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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 January 1 was 1676.0 millimetres, two per cent below the normal figure of 1707.4 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 August 22 and necessitated the hoisting of Gale or Storm Signal No 8. The Black Rainstorm Warning Signal was issued on August 23 and 24 and the Landslip Warning was also issued on August 22 and 25.
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 August 2. Showers became thundery on August 3 although sunny intervals also managed to appear.
Over 100 millimetres of rainfall were recorded in Tseung Kwan O and Cheung Sha Wan on August 4 and 5 respectively. There were 15 reports of flooding in Kowloon and the New Territories on August 5. 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 August 6 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 August 8, unsettled weather continued to affect Hong Kong and there were isolated heavy showers over Lantau. Showers were heavy or thundery again from August 9 till August 14 and a minor landslip occurred in Cha Kwo Ling on August 12.
Fine weather eventually prevailed on August 15 when a ridge of high pressure extended westwards to cover southeastern China. It was hot on August 16. Fine and sunny weather persisted for the next few days. It was hazy on August 20 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 August 21. Winds freshened from the north that evening and became strong early next morning. Heavy rain began to buffet the territory on the afternoon of August 22 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 August 23. A total of 609.4 millimetres of rainfall was recorded from August 22 to 25. Considerable damage was inflicted on Hong Kong by Sam and the rainstorms that followed in its wake. There were 310 cases of flooding and 200 cases of landslides.
Sunny periods developed at last on August 26 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 August 28. Showers eased off on August 30 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.
Details of the issuance/ hoisting and cancellation/lowering of various warnings/signals in the month are summarised in Table 1.1. Monthly meteorological figures and departures from normal of August are tabulated in Table 1.2.
Table 1.1 Warnings and Signals in August 1999 Beginning Time Ending Time Type of Warnings/Signals day/month hour day/month hour
Tropical Cyclone Warning Signals
Name of Signal
Tropical Cyclone Number
Sam 1 20/8 1615 22/8 0230
3 22/8 0230 22/8 1230
8 NW 22/8 1230 22/8 2010
8 SW 22/8 2010 23/8 0350
3 23/8 0350 23/8 2100
Rainstorm Warning Signals
Amber 4/8 0735 4/8 0940
Amber 5/8 1825 5/8 2130
Amber 10/8 0500 10/8 0700
Amber 11/8 1735 11/8 1925
Amber 22/8 1355 23/8 0525
Red 23/8 0525 23/8 0613
Black 23/8 0613 23/8 1200
Amber 23/8 1200 23/8 1900
Amber 24/8 0205 24/8 0300
Red 24/8 0300 24/8 0435
Black 24/8 0435 24/8 1000
Red 24/8 1000 24/8 1200
Amber 25/8 0330 25/8 0405
Red 25/8 0405 25/8 0545
Amber 25/8 0545 25/8 1600
Landslip Warnings 22/8 2145 24/8 1345
25/8 0350 26/8 0550
Thunderstorm Warnings 2/8 1520 2/8 1720
3/8 0900 3/8 1200
3/8 1800 3/8 2000
4/8 0410 4/8 1000
4/8 2330 5/8 0600
5/8 1255 5/8 2255
6/8 1340 6/8 1640
6/8 2115 6/8 2315
7/8 1110 7/8 1200
8/8 0445 8/8 0945
9/8 0055 9/8 1030
9/8 1555 9/8 1755
9/8 2300 10/8 0800
10/8 1145 10/8 1445
11/8 0055 11/8 0900
11/8 1130 11/8 2000
12/8 0535 12/8 1430
12/8 2230 13/8 1630
13/8 1810 14/8 1030
21/8 1505 21/8 1805
24/8 0035 24/8 1200
25/8 0135 25/8 1600
28/8 0845 28/8 1600
28/8 2300 29/8 0800
29/8 0850 29/8 1145
31/8 0345 31/8 0545
Table 1.2 Figures and Departures from Normal - August
1999
Meteorological Element Figure of Departure from
normal
the month above normal below
normal
Total Bright Sunshine 138.5 ---- 68.5
Duration(hours)
Mean Daily Global Solar 12.56 ---- 5.05
Radiation (MJ/Square Metre)
Total Rainfall (mm) 892.0 500.6 ----
Mean Cloud Amount (%) 80 14 ----
Mean Relative Humidity (%) 82 1 ----
Mean Daily Maximum Air 30.8 ---- 0.5
Temperature
(Degree Celsius)
Mean Air Temperature 28.3 ---- 0.1
(Degree Celsius)
Mean Daily Minimum Air 26.2 ---- 0.1
Temperature (Degree Celsius)
Mean Dew Point 24.9 0.1 --
--
(Degree Celsius)
Total Evaporation (mm) 135.2 ---- 21.7
End/Friday, September 10, 1999
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