The Weather of April 2015

    April 2015 was characterized by sunny, warm and relative dry weather, in particular during the second half of the month.  Overall, the total duration of sunshine in April 2015 was 159.2 hours, 57.5 hours above the normal figure of 101.7 hours.  The mean temperature of the month was 23.6 degrees, 1.0 degree higher than the normal figure of 22.6 degrees.  The monthly mean relative humidity was 77 percent, the third lowest for April since 1961.

    The monthly rainfall was also less than usual with a deficit of about 63 percent compared to the normal figure of 174.7 millimetres.  The accumulated rainfall of 166.6 millimetres since 1 January was only about one-half of the normal figure of 336.1 millimetres for the same period.

    Under the influence of a maritime airstream, the weather in Hong Kong was mainly cloudy and humid with coastal fog for the first four days in the month.  With the clouds thinning out, it turned fine on 5 - 6 April with fog patches in the morning and hot conditions during the day.

    Meanwhile, a cold front crossed the coastal areas of Guangdong on 7 April and local weather turned cloudy that night with rain patches in the following days.  The northeast monsoon behind the cold front brought significantly cooler weather to Hong Kong and temperatures at the Observatory fell to a minimum of 15.9 degrees on 9 April, the lowest of the month.  Affected by an upper-air disturbance, the weather was overcast and rainy on 11 April with more than 30 millimetres of rainfall generally recorded over the territory.  The passage of the disturbance also helped to clear the clouds that had been lingering over Guangdong, as a dry continental airstream advanced southwards to reach the south China coast on 12 April.  Daytime relative humidity at the Observatory fell below 40 percent in the next three days, and sunny and dry conditions persisted till 17 April.

    The return of a maritime airstream brought cloudy and more humid weather to Hong Kong on 18 - 20 April along with a few showers.  Meanwhile, a weak cold front crossed the coastal areas of southern China on the night of 20 April and the northeast monsoon brought slightly cooler weather to Hong Kong in the next three days.  With the moderation of the northeast monsoon, the weather turned fine on 24 April.  Generally fine weather persisted towards the end of the month and daytime conditions became hot under the influence of a warm southerly airstream, with temperatures at the Observatory reaching a maximum of 31.9 degrees on 30 April, the highest of the month.
    Two tropical cyclones occurred 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 1.2.  Monthly meteorological figures and departures from normal for April are tabulated in Table 2.