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The Weather of May 2007

    May 2007 was warmer and sunnier than usual. The mean temperature of 26.4 degrees was 0.5 degree above normal. The total bright sunshine duration of 183.4 hours was about 19 percent above normal. It was also slightly drier with a monthly rainfall amount of 288.3 millimetres, about 9 percent below normal. The accumulated rainfall since the beginning of the year was 477.9 millimetres, about 22 percent below normal for the same period.

    The weather in Hong Kong was generally fine under the influence of a ridge of high pressure on the first three days of the month. The weather turned cloudy with some rain on 4 and 5 May when a trough of low pressure over southern China moved cross the coastal areas.

    The weather became fine and dry on 6 and 7 May as another ridge of high pressure established over southern China. The relative humidity dropped below 40 % during the day. In the following ten days, it was generally fine and hot apart from a few brief showers on 16 and 17 May. It was also hazy on 13 and 14 May.

    The weather deteriorated later on 18 May when a trough of low pressure developed over inland Guangdong and moved across the south China coast. A squall line swept across Hong Kong from northwest to southeast that evening, bringing heavy showers and severe gusts to the territory. Peak gust over 100 kilometres per hour was recorded at Waglan Island. In Tin Shui Wai, four persons were injured when more than 20 stacked containers collapsed in gusty winds. With the tough of low pressure lingering along the south China coast, there were heavy showers and thunderstorms between 19 and 22 May. During the heavy downpour on 20 May, two landslips and 15 cases of flooding were reported.

    Sunshine returned on 23 May when the trough of low pressure weakened. Under the influence of a southerly airstream, it was hot with sunny periods in the next three days.

    The weather became unstable with heavy showers and squally thunderstorms on 27 May, but improved with some sunny periods in the following day. Under the influence of a ridge of high pressure over the coastal areas of Guangdong, it was fine, clear and hot on 29 and 30 May. Apart from a few showers in the morning, the weather stayed mainly fine and hot on the last day of the month.

    Only one tropical cyclone occurred in the western North Pacific in the month.

The radar image of a squall line moved across Hong Kong at 5:18 p.m. on 18 May 2007.

 

    Details of issuance and cancellation of various warnings/signals in the month are summarized in Tables 1.1 to 1.5.  Monthly meteorological figures and departures from normal for May are tabulated in Table 2.


 

Warnings and Signals issued in May 2007


Table 1.1   Rainstorm Warning Signals

Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Amber 19 / 5 1630 19 / 5 1830
Amber 20 / 5 0710 20 / 5 1155
Amber 27 / 5 0435 27 / 5 0535
Amber 27 / 5 1445 27 / 5 1600


Table 1.2   Landslip Warning

Beginning Time Ending Time
Day/Month HKT Day/Month HKT
20 / 5 0900 20 / 5 1430


Table 1.3   Thunderstorm Warning

Beginning Time Ending Time
Day/Month HKT Day/Month HKT
16 / 5 1935 16 / 5 2045
18 / 5 1715 18 / 5 2015
19 / 5 1250 19 / 5 2000
20 / 5 0620 20 / 5 1430
22 / 5 0440 22 / 5 1400
23 / 5 0715 23 / 5 0915
27 / 5 0120 27 / 5 0700
27 / 5 1155 27 / 5 1700
28 / 5 0300 28 / 5 0700
28 / 5 1040 28 / 5 1500
30 / 5 1250 30 / 5 1400
31 / 5 0530 31 / 5 0900


Table 1.4   Fire Danger Warnings

Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Red 2 / 5 1130 2 / 5 1800
Red 6 / 5 1100 8 / 5 0600


Table 1.5   Very Hot Weather Warning

Beginning Time Ending Time
Day/Month HKT Day/Month HKT
26 / 5 1245 26 / 5 1620
29 / 5 1330 30 / 5 1620


Table 2   Figures and Departures from Normal - May 2007

Meteorological Element Figure of the Month Departure from Normal*
Mean Daily Maximum Air Temperature 29.7 degrees C 1.0 degree above normal
Mean Air Temperature 26.4 degrees C 0.5 degree above normal
Mean Daily Minimum Air Temperature 24.3 degrees C 0.4 degree above normal
Mean Dew Point Temperature 22.4 degrees C 0.2 degree below normal
Mean Relative Humidity 80 % 3 % below normal
Mean Cloud Amount 73 % 1 % below normal
Total Rainfall 288.3 mm 28.4 mm below normal
Number of hours of Reduced VisibilityΔ 158 hours 114.1 hours above normal§
Total Bright Sunshine Duration 183.4 hours 29.6 hours above normal
Mean Daily Global Solar Radiation 16.60 Megajoule / square metre 0.48 Megajoule above normal
Total Evaporation 120.4 mm 17.3 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.


 

The calibration factors of the pyranometer at King's Park in 2006 and 2007 have been re-computed based on the results of a regional instrument comparison in Asia in 2007. The readings of total global solar radiation at King's Park in 2006 and 2007 displayed in this web page before 13 June 2008 have been revised on 13 June 2008 using the new calibration factors.

  *   Departure from 1961-1990 climatological normal, except for number of hours of reduced visibility.
  §   Departure from mean value between 1997 and 2006.
daily values of selected meteorological elements for HK for May 2007

Extract of Meteorological Observations in Hong Kong for May 2007