The Weather of May 1999

    May was cooler and drier than usual. The monthly mean temperature of 24.9 degrees was one degree below normal while the month's total rainfall of 177.8 millimetres was 138.9 millimetres below. The accumulated rainfall of 382.8 millimetres since 1 January was about 38 per cent below the normal figure. The Number 8 Northeast Gale or Storm Signal was hoisted for the first time this year on 2 May. It was also the earliest date on which a Number 8 signal was hoisted since 1946.

    During the first two days of the month, Hong Kong was under the threat of Typhoon Leo. The weather was cloudy with periods of rain. Strong northerly winds affected most parts of Hong Kong, reaching gale force offshore and on high ground. As Leo dissipated overland on the evening of 2 May, winds subsided rapidly and the weather improved the next day with plenty of sunshine. No severe damage was incurred during the approach of Leo.

    A trough of low pressure crossed the south China coast on 4 May, bringing rainy weather to the coastal areas during the next couple of days. It was overcast with occasional heavy rain and isolated thunderstorms on the morning of 5 May.

    A northeast monsoon reached the south China coastal areas on 6 May. As winds freshened from the northeast, there was continual rain and temperatures fell to 17.0 degrees, the lowest in the month, late in the afternoon. This was the fourth lowest daily minimum temperature recorded in May. It remained cloudy to overcast with periods of rain during the next few days. Winds were fresh offshore till 9 May.

    A dry continental airstream brought fine weather to southern China on 10 May. Generally fine weather persisted for the next few days. It became progressively warmer as a maritime airstream began to affect the south China coastal areas on 15 May and there were some isolated showers. The maximum temperature rose to 31.5 degrees on 17 May, the highest so far this year.

    A trough of low pressure crossing the south China coast on 18 May brought heavy rain and thunderstorms to Hong Kong the next morning. Under the influence of the fresh easterly airstream behind the trough, the weather remained generally cloudy with some rain for the next couple of days. Temperatures varied within only a few degrees during this period.

    The easterly winds were replaced by a maritime southeasterly airflow on 22 May which brought patches of rain in the morning. The weather became fine with abundant sunshine the next day. It turned cloudy again on 24 May with a few isolated showers.

    Hong Kong came under the influence of a trough of low pressure over Guangdong from 25 May onwards. The weather was cloudy with showers which were heavy and thundery on the early morning of 25 May. There were heavy showers again on 27 May as the trough crossed the south China coast. It remained cloudy to overcast with a few isolated showers during the next couple of days. The weather became fine and sunny on the last day of the month.

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

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