The Weather of April 2011 - 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 17 April 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 1 January 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 3 April.  Meanwhile, a cold front formed over inland Guangdong and crossed the coastal areas in the morning on 4 April 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 7 to 11 April apart from morning mist on 8 and 9 April. The temperature at the Observatory rose to a maximum of 29.7 degrees on 10 April, the highest of the month.  Freshening of easterlies over the coastal areas of Guangdong brought cooler and cloudy weather to Hong Kong on 12 April. After the moderation of the easterly airstream, local weather was generally fine and hot apart from some mist for the next four days.    

     On 17 April, 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 3 p.m 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 19 April.

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

     A trough of low pressure moved southwards and crossed Guangdong on 22 April, bringing showery weather to Hong Kong that night and on 23 April.  Under the influence of a continental airstream, local weather became sunny and very dry on 24 April. The mean relative humidity of that day was only 50%.  It remained fine and dry until the evening on 27 April 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.

     There was no tropical cyclone 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 summarized in Tables 1.1 to 1.3.  Monthly meteorological figures and departures from normal for April are tabulated in Table 2.