ENG-1983 — Page 315

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THE ENVIRONMENT

affected, although gales can occur any time between May and November. When a tropical cyclone is about 700–1 000 kilometres southeast of Hong Kong the weather is usually fine and exceptionally hot, but isolated thunderstorms sometimes occur in the evenings. If the centre moves closer to Hong Kong winds increase and rain can become heavy and widespread. Heavy rain from tropical cyclones may last for a few days and consequent landslips and flooding sometimes cause more damage than the winds.

The mean annual rainfall ranges from around 1 200 millimetres at Waglan Island to more than 3 000 millimetres in the vicinity of Tai Mo Shan. About 80 per cent of the rain falls between May and September. The wettest month is June when rain occurs about two days out of three and the average monthly rainfall at the Royal Observatory is 431.8 millimetres. The driest month is December when the monthly average is only 25.3 millimetres and rain falls on only about five days in the month. Climatological information on Hong Kong's weather is given at Appendix 39.

Severe weather phenomena that can affect Hong Kong include tropical cyclones, strong winter monsoon winds, and thunderstorms with associated squalls that are most frequent from April to September. Water-spouts, hailstorms and snow occur infrequently and tornadoes are rare.

The Year's Weather

1983 was a year of both records and contrasts. During the first half of the year, rainfall was above normal every month. The total rainfall from January to April broke all previous records for the same period. In contrast, the year ended with a long dry spell with no measurable rain between October 27 and December 27. Unusually, there was not a single tropical cyclone over the western North Pacific until nearly the end of June. This rare situation last occurred in 1973. However in September Hong Kong suffered a direct hit by Typhoon Ellen, resulting in extensive damage. The year as a whole was wetter than normal and a total rainfall of 2 893.8 mm was recorded at the Royal Observatory.

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January was wet and cloudy. The total of 76.3 mm of rain recorded was nearly three times the average figure while the total duration of sunshine, 75 hours, was only about half the normal value. The month was cooler than usual and temperatures fell below 10°C between January 20 and 24. A minimum of 7.1°C was recorded at the Royal Observatory on January 22 and -1.0°C was reported on Tai Mo Shan on the same morning. Coastal fog patches occurred towards the end of the month.

February was again wet. New rainfall records were set with 241 mm of rain for the month, the highest figure since records began at the Royal Observatory in 1884 and about six times the average amount. The number of days with measurable rain, totalling 24, exceeded all previous records. The month was also cooler than usual. Fog was reported on a number of occasions, resulting in the diversion of six aircraft from Hong Kong International Airport. Ferry services were also affected: on February 1, a triple-decker ferry with more than 430 passengers went aground in fog off Kau Yi Chau and two people were hospitalised.

March was the second consecutive month in the year with a record rainfall: 428 mm compared with an average of 54.8 mm. Most of the rain was associated with thunderstorms which occurred on 10 days of the month, the highest frequency for March since 1947. Frequent thunderstorms on March 27 brought 98.6 mm of rain, making it the wettest day in March since 1884. Hail, reported on March 1, 2, 12, 25 and 26, was more frequent than has been observed in any single year in the past. The month was again cooler than usual. Temperatures dropped to 9.6°C on March 6 following an outburst of the winter monsoon

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