ENG-1993 — Page 464

THE ENVIRONMENT

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The mean annual rainfall ranges from around 1 300 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 August when rain occurs about four days out of seven for which the monthly average at the Royal Observatory is 391.4 millimetres. The driest month is January, when the monthly average is only 23.4 millimetres and rain falls only about six days in the month.

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

Climatological data are given in Appendix 45.

The Year's Weather

The year 1993 was eventful. Extremes in seasonal temperature variations were marked by a prolonged cold spell in January and an unusually hot and dry summer in July and August. In a year of frequent tropical cyclone activity, the No. 8 Gale or Storm signals were hoisted four times, just once short of the record set in 1964. The rather long typhoon season spanned from late June to early November. Apart from seasonal monsoon rain in June, severe rainstorms also occurred with the passage of tropical cyclones, most notably during Dot in September and Ira in November. The total rainfall during the year amounted to 2 343.9 millimetres, six per cent above the annual mean of 2 214.3 millimetres.

One of the most persistent cold spells ever experienced in Hong Kong occurred in January. Daily minimum temperatures were below 12°C for 16 consecutive days from January 15, and daily mean temperatures were below 12°C for 11 consecutive days, the longest duration since 1887. Freezing conditions were experienced on high ground on the morning of January 16 and temperatures at Tai Mo Shan went down to -2°C. The monthly rainfall of 33.5 millimetres was 43 per cent above the January normal.

Without significant surges of cold air from the north, February was unseasonably mild. The monthly mean temperature of 18°C and the monthly mean minimum temperature of 16.3°C were, in their respective categories, the fourth highest on record for the month of February. The monthly rainfall of one millimetre was only about two per cent of the February normal of 48 millimetres, making it the seventh driest February on record.

March was warmer, cloudier, and more humid than normal. The first thunderstorm of the year occurred on March 16.

April was relatively cool and cloudy, with the northeast monsoon active late in the season. Although rainfall was recorded at the Royal Observatory for 23 days in the month, the monthly total of 136.3 millimetres was still 16 per cent below the April normal of 161.5 millimetres.

The weather in May was typical of the season in both temperature and precipitation. Most of the rain was associated with monsoon troughs which affected the South China coastal areas from time to time.

With moist southwest monsoons and the presence of active troughs near the South China coast, June was cloudier than normal and unusually humid. The mean dew point of 25.2°C was the second highest ever recorded in June. Torrential downpours on June 11 and 16 caused widespread flooding in the urban area, necessitating the issuance of Rainstorm Red Warnings on both occasions and a Rainstorm Black Warning in the latter case. The