THE ENVIRONMENT
year's first tropical cyclone to affect Hong Kong was Typhoon Koryn, which necessitated the hoisting of the No. 8 Gale or Storm Signal on June 27. With only one rain-free day in the month, the monthly total rainfall of 485.2 millimetres was 29 per cent above the June normal of 376.0 millimetres.
July was unusually hot. The mean minimum temperature of 27.6°C and the mean air temperature of 29.4°C ranked, respectively, the highest and second highest on record for July. Overall, it was a relatively dry month although more than 100 millimetres of rain fell during the passage of Severe Tropical Storm Lewis. The monthly rainfall of 213.7 millimetres was 34 per cent below the July normal of 323.5 millimetres.
The very hot and dry conditions continued into August. The monthly mean minimum temperature of 26.8°C was the fifth highest on record for August. The monthly rainfall amounted to 182.8 millimetres only, less than half of the August normal of 391.4 millimetres. This was despite the stormy weather brought by Typhoon Tasha on August 20, when the No. 8 Gale or Storm signal was hoisted for the second time in the year.
Then came three typhoons in quick succession within a fortnight in September. Becky and Dot necessitated the hoisting of the No. 8 Gale or Storm signals on September 17 and 26, respectively. The 223.9 millimetres of rain brought by Dot on September 26 was the third highest daily amount on record for September. By early next morning, the northern part of the New Territories was inundated with flood water as deep as seven metres in some areas. With a monthly rainfall of 655.9 millimetres, more than twice the September normal of 299.7 millimetres, September 1993 became the fifth wettest September on record.
October was much drier and slightly cooler than normal. This was due to the dominance of continental anti-cyclones which led to the third highest October monthly mean pressure of 1 016.2 hectopascals. The monthly rainfall of 87.8 millimetres was 39 per cent below the October normal of 144.8 millimetres.
November was unusually cloudy and humid. The monthly rainfall of 144.6 millimetres, more than four times the November normal of 35.1 millimetres, and the 131.2 hours of bright sunshine in the month were, respectively, the fifth highest and sixth lowest on record for November. The most significant weather event was the late-season passage of Typhoon Ira. Rain associated with Ira was quite exceptional as intense convection embedded within one of its trailing rainbands brought concentrated heavy rain of more than 700 millimetres to the western part of the territory on November 4 and 5. There were extensive landslides on Lantau Island and widespread flooding in Tuen Mun and Yuen Long. An inundated water treatment plant in Tuen Mun and burst water mains on Cheung Chau Island resulted in a cut-off of fresh water supply to these areas for a period of four days.
December was slightly cooler and cloudier than normal. Apart from a humid spell early in the month, it was rather dry. The monthly rainfall of 15.7 millimetres was 42 per cent below the December normal of 27.3 millimetres. The winter monsoon was active before Christmas, giving rise to some cold mornings and days of low humidity. The weather turned noticeably milder during the period leading up to the New Year.
Topography and Geology
The topography of Hong Kong is characterised by steep granitic and volcanic mountains. Much of the terrain at the foot of the mountains is blanketed by slope deposits transported by erosion and mass movement from the hillsides. Some 40 per cent of the exposed land is
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