The Weather of May 2010

     The mean temperature for May 2010 was 25.6 degrees, close to the normal figure of 25.8 degrees.  The rainfall over the territory was highly uneven.  The total rainfall recorded at the Hong Kong Observatory Headquarters in the month was only 176.6 millimetres, about 54% of the normal figure of 329.5 millimetres, whereas over 400 millimetres and 300 millimetres of rainfall were recorded at the eastern part of the New Territories and western Lantau respectively.  The accumulated rainfall since 1 January was 410.7 millimetres, about 38 percent below the normal figure of 666.6 millimetres for the same period.

     Under the influence of a ridge of high pressure, the weather was generally fine for the first three days of the month.  An easterly airstream brought mist and light rain patches to the territory for the next two days.  Affected by a trough of low pressure over southern China, local weather became cloudy with showers on 6 May.  With the trough approaching the coast of Guangdong, local weather deteriorated further with heavy rain and squally thunderstorms on the morning of 7 May.   Dominated by a humid maritime airstream over the coastal areas of Guangdong, it was mainly cloudy with coastal fog on 8 May.
 
     A trough of low pressure formed over southern China on 8 May.  It moved southwards and brought some showers to the territory the next day.  With the passage of the trough of low pressure across the coast of Guangdong, it was rainy and thundery on 10 May. Although there were still some showers, local weather improved with some sunny intervals from 11 to 13 May with the establishment of a weak ridge of high pressure over the coastal areas of southeastern China.  Affected by a humid easterly airstream, there were a few light rain and fog patches on 14 May.  Another trough of low pressure over southern China brought some thundery showers to Hong Kong the next day.  Affected by an easterly airstream, the weather became mainly cloudy with a few rain patches and coastal mist on 16 and 17 May.

     A ridge of high pressure over the south China coastal areas brought generally fine and hot weather to the territory on 18 May.  Under the influence of a trough of low pressure, it turned rainy with a few squally thunderstorms for the ensuing two days.

     A moist maritime airstream affected the coastal areas of Guangdong and brought hot weather to Hong Kong on 21 May.  The temperature at the Hong Kong Observatory rose to a maximum of 30.7 degrees on that day, the highest of the month.  Local weather became cloudy with some showers and thunderstorms on 22 and 23 May as an active trough of low pressure across the south China coast on the morning of 23 May.  After its passage, a dry continental airstream brought fine and dry conditions to the territory for the next three days. 

     Dominated by an easterly airstream over the south China coastal areas, local weather turned mainly cloudy with a few rain patches on 27 May.  There were some sunny periods on 28 May.  A trough of low pressure over the south China coastal areas brought an episode of thundery showers to Hong Kong on the evening of 29 May.  Local weather remained showery with squally thunderstorms the next day.  Over 100 millimetres of rainfall was recorded at Sha Tin and Sai Kung in that afternoon.  There were still a few rain patches on the last day of the month. 

Figure 1.  Distribution of rainfall in Hong Kong for May 2010.

     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 1.3.  Monthly meteorological figures and departures from normal for May are tabulated in Table 2.