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The Weather of May 2009

    May 2009 was drier and sunnier than usual. The total rainfall of 245.2 millimetres in the month, which was mainly recorded in the last 10 days, was about 26 percent below the normal figure of 329.5 millimetres. The accumulated rainfall since 1 January was 475.7 millimetres, about 29 percent below the normal figure of 666.6 millimetres for the same period. The total bright sunshine duration was 169.8 hours, about 23 percent above normal. The mean temperature was 25.5 degrees, 0.3 degrees below the normal figure of 25.8 degrees.

    Under the influence of a ridge of high pressure over southeastern China, the weather in Hong Kong was fine and dry on the first three days of the month. A cloud band developed over southern China, moved southwards to the coastal areas and brought a few light rain patches to Hong Kong on 4 May. With dissipation of the cloud band, local weather turned fine on 5 May. Fine and dry weather persisted for the ensuing four days.

    Another cloud band moved from west to east across southern China and brought a few light rain patches to Hong Kong on 10 May. After the passage of the cloud band, the weather turned fine again on 11 May. It was sunny and hot for the following two days. Affected by an easterly airstream, local weather turned mainly cloudy with a few rain patches from 14 to 16 May.

    The easterly airstream was replaced by a southwesterly airstream on 17 May. Local weather became sunny and hot apart from a few showers from 17 to 19 May. A trough of low pressure developed over the south China coastal areas and brought cloudy weather and a few thundery showers to the territory for the next three days.

    Affected by an area of low pressure near Hainan, local weather became more unsettled with occasional heavy downpour and squally thunderstorms from 23 to 25 May. The rain eased off gradually with some bright periods on 26 May. A fresh to strong easterly airstream associated with a ridge of high pressure over southeastern China brought cloudy and rainy weather to the territory for the following three days. Under the influence of a dry continental airstream, it was dry and mainly fine for the last two days of the month.

    Two tropical cyclones occurred in the western North Pacific and the South China Sea 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 May are tabulated in Table 2.


 

Warnings and Signals issued in May 2009

Table 1.1   Strong Monsoon Signal
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
23 / 5 0630 23 / 5 1945
24 / 5 0630 24 / 5 1530
27 / 5 1715 28 / 5 0845


Table 1.2   Rainstorm Warning Signals
Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Amber 21 / 5 0550 21 / 5 0830
Amber 23 / 5 1645 23 / 5 1900
Amber 24 / 5 1305 24 / 5 1500


Table 1.3   Thunderstorm Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
19 / 5 0215 19 / 5 0615
20 / 5 1745 20 / 5 2045
21 / 5 0155 21 / 5 0830
21 / 5 1025 21 / 5 1230
21 / 5 1340 21 / 5 1500
21 / 5 1550 21 / 5 1800
23 / 5 0050 23 / 5 0930
23 / 5 1145 24 / 5 0145
24 / 5 0705 24 / 5 1730
25 / 5 0615 25 / 5 0715
25 / 5 1120 25 / 5 1500
26 / 5 0130 26 / 5 0400
27 / 5 1410 27 / 5 1515


Table 1.4   Fire Danger Warnings
Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Red 7 / 5 0600 8 / 5 0600
Red 30 / 5 1100 31 / 5 0600


Table 2   Figures and Departures from Normal - May 2009
Meteorological Element Figure of the Month Departure from Normal*
Mean Daily Maximum Air Temperature 28.8 degrees C 0.4 degree above normal
Mean Air Temperature 25.5 degrees C 0.3 degree below normal
Mean Daily Minimum Air Temperature 23.5 degrees C 0.4 degree below normal
Mean Dew Point Temperature 21.2 degrees C 1.5 degrees below normal
Mean Relative Humidity 78 % 6 % below normal
Mean Cloud Amount 70 % 7 % below normal
Total Rainfall 245.2 mm 84.3 mm below normal
Number of hours of Reduced VisibilityΔ 17 hours 40.8 hours below normal§
Total Bright Sunshine Duration 169.8 hours 31.2 hours above normal
Mean Daily Global Solar Radiation 17.00 Megajoule / square metre 2.65 Megajoule above normal
Total Evaporation 124.5 mm 6.1 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
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