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A dry and cooler April
     Mainly attributing to the stronger than usual northeast monsoon over southern China, April 2020 was slightly cooler than usual in Hong Kong. The monthly mean temperature was 22.0 degrees, 0.6 degree below the normal figure of 22.6 degrees.  With weaker southerlies and less moisture in the lower atmosphere over southern China, the month was also drier than usual in Hong Kong. The total rainfall in the month was 77.8 millimetres, about 55 per cent below the normal figure of 174.7mm. The accumulated rainfall recorded in the first four months of the year was 213.7mm, about 36 per cent below the normal figure of 336.1mm for the same period.
     Under the influence of the northeast monsoon and the subsequent fresh to strong easterly airstream, it was cloudy with a few rain patches from April 1 to 4. With the strengthening of the easterlies and the setting in of an upper-air disturbance, local weather became cooler with more showers on April 5 and 6. More than 20mm of rainfall were generally recorded over the territory on these two days, with the rainfall recorded over parts of the New Territories even exceeding 40mm. The temperature at the Hong Kong Observatory dropped to 16.1 degrees with rain on April 6, the lowest of the month.
     With the departure of the upper-air disturbance, showers over Hong Kong eased off gradually and there were bright periods during the day on April 7. Under the influence of a relatively dry easterly airstream and the setting in of a ridge of high pressure, the cloud band covering Hong Kong thinned out and the weather was generally fine and dry in the next few days. The weather turned cloudier on April 11 with the approach of a cold front. The cold front moved across the coastal areas of Guangdong and brought showery weather to the region on the night of April 11. Locally, more than 10mm of rainfall were recorded over the territory that night. Under the influence of a continental airstream behind the cold front, the weather of Hong Kong became fine and very dry with cool mornings on April 12 and 13. The relative humidity once fell below 30 per cent during the day on April 13.
     After a mainly cloudy day on April 14, local weather turned fine under the influence of a ridge of high pressure on April 15 and remained so in the next two days. Affected by a maritime airstream, it was mainly fine and hot during the day from April 18 to 21. There was also coastal fog on the mornings of April 19 and 21.
     A cold front moved across the coast of Guangdong on the morning of April 22. Under the influence of the northeast monsoon and upper-air disturbances, local weather became cloudy with rain on April 22. More than 40mm of rainfall were recorded in Tseung Kwan O and eastern Hong Kong Island respectively on that day. The weather of Hong Kong remained cloudy with some rain and became cooler in the next three days. With the moderation of the northeast monsoon, the temperature rose gradually on April 26 and 27 with sunny periods. Under light wind conditions, there was some haze during the day on these two days. The visibility at Chek Lap Kok once fell to around 3 000 metres on April 26. Under the influence of a continental airstream, local weather continued to improve and became generally fine and dry on April 28. With the setting in of an anticyclone aloft, generally fine weather prevailed on the last two days of the month. With plenty of sunshine during the day, the maximum temperature at the Hong Kong Observatory soared to 30.3 degrees on April 30, the highest of the month.
     There were no tropical cyclones over the South China Sea and the western North Pacific in April 2020.
     Details of issuance and cancellation of various warnings/signals in April are summarised in Table 1. Monthly meteorological figures and departures from normal for April are tabulated in Table 2.
Ends/Tuesday, May 5, 2020
Issued at HKT 12:35
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