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An extremely hot and sunny May
     Under the influence of an upper-air anticyclone over the northern part of the South China Sea, Hong Kong experienced an exceptionally hot and dry May this year with a 20-day fine spell that persisted till the end of the month. There were in total 16 very hot days and six hot nights in the month, breaking the records for May.
     The monthly mean temperature of 28.3 degrees was 2.4 degrees above the normal figure and the highest recorded for May. The monthly mean minimum and maximum temperatures were 26.1 degrees and 31.7 degrees respectively, which were 2.0 degrees and 3.3 degrees above their respective normal figures, and were the highest and second highest on record for May. The total duration of bright sunshine was 236.9 hours, 96.5 hours above the normal figure of 140.4 hours and the fourth highest on record for May. The mean cloud amount of 62 per cent and the mean relative humidity of 77 per cent were the fifth and the third lowest on record respectively for May. Rainfall in the month amounted to only 57.5 millimetres, less than one-fifth of the normal of 304.7 millimetres. The accumulated rainfall recorded in the first five months of the year was 175.0 millimetres, a deficit of 73 per cent compared to the normal figure of 640.8 millimetres and the second lowest recorded for the same period.
     Apart from some isolated showers and coastal fog on May 1, the month began with generally fine weather. After a very hot day on May 3, the weather turned cloudy with some scattered showers towards the evening as winds strengthened from the east. The easterly airstream brought much cooler conditions and the month's lowest temperature of 22.1 degrees was recorded on May 4. As the easterly airstream subsided, the weather turned brighter over the next two days despite the occurrence of some showers, particularly over the eastern part of the territory on May 6.
     With a trough of low pressure over Guangdong moving towards the coastal areas, the weather in Hong Kong turned cloudy with showers and thunderstorms later on May 7. More than 50 millimetres of rain fell over Hong Kong Island and Sai Kung on the morning of May 8 as the trough swept across the territory. Cloudy and showery conditions continued for the next three days as a strengthening easterly airstream once again brought cooler weather to the south China coastal areas.
     As the easterly airstream subsided and an upper-air anticyclone became firmly established over the northern part of the South China Sea, a spell of fine weather set in on May 12 and persisted till the end of the month. The territory was mostly rain-free during the period, apart from some isolated showers on May 21, 22, 25 and 30, and showers on the mornings of May 26 and 27 as a trough of low pressure developed over southern China. With long hours of sunshine, a heat wave tightened its grip on the territory in the latter half of the month with daytime temperatures at the Hong Kong Observatory reaching 33.0 degrees or above for 15 consecutive days from May 17 to 31 and soaring to a peak of 35.4 degrees on May 30, the highest temperature of the month and the second highest on record for May. The Very Hot Weather Warning remained in force for more than 320 hours, a record since the introduction of the warning in 2000.
     There was no tropical cyclone over the South China Sea and the western North Pacific in May 2018.
     Details of issuance and cancellation of various warnings/signals in May are summarised in Table 1. Monthly meteorological figures and departures from the normal for May are tabulated in Table 2.
Ends/Monday, June 4, 2018
Issued at HKT 17:02
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