The Weather of September 1998

    September 1998 was warmer than usual. The mean minimum temperature of 26.2 degrees was the fourth highest for September and the mean temperature of 27.8 degrees was 0.2 degrees above normal. The total rainfall of 230.9 millimetres in the month was 68.8 millimetres below normal. However, the accumulated rainfall since 1 January, 2 388.2 millimetres, was still 19 per cent more than the normal for the same period. The Standby Signal No. 1 was hoisted on one occasion in the month.

    Under the influence of a trough of low pressure over the northern part of the South China Sea, heavy thundery showers affected Hong Kong on the first day of September. Weather gradually improved and there were sunny intervals apart from isolated showers in the next few days. Thunderstorms returned on 6 September as another trough of low pressure approached from the north. Showers moderated the next couple of days but it became thundery again on 9 September. Thundery conditions persisted the next few days as an area of low pressure developed over the northern part of the South China Sea.

    On 12 September, the area of low pressure intensified into a tropical depression. Locally, winds freshened from the northeast and there were occasional squally showers. As the tropical depression moved away towards the west, fresh to strong easterlies set in on 14 September, bringing thundery showers in the early morning. Showers eased off that afternoon and there were sunny periods. Fine weather continued for the next couple of days.

    On the evening of 16 September, winds strengthened from the east again as a ridge of high pressure extended southwards across the Taiwan Strait. Winds moderated the next day. It was sunny apart from some isolated showers. On 19 September, a dry northerly airstream arrived, bringing fine weather to southern China for the next few days. Temperatures rose to 32.1 degrees on 20 September, the highest in the month. Haze was reported over parts of Hong Kong on 21 and 22 September.

    On 22 September, winds turned easterly bringing cloudier weather and some light showers the next morning. Fine weather returned in the afternoon and then lasted the next few days. On 27 September, another northerly airstream affected Hong Kong and haze was again observed. The weather remained fine and dry till the end of the month. Temperatures dropped to 24.3 degrees on 30 September, the lowest in the month.

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

    During the month, a total of 2 aircraft was diverted due to adverse weather.