Mean Air Temperature 23.6 degrees C 1.0 degree above normal
Mean Daily Minimum Air Temperature 21.7 degrees C 0.9 degree above normal
Mean Dew Point Temperature 19.4 degrees C normal
Mean Relative Humidity 78 % 5 % below normal
Mean Cloud Amount 71 % 10 % below normal
Total Rainfall 28.1 mm 146.6 mm below normal
Number of hours of Reduced VisibilityΔ 26 hours 57.1 hours below normal§
Total Bright Sunshine Duration 143.5 hours 41.8 hours above normal
Mean Daily Global Solar Radiation 14.71 Megajoule / square metre 3.11 Megajoule above normal
Total Evaporation 98.2 mm 14.4 mm above 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 April 2018

The percentile map of mean temperature of April 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