The warmest April

     Mainly attributing to the warmer than normal sea surface temperature and stronger than usual southerly flow in the lower atmosphere over the northern part of the South China Sea, April 2024 was much warmer than usual. The monthly mean maximum temperature of 28.9 degrees, monthly mean temperature of 26.4 degrees and monthly mean minimum temperature of 24.5 degrees were 3.3 degrees, 3.4 degrees and 3.4 degrees above their corresponding normals and all of them were the highest on record for April. Affected by a number of troughs of low pressure, the month was also wetter than usual with a total rainfall of 257.1 millimetres, about 68 per cent more than the normal figure of 153.0 millimetres. The accumulated rainfall recorded in the first four months of the year was 289.5 millimetres, about 4 per cent below the normal figure of 300.4 millimetres for the same period.
 
     Under the influence of a southerly airstream, the weather was hot with sunny intervals during the day in Hong Kong on the first four days of the month. The visibility was rather low in the morning and at night on April 1. The daily mean temperature of 27.2 degrees and daily minimum temperature of 26.5 degrees on April 4 were both the highest on record for the Ching Ming Festival. Affected by a trough of low pressure over Guangdong, the local weather was mainly cloudy with a few showers and thunderstorms from April 5 to 7. The showers were heavier on April 6 with more than 40 millimetres of rainfall recorded over the northern and the eastern parts of the New Territories. With the weakening of the trough of low pressure, showers eased off gradually in Hong Kong on April 8. The visibility was also rather low in some areas.
 
     Under the influence of a fresh to strong easterly airstream, it was relatively cooler with a few light rain patches on April 9. With the band of clouds covering the coastal areas of Guangdong thinning out gradually, the local weather became dry with sunny periods during the day on April 10. Affected by an anticyclone aloft and the subsequent southerly airstream, it was generally fine and hot during the day from April 11 to 17. With plenty of sunshine, the temperatures at the Observatory rose to a maximum of 31.9 degrees on the afternoon of April 13, the highest of the month. There was also coastal fog at first on April 15. Under the influence of a trough of low pressure, it was mainly cloudy with showers and squally thunderstorms on April 18. The showers were heavy at times and more than 30 millimetres of rainfall was recorded over the eastern part of the territory and parts of the New Territories. A southerly airstream brought a few showers and isolated thunderstorms to Hong Kong on the next day.
 
     An active trough of low pressure over the northern part of South China edged south on April 20 and lingered along the coastal areas of Guangdong in the next three days. There were heavy showers and squally thunderstorms in Hong Kong from April 20 to 23. More than 100 millimetres of rainfall was generally recorded over the territory and the rainfall even exceeded 200 millimetres over North, Tai Po, Tsuen Wan and Sai Kung Districts on these four days. With the rain, temperatures at the Observatory dropped to a minimum of 21.5 degrees on April 21, the lowest of the month but the highest monthly absolute minimum temperature on record for April. Moreover, waterspout was spotted near Clear Water Bay that morning. With the trough of low pressure shifting to the south of Hong Kong and the onset of an easterly airstream over the coastal areas of Guangdong, local showers abated on April 24. 
 
     The trough of low pressure returned and lingered over the coast of Guangdong again from April 25 to 28. The weather of Hong Kong became unsettled with occasional showers and squally thunderstorms. The showers were particularly heavy on April 26. More than 30 millimetres of rainfall was generally recorded over the territory and the rainfall even exceeded 100 millimetres over Sai Kung and Sha Tin Districts on that day. Meanwhile, the daily mean temperature of 28.8 degrees and daily minimum temperature of 27.7 degrees on April 27 were both the highest on record for April. Under the influence of a southerly airstream, the local weather was hot with sunny intervals during the day on April 29. While there were sunny intervals during the day on the last day of the month, affected by an area of intense thundery showers associated with a trough of low pressure, the weather deteriorated with outbreaks of heavy showers and severe squally thunderstorms that night. About 30 millimetres of rainfall was recorded over many places of the territory. Hail was reported in Yuen Long. Violent gusts of around 110 kilometres per hour were once recorded at Tai O. A taxi in Aberdeen was damaged by a fallen tree.
 
     There was no tropical cyclone over the South China Sea and the western North Pacific in April 2024.
 
     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 April are tabulated in Table 2.