THE ENVIRONMENT
348
February was warmer and sunnier than normal. The rain was mostly light and patchy. The monthly total rainfall of 8.2 millimetres was 83 per cent below the February normal of 48.0 millimetres.
March was warmer and also more humid. The monthly mean minimum temperature of 18.6 degrees was the fifth highest on record for March. Rain that fell was mostly associated with late-season surges of the winter monsoon. The first major downpour and thunder- storm of the year occurred on March 22. However, the monthly total rainfall of 51.5 milli- metres was 23 per cent below the March normal of 66.9 millimetres.
The weather in April was sunnier and drier. It was the seventh driest April on record. The monthly total rainfall of 34.7 millimetres was 79 per cent below the April normal of 161.1 millimetres. Most of the rain fell on the last day of the month and could be attributed to a dissipating tropical cyclone over western Guangdong.
May was one of the driest and warmest Mays on record. The monthly rainfall of 60.9 millimetres, the eighth lowest ever, was about one-fifth of the May normal. The monthly mean pressure of 1 011.6 hectopascals in Hong Kong was a record high for the month of May. Intense solar heating caused the temperature to soar to a maximum of 33.4 degrees on May 28, making it the fifth hottest day ever in the month of May.
June was the seventh warmest June on record with a monthly mean temperature of 28.4 degrees and a monthly mean minimum temperature of 26.4 degrees. Less spectacular, but more significant, was its near-normal rainfall following a substantial deficit in the first five months of the year. The monthly total rainfall of 371.7 millimetres represented about 99 per cent of the June normal of 376.0 millimetres.
Three tropical cyclones affected Hong Kong in July but the monthly rainfall of 293.6 millimetres still fell short of the July normal of 323.5 millimetres by nine per cent. On July 23 and 24, Severe Tropical Storm Brendan prompted the hoisting of the No. 8 Gale or Storm Signal. At the end of the month, torrential rain associated with the active south-west monsoon brought extensive flooding to the Yuen Long area where the flood water in places exceeded one metre in depth.
August was characterised by the passage of Typhoon Fred mid-month and a spell of light wind and hazy conditions in the second half of the month. Fred necessitated the Strong Wind Signal No. 3 for nearly 34 hours. An offshore oilfield barge capsized in the stormy weather on August 15 about 100 kilometres east of Hong Kong. The monthly rainfall of 302.3 millimetres was 23 per cent below the August normal of 391.4 millimetres.
September was relatively warm and dry. The monthly mean temperature of 28.1 degrees and the monthly minimum temperature of 25.8 degrees were respectively the fourth and eighth highest ever for September. The monthly rainfall of 178.7 millimetres fell short of the September normal of 299.7 millimetres by 40 per cent. Over half the month's rain occurred on the night of September 14 in association with a fresh easterly airstream. In contrast, the two passing tropical cyclones, Joel and Nat, disappointed by bringing relatively little rain.
In spite of long hours of sunshine and below-normal humidity, October 1991 was a month of abundant rain. The monthly rainfall of 294.3 millimetres was more than double the October normal of 144.8 millimetres and was the 10th highest on record for the month of October. Nearly all the rain fell within the 4-day period of October 14 to 17. The lowest relative humidity of 21 per cent on October 27 and 28 equalled the record previously set for October in 1968 and 1973.