ENG-1990 — Page 413

THE ENVIRONMENT

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In recent years, the vagaries of the local weather have become better understood through experience gained in analysing and interpreting the outputs of numerical weather pre- diction models run at the observatory and major meteorological centres abroad. With advances in the reliability of numerical model outputs, useful weather outlooks beyond the first day may now be issued with greater confidence.

The Year's Weather

In 1990, six of the 13 tropical cyclones which occurred over the South China Sea required the hoisting of tropical cyclone signals in Hong Kong although none of them necessitated hoisting gale or storm signals. With August 1990 the hottest month on record and August 18 the hottest day ever experienced in Hong Kong, the year's mean temperature of 23.1 °C was 0.3 degree higher than the normal figure. The total rainfall in 1990 was 2046.9 millimetres, eight per cent below the normal of 2 224.7 millimetres.

January 1990 was generally cloudy, humid and wet. There are only seven days in the month with no rain recorded and the total rainfall for the month amounted to 47.5 millimetres, which was 77 per cent above normal.

February was even wetter and more humid. The monthly total rainfall of 195.7 millimetres was more than four and a half times the normal.

The weather in March turned drier and brighter. The monthly total rainfall of 29.9 - millimetres was 45 per cent below normal. Apart from the last 10 days of the month, the daily mean pressure stayed consistently on the high side as a continental anticyclone prevailed over China. As a result, the monthly mean pressure of 1018.2 hectopascals was the fifth highest on record for March.

April was gloomy and wet. There were only three days in the month when no rainfall was recorded, the total amounting to 257.6 millimetres which was 85 per cent above normal. On the morning of April 11, gusts of up to 135 kilometres per hour were recorded at Waglan Island in severe squalls accompanying heavy downpour. This was the strongest gust ever recorded at that station in the month of April.

May was relatively dry with a monthly total rainfall of 102.4 millimetres. Tropical cyclone signals were hoisted on May 17 for Typhoon Marian, but it failed to come close enough to bring any significant rain to the territory.

In June, tropical cyclone signals were hoisted during the passages of Severe Tropical Storm Nathan and Typhoon Percy but the monthly rainfall total of 448.1 millimetres was only slightly above normal. On June 30, an unstable south-westerly airstream in the wake of Percy brought about periods of heavy rain and thunderstorms, resulting in three incidents of landslips and over 30 reports of flooding in Kowloon and the New Territories. A boy was washed away in a flooded culvert in Kwai Chung.

July was hotter and drier than normal. The weather was basically fine except for showers and periods of rain brought by an unstable south-westerly airstream in mid-July and by Severe Tropical Storm Tasha which necessitated the hoisting of the Strong Wind Signal No. 3 towards the end of the month.

August was the hottest month and the driest August ever experienced in Hong Kong. The monthly mean temperature of 29.5°C and the monthly mean minimum temperature of 27.4°C were the highest on record, while the monthly mean maximum of 32.1°C was the second highest for August. With a daily mean temperature of 31.9°C and a daily mini- mum temperature of 30.1°C, August 18 was the hottest day on record. The maximum