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The Weather of May 2008

    May 2008 was cooler and drier than usual. The monthly mean temperature of 25.3 degrees was 0.5 degrees below normal. The total rainfall of the month was 191.9 millimetres, about 137.6 millimetres below normal. The accumulated rainfall since 1 January was 564.9 millimetres, about 15 percent below the normal figure of 666.6 millimetres for the same period.

    Under the influence of a humid easterly airstream, the weather was cloudy with mist and a few showers for the first four days of May. A trough of low pressure over south China reached the coast in the evening of 5 May and brought thundery showers to the territory. It remained cloudy with a few rain patches for the next two days.

    Another humid maritime airstream brought coastal fog to Hong Kong on the morning of 8 May. It was sunny and hot during the day on 9 May. A cold front crossed the south China coast that evening. The northeast monsoon behind the cold front brought cooler weather to the territory for the next two days.

    A dry northeast monsoon dominated over south China from 12 to 17 May and brought fine and dry weather to Hong Kong. The weather turned cloudy with rain for the ensuing five days when a trough of low pressure affected the south China coastal areas. Under the influence of a southwesterly airstream, the weather was hot with sunny periods on 23 May. The maximum temperature at the Hong Kong Observatory rose to 31.4 degrees, the highest of the month. Local weather became progressively cloudier and more showery from 24 to 28 May when the southwesterly airstream became more active.

    A trough of low pressure lingering over the south China coastal areas brought rainy and thundery weather to the territory for the rest of the month.

    Four 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 2008


Table 1.1   Strong Monsoon Signal

Beginning Time Ending Time
Day/Month HKT Day/Month HKT
5 / 5 2130 6 / 5 0600
20 / 5 0915 21 / 5 0005


Table 1.2   Rainstorm Warning Signals

Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Amber 29 / 5 0905 29 / 5 1140


Table 1.3   Thunderstorm Warning

Beginning Time Ending Time
Day/Month HKT Day/Month HKT
5 / 5 1410 5 / 5 1515
5 / 5 2005 5 / 5 2400
9 / 5 2145 9 / 5 2325
24 / 5 0255 24 / 5 0645
25 / 5 0915 25 / 5 1300
26 / 5 0937 26 / 5 1100
26 / 5 1410 26 / 5 1645
26 / 5 1730 26 / 5 1900
28 / 5 1005 28 / 5 1300
29 / 5 0720 29 / 5 1300
30 / 5 0315 30 / 5 0730
30 / 5 0835 30 / 5 1900
31 / 5 1145 31 / 5 1345


Table 1.4   Fire Danger Warnings

Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Red 13 / 5 0600 14 / 5 0600


Table 2   Figures and Departures from Normal - May 2008

Meteorological Element Figure of the Month Departure from Normal*
Mean Daily Maximum Air Temperature 28.1 degrees C 0.3 degree below normal
Mean Air Temperature 25.3 degrees C 0.5 degree below normal
Mean Daily Minimum Air Temperature 23.3 degrees C 0.6 degree below normal
Mean Dew Point Temperature 22.1 degrees C 0.6 degree below normal
Mean Relative Humidity 83 % 1 % below normal
Mean Cloud Amount 75 % 2 % below normal
Total Rainfall 191.9 mm 137.6 mm below normal
Number of hours of Reduced VisibilityΔ 97 hours 42.7 hours above normal§
Total Bright Sunshine Duration 124.8 hours 13.8 hours below normal
Mean Daily Global Solar Radiation 14.05 Megajoule / square metre 0.3 Megajoule below normal
Total Evaporation 103.4 mm 15.0 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 1971 - 2000 climatological normal, except for number of hours of reduced visibility

  §   Departure from mean value between 1997 and 2007
daily values of selected meteorological elements for HK for May 2008

Extract of Meteorological Observations in Hong Kong for May 2008