5 May 2020

Mainly attributing to the stronger than usual northeast monsoon over southern China, April 2020 was slightly cooler than usual with the mean temperature of 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 percent below the normal figure of 174.7 millimetres. The accumulated rainfall recorded in the first four months of the year was 213.7 millimetres, about 36 percent below the normal figure of 336.1 millimetres 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 on 1 - 4 April. With the strengthening of the easterlies and the setting in of an upper-air disturbance, local weather became cooler with more showers on 5 – 6 April. More than 20 millimetres of rainfall were generally recorded over the territory, rainfall even exceeded 40 millimetres over parts of the New Territories on these two days. The temperature at the Observatory dropped to 16.1 degrees under the rain on 6 April, 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 7 April. 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 became generally fine and dry in the next few days. The weather turned cloudier again on 11 April 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 11 April. Locally, more than 10 millimetres 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 12 – 13 April. The relative humidity once fell below 30 percent during the day on 13 April.

After a mainly cloudy day on 14 April, local weather turned fine again under the influence of a ridge of high pressure on 15 April and remained so in the next two days. Affected by a maritime airstream, it was mainly fine and hot during the day on 18 – 21 April. There was also coastal fog on the morning of 19 and 21 April.

A cold front moved across the coast of Guangdong on the morning of 22 April. Under the influence of the northeast monsoon and upper-air disturbances, local weather became cloudy with rain on 22 April. More than 40 millimetres of rainfall were recorded at Tseung Kwan O and eastern Hong Kong Island 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 26 – 27 April with sunny periods. Under light wind conditions, there was some haze during the day on these two days. Visibility at Chek Lap Kok once fell to around 3000 metres on 26 April. Under the influence of a continental airstream, local weather continued to improve and became generally fine and dry on 28 April. 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 Observatory soared to 30.3 degrees on 30 April, the highest of the month.

There was no tropical cyclone over the South China Sea and the western North Pacific in April 2020.

   
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 April are tabulated in Table 2.