The Weather of August 1999

    August 1999 was very wet. The total monthly rainfall amounted to 892.0 millimetres and was the second highest for August. The accumulated rainfall since 1 January was 1 676.0 millimetres, two per cent below the normal figure of 1 707.4 millimetres for the same period. August was also cloudier than normal. The mean cloud amount stood at 80 per cent and was the fourth highest for the month. The total bright sunshine duration was only 138.5 hours, the fifth lowest for August.

    Typhoon Sam crossed the northeastern part of the New Territories on 22 August and necessitated the hoisting of Gale or Storm Signal No. 8. The Black Rainstorm Warning Signal was issued on 23 and 24 August and the Landslip Warning was also issued on 22 and 25 August.

    The weather was fine and sunny on the first day of the month. Under the influence of an area of low pressure over the northern part of the South China Sea, the weather turned cloudy with some isolated heavy showers on 2 August. Showers became thundery on 3 August although sunny intervals also managed to appear. Over 100 millimetres of rainfall were recorded in Tseung Kwan O and Cheung Sha Wan on 4 and 5 August respectively. There were 15 reports of flooding in Kowloon and the New Territories on 5 August. It remained cloudy with isolated heavy or thundery showers for the next couple of days. The low pressure area developed into Tropical Depression Rachel on 6 August and moved towards Taiwan without further affecting Hong Kong.

    As the southwest monsoon became established over the northern part of the South China Sea on 8 August, unsettled weather continued to affect Hong Kong and there were isolated heavy showers over Lantau. Showers were heavy or thundery again from 9 August till 14 August and a minor landslip occurred in Cha Kwo Ling on 12 August.

    Fine weather eventually prevailed on 15 August when a ridge of high pressure extended westwards to cover southeastern China. It was hot on 16 August. Fine and sunny weather persisted for the next few days. It was hazy on 20 August and very hot the next day with temperatures rising to 35.1 degrees, the highest so far this year, as Severe Tropical Storm Sam approached Hong Kong from the southeast.

    The first rainband of Sam affected Hong Kong on 21 August. Winds freshened from the north that evening and became strong early next morning. Heavy rain began to buffet the territory on the afternoon of 22 August and gale force winds from the northwest affected the western areas of Hong Kong. Sam crossed the northeastern part of the New Territories that evening. Gale force winds turned southwesterly in the evening and moderated the next day but heavy rain continued. Temperatures dropped to 23.7 degrees, the lowest in the month, in heavy showers on 23 August. A total of 609.4 millimetres of rainfall was recorded from 22 to 25 August. Considerable damage was inflicted on Hong Kong by Sam and the rainstorms that followed in its wake. An accident occurred while an aircraft was trying to land at the airport at Chek Lap Kok. There were 310 cases of flooding and 200 cases of landslides.

    Sunny periods developed at last on 26 August as a ridge of high pressure became established over southeastern China. It remained fine the next day.

    The weather turned cloudy with heavy showers and thunderstorms on 28 August. Showers eased off on 30 August and there were sunny periods. It remained cloudy with sunny periods apart from isolated thundery showers on the last day of the month.

    Seven tropical cyclones occurred in the South China Sea and the western North Pacific in the month.

    During the month, a total of 85 aircraft was diverted due to adverse weather.