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  • The Weather of April 2017

  • 4 May 2017

    April 2017 was warmer than usual with a mean temperature of 23.3 degrees, 0.7 degree above the normal of 22.6 degrees. The month was also drier than usual in terms of rainfall with 58.8 millimetres recorded in the month, only about one-third of the normal of 174.7 millimetres. The accumulated rainfall recorded in the first four months of the year was 134.5 millimetres, a deficit of 60 percent compared to the normal of 336.1 millimetres for the same period.

    Under the influence of a continental airstream brought by the northeast monsoon, the weather in Hong Kong was sunny and dry on the first four days of the month. The month started off with a cool morning on 1 April and the minimum temperature of 15.5 degrees recorded at the Hong Kong Observatory was the lowest of the month. With the moderation of the northeast monsoon, the weather turned cloudier on 5 April with a mixture of sunny periods, coastal fog patches and isolated showers over the next five days. Temperatures climbed steadily during the period, leading to a rather warm night on 9 April under the influence of a southerly airstream.

    Upon the passage of a trough of low pressure, widespread rain affected the territory on 11 April. Rain was heavy at times on 12 April, with more than 30 millimetres falling in some places, before easing off the next day. Meanwhile, the weather turned cooler as well, as a replenishment of the northeast monsoon reached the south China coast. Following the moderation of the northeast monsoon, the weather was mainly fine from 14 to 19 April apart from some mist and haze. With plenty of sunshine, daytime conditions became rather hot with temperatures at the Hong Kong Observatory rising to a maximum of 30.7 degrees on 18 April, the highest of the month.

    The setting in of unsettled weather associated with a southerly airstream on 20 April marked the beginning of a showery spell that lasted for more than a week. Thundery showers brought over 3000 lightning strokes to the territory on 21 April ahead of the passage of a cold front later that day. The weather then became appreciably cooler on 22 and 23 April. A freshening of easterly winds on 24 April was followed by more showers and squally thunderstorms the next day. After a lull in the northeast monsoon and a relatively warm day on 26 April, the passage of a trough of low pressure brought another replenishment of the northeast monsoon to the coast of Guangdong and cooler weather to Hong Kong over the next couple of days.

    With the northeast monsoon subsiding, the weather was generally fine and rather warm on the last two days of the month.

    Two tropical cyclones 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 1.4. Monthly meteorological figures and departures from normal for April are tabulated in Table 2.

Warnings and Signals issued in April 2017


Table 1.1   Strong Monsoon Signal
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
31 / 3 1615 1 / 4 0600


Table 1.2   Rainstorm Warning Signals
Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Amber 21 / 4 1520 21 / 4 1700


Table 1.3   Thunderstorm Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
12 / 4 1450 12 / 4 1800
20 / 4 1250 20 / 4 1535
21 / 4 1500 21 / 4 1900
22 / 4 1110 22 / 4 1330
25 / 4 1510 25 / 4 2000
26 / 4 1240 26 / 4 1430


Table 1.4   Fire Danger Warnings
Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Yellow 1 / 4 0600 1 / 4 1930
Yellow 2 / 4 0600 2 / 4 1030
Red 2 / 4 1030 2 / 4 2315
Yellow 4 / 4 0600 4 / 4 1800
Yellow 9 / 4 1145 9 / 4 1800
Yellow 14 / 4 1100 14 / 4 1800
Yellow 16 / 4 1215 16 / 4 1800
Yellow 17 / 4 1100 17 / 4 1800
Yellow 29 / 4 0800 29 / 4 1800
Yellow 30 / 4 0600 30 / 4 1800


Table 2   Figures and Departures from Normal - April 2017
Meteorological Element Figure of the Month Departure from Normal*
  Mean Daily Maximum Air Temperature 26.2 degrees C 1.2 degree above normal
  Mean Air Temperature 23.3 degrees C 0.7 degree above normal
  Mean Daily Minimum Air
Temperature
21.2 degrees C 0.4 degree above normal
  Mean Dew Point Temperature 19.8 degrees C 0.4 degree above normal
  Mean Relative Humidity 81 % 2 % below normal
  Mean Cloud Amount 69 % 12 % below normal
  Total Rainfall 58.8 mm 115.9 mm below normal
  Number of hours of Reduced VisibilityΔ 8 hours 78.9 hours below normal§
  Total Bright Sunshine Duration 130.3 hours 28.6 hours above normal
  Mean Daily Global Solar Radiation 14.10 Megajoule / square
metre
2.50 Megajoule above normal
  Total Evaporation 84.1 mm 0.3 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 2016
daily values of selected meteorological elements for HK and the percentile map of mean temperature for April 2017


The percentile map of mean temperature of April 2017
  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


Rainfall Map For April 2017 (isohyets are in millimetres)

Extract of Meteorological Observations in Hong Kong for April 2017