The Weather of December 1999

    December was cooler than normal. The monthly mean temperature of 16.8 degrees was 0.8 degrees lower than normal. Under the influence of an intense cold surge, the daily mean temperature of 8.5 degrees on 22 December was the ninth lowest for December. For four consecutive days from 21 to 24 December, the minimum temperatures stayed below 10 degrees. Formation of ice and thin frost were reported.

    It was also a dry month with the mean relative humidity of only 60 per cent, the fifth lowest for December. The monthly rainfall amounted to 32.9 millimetres, 21 per cent above normal. However, the annual total in 1999 was 2 129.1 millimetres, close to the normal of 2 214.3 millimetres.

    It was fine, sunny and dry on 1 December with a maximum temperature in the month of 23.6 degrees. As the northeast monsoon over the south China coastal areas strengthened, it became cloudy on the evening of 2 December with strong winds affecting offshore areas. Rain set in the next afternoon and winds turned northerly, moderating in the evening. Cloudy weather with light rain continued the next couple of days. Northerly winds strengthened again in the afternoon of 5 December and moderated the next day. It was hazy over the airport on 6 December.

    The weather became fine and very dry on 7 December. These conditions persisted until 12 December when it became cloudy again as winds turned easterly. It remained cloudy the next day.

    Sunny and very dry weather returned on 14 December. A broad band of rain over the South China Sea brought overcast weather with periods of rain to Hong Kong on 16 December. This rainy episode lasted for four days.

    An intense surge of the winter monsoon reached the south China coastal areas on 19 December with winds strengthening from the north. Under the influence of a persistently cold and dry airstream, the sky was almost cloud-free the next six days. Temperatures dropped to 5.8 degrees on the morning of 23 December, the lowest in the month. On Tai Mo Shan, the minimum temperature fell to -3.3 degrees. During the three-day period from 22 to 24 December, formation of ice was reported at Tai Mo Shan, Tate's Cairn and the northern part of the New Territories. Thin frost was also reported at Kwu Tung and Sheung Shui on 23 and 24 December. Under the very dry conditions, a hillfire broke out late on Christmas Eve at Fei Ngo Shan. An area of 1.2 million square metres was affected and the fire was finally put out after 12 hours.

    The cloud amount increased on the evening of 26 December. There were some light rain patches on the morning of 27 December. Clouds dispersed the next day and long sunny periods developed. However, it was hazy over the western areas. Cloudy periods returned on 29 December. Hazy conditions extended to affect the harbour on 30 December although the weather became sunny again. Visibility was reduced to 1 100 metres at the airport. With moderate easterly winds, haze cleared in the evening and it became fine and dry on the last day of 1999.

    Only one tropical cyclone occurred in the South China Sea in the month.

    During the month, no aircraft was diverted due to adverse weather.