The Weather of April 1999

    April 1999 was very warm with a mean temperature of 24.3 degrees, the third highest for the month. The mean minimum and mean maximum temperatures of 22.4 and 26.9 degrees were the third and sixth highest respectively for the month. It was also relatively dry with the mean relative humidity of 79 per cent, the fourth lowest for the month. Although the monthly rainfall of 176.9 millimetres was slightly above normal, the accumulated rainfall of 205.0 millimetres since 1 January was still 94.8 millimetres below the normal value.

    The first Amber Rainstorm Signal of the year was issued on 12 April and the first Tropical Cyclone Warning Signal was hoisted on 29 April. It was also the first time that the Strong Wind Signal No. 3 was hoisted as early as 30 April.

    It was mainly fine during the first two days of April though there were also a few light rain patches. Winds strengthened from the east as a cold front crossed the coast of Guangdong late on 2 April. Winds moderated gradually the next day but cloudy weather prevailed for a few more days.

    Long sunny periods developed on 5 April as Hong Kong came under the influence of a dry continental airstream. During the Easter Holidays and Ching Ming Festival, about 880 hectares of vegetation and 5 400 trees were burnt in hill fires. There were 475 cases of hill fire from 4 to 7 April.

    Cloudy weather with light rain patches returned later on 8 April and cloudy conditions persisted the next few days. It turned fine with long sunny periods on 10 April but the weather was cloudy with rain again the next day. Thunderstorms and heavy rain associated with a trough of low pressure affected Hong Kong early on 12 April. Rain was heaviest over Hong Kong Island and eastern Lantau where over 80 millimetres of rainfall were recorded. A minor landslip occurred in Lai King.

    The weather was fine and sunny on 13 April but it became cloudy with some light rain patches again that evening as an intense replenishment of the northeast monsoon reached the south China coastal areas. Winds strengthened from the east and persisted the next day. The weather improved on 15 April and winds moderated. Isolated showers developed on 17 April especially over the New Territories.

    A trough of low pressure crossed the south China coast on 19 April bringing thunderstorms and heavy rain to Hong Kong. About 30 millimetres of rain were recorded in the eastern areas. Rain became widespread on the evening of 21 April. The weather improved the next day with long sunny periods. Temperatures rose to 31.1 degrees on 23 April, the highest in the month.

    Under the influence of a maritime airstream, isolated heavy showers affected the northeastern part of the New Territories on 24 April. The weather improved with long sunny periods on 25 April apart from some light rain. Winds freshened from the east on the evening of 26 April and it turned cloudy with some showers again.

    A tropical depression named Leo developed over the central part of the South China Sea on 28 April. The outer rainbands of this system began to affect the coastal areas on 29 April. It was cloudy to overcast with heavy rain at times and offshore winds became strong. The weather remained cloudy to overcast with periods of rain on the last day of the month. Temperatures dropped to 18.3 degrees in the morning, the lowest in the month.

    Two tropical cyclones occurred in the South China Sea and the western North Pacific in the month.

    During the month, no aircraft was diverted due to adverse weather.