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An unusually hot May with much clouds and rain

     May 2015 was the fourth hottest May since records began in 1884.  Its monthly mean temperature of 27.5 degrees was 1.6 degrees higher than the normal figure of 25.9 degrees. Affected by active troughs of low pressure in the second half of the month, May 2015 was actually cloudier than usual and rather rainy as well. Bright sunshine during the month amounted to 93.5 hours only, about 33 per cent below the normal figure of 140.4 hours, the seventh lowest for May on record. The monthly total rainfall was 513.0 millimetres, a surplus of 68 per cent against the normal figure of 304.7 millimetres. The accumulated rainfall of 679.6 millimetres since January 1 was about 6 per cent above the normal figure of 640.8 millimetres for the same period.

     Under the influence of a southerly airstream, the weather in Hong Kong was hot with sunny periods on the first four days of the month. A trough of low pressure brought generally cloudy weather with a few showers to the territory on May 5 to 7. After a cloudy but relatively rain-free day on May 8, another trough of low pressure brought heavier showers and thunderstorms to Hong Kong on May 9 to 10.  

     A cold front over southern China moved across the coast of Guangdong on May 11. A squall line associated with the cold front brought heavy showers and squally thunderstorms to the territory later that day (please see the diagram). A maximum gust of over 100 kilometres per hour was recorded at Lau Fau Shan during the passage of the squall line. Affected by a continental airstream, local weather became relatively cool and dry the next day. Temperature at the Observatory fell to 22.6 degrees on May 12, the lowest of the month. As winds veered to southerly gradually, the weather became mainly fine and hot on May 14 to 15. Temperature at the Observatory rose to 32.6 degrees on May 15, the highest of the month.

     With a trough of low pressure lingering over the coastal areas of Guangdong, local weather remained unsettled with occasional heavy showers and squally thunderstorms over the next 12 days. Two Red rainstorm episodes on May 20 and 23 brought more than 150 millimetres of rain to most parts of the territory. Flooding was reported in Sha Tin, Ma On Shan and Tuen Mun on May 20, and also at the low-lying areas in Sheung Shui and Yuen Long on May 23. Meanwhile, a fresh to strong easterly airstream also brought windy and cooler conditions to the territory on May 21 to 22. Another rapidly developed rainstorm brought more than 70 millimetres of rain to the urban areas, and more than 100 millimetres of rain to Sha Tin, Tsuen Wan, Sai Kung and Ma On Shan on May 26, necessitating the first issuance of Black Rainstorm Warning of the year. Flooding was reported in Ho Man Tin, Sha Tin and Sai Kung.

     With the weakening of the trough, the weather improved gradually with sunny periods on May 28 to 30. However, the weather turned cloudy again with isolated showers and thunderstorms on May 31 as another trough of low pressure from southern China moved towards the coastal areas.

     Two tropical cyclones occurred 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 summarised in Table 1. Monthly meteorological figures and departures from normal for May are tabulated in Table 2.

Ends/Tuesday, June 2, 2015
Issued at HKT 20:16


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