Skip Content

The Weather of April 2009

    April 2009 was drier and cooler than usual. The mean temperature was 22.0 degrees, 0.5 degrees below the normal figure of 22.5 degrees. The total rainfall in the month was 108.7 millimetres, about 42 percent below the normal figure of 188.5 millimetres. The accumulated rainfall since 1 January was 230.5 millimetres, about 32 percent below the normal figure of 337.1 millimetres for the same period.

    Under the influence of a dry northeast monsoon, it was dry with sunny periods for the first two days of the month. With winds strengthening from the east, local weather became cloudy with some rain patches on 3 April. With the easterly winds moderating, sunny intervals developed on 4 and 5 April.

    A cold front formed over the northern part of southern China on 4 April and moved across the south China coastal areas in the afternoon of 5 April. Locally, the weather became cooler and rainy on 6 and 7 April. Affected by a dry easterly airstream over the coastal areas of southern China, it was mainly fine for the ensuing four days.

    A trough of low pressure over the south China coastal areas brought showery weather to Hong Kong from 12 to 16 April. With the trough of low pressure dissipating, local weather turned mainly fine on 17 April. A fresh to strong easterly airstream over southern China brought windy and rainy weather to the territory in the next two days. With winds subsiding gradually, local weather became sunny and hot during the day on 20 April. The temperature at the Hong Kong Observatory rose to 30.1 degrees on that day, the highest of the month.

    Affected by another fresh to strong easterly airstream over the south China coastal areas, it was windy and rainy in Hong Kong from 21 to 23 April. With moderation of the easterly airstream, it was cloudy and misty on 24 April. An active trough of low pressure formed over central China on 24 April and crossed the coast of Guangdong the next day, bringing heavy rain and squally thunderstorms to Hong Kong on the morning of 25 April.

    Under the influence of the northeast monsoon, it was cooler and rainy in Hong Kong on 26 April. A dry easterly airstream associated with a ridge of high pressure over the southeastern coast of China brought mainly fine and dry weather to Hong Kong for the last four days of the month.

    There was no tropical cyclone 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 March are tabulated in Table 2.


 

Warnings and Signals issued in April 2009


Table 1.1   Strong Monsoon Signal

Beginning Time Ending Time
Day/Month HKT Day/Month HKT
1 / 4 2315 3 / 4 0800
18 / 4 0445 18 / 4 1130
23 / 4 0845 23 / 4 1145
28 / 4 0915 28 / 4 1145


Table 1.2   Rainstorm Warning Signals

Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Amber 16 / 4 0205 16 / 4 0325
Amber 25 / 4 1020 25 / 4 1240


Table 1.3   Thunderstorm Warning

Beginning Time Ending Time
Day/Month HKT Day/Month HKT
13 / 4 1505 13 / 4 1715
16 / 4 0040 16 / 4 0600
25 / 4 0340 25 / 4 1330


Table 1.4   Fire Danger Warnings

Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Yellow 4 / 4 0630 4 / 4 1800
Yellow 5 / 4 0600 5 / 4 1800
Red 9 / 4 0600 11 / 4 0600
Yellow 11 / 4 0600 12 / 4 1800
Red 28 / 4 0600 29 / 4 0615


Table 2   Figures and Departures from Normal - April 2009

Meteorological Element Figure of the Month Departure from Normal*
Mean Daily Maximum Air Temperature 24.8 degrees C 0.3 degree below normal
Mean Air Temperature 22.0 degrees C 0.5 degree below normal
Mean Daily Minimum Air Temperature 20.2 degrees C 0.4 degree below normal
Mean Dew Point Temperature 17.6 degrees C 1.8 degrees below normal
Mean Relative Humidity 77 % 6 % below normal
Mean Cloud Amount 79 % 1 % below normal
Total Rainfall 108.7 mm 79.8 mm below normal
Number of hours of Reduced VisibilityΔ 148 hours 52.6 hours above normal§
Total Bright Sunshine Duration 119.3 hours 17.5 hours above normal
Mean Daily Global Solar Radiation 13.63 Megajoule / square metre 1.8
Total Evaporation 100.1 mm 6.9 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 1971 - 2000 climatological normal, except for number of hours of reduced visibility

  §   Departure from mean value between 1997 and 2008
daily values of selected meteorological elements for HK for April 2009

Extract of Meteorological Observations in Hong Kong for April 2009