A sunny and dry April

     Due to the stronger than normal northeast monsoon over southern China, April 2011 was sunnier and drier than usual in Hong Kong. The monthly total bright sunshine duration was 191.6 hour, 89.8 hour above normal, while the month's mean relative humidity was 76%, the lowest in April since 1970.

     Despite an episode of squally and thundery showers on April 17 which necessitated the issuance of the first amber rainstorm warning and thunderstorm warning for the year, there were only 36.0 millimetres of rainfall recorded in the month, a deficit of about 81 percent comparing with the normal figure of 188.5 millimetres. The accumulated rainfall since January 1 was 85.6 millimetres, only about 25 percent of the normal figure of 337.1 millimetres for the same period.

     Under the influence of a ridge of high pressure, the weather in Hong Kong was generally fine and dry for the first two days of the month. Apart from some fog patches in the morning, it remained fine on April 3. Meanwhile, a cold front formed over inland Guangdong on April 3 and crossed the coastal areas in the morning on April 4 bringing cooler weather and some light rain patches to the territory. The weather turned dry with clouds clearing up under the influence of a fresh and dry easterly airstream for the next two days.

     With the setting in of a relatively warm maritime airstream, local weather was mainly fine from April 7 to 11 apart from morning mist on April 8 and 9. Freshening of easterlies over the coastal areas of Guangdong brought cooler and cloudy weather to Hong Kong on April 12. After the moderation of the easterly airstream, local weather was generally fine and hot apart from some mist for the next four days. The temperature at the Observatory rose to a maximum of 29.6 degrees on April 14, the highest of the month.  

     On April 17, an active trough of low pressure swept southwards across Guangdong and brought thundery showers, squalls and hailstones to the region. The associated area of thunderstorms weakened slightly while moving across the territory in the afternoon. Gusts of 85 km/h were registered in Ta Kwu Ling around 3pm and more than 30 millimetres of rainfall were recorded in the northern part of the New Territories and Lantau Island. With the trough of low pressure moving further south over the northern part of the South China Sea and weakening, there were some sunny intervals in Hong Kong the next day. The weather turned fine and dry on April 19.

     Freshening of the easterlies brought cloudy weather to Hong Kong on April 20. The cloud thinned out and the weather turned fine again on April 21 when the easterly winds subsided.

     A trough of low pressure moved southwards and crossed Guangdong on April 22, bringing showery weather to Hong Kong that night and on April 23. Under the influence of a continental airstream, local weather became sunny and very dry on April 24. The mean relative humidity of that day was only 50%. It remained fine and dry until the evening on April 27 when a broad band of clouds set in from the southwest. Local weather was mainly cloudy with mist and rain patches on the last three days of the month.

     Only one tropical cyclone occurred over the South China Sea and the western North Pacific in the month.

     Details of the issuance and cancellation of various warnings/signals in the month are summarised in Table 1. Monthly meteorological figures and departures from normal of April are tabulated in Table 2.

Ends/Wednesday, May 4, 2011
Issued at HKT 20:17