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April 2000 was marked by unsettled weather and heavy rain due to the frequent passages of troughs of low pressure and a very active southwest monsoon.
The total rainfall of 547.7 millimetres, more than three times the normal figure of 161.5 millimetres, was the highest for April since records began in 1884. The daily rainfall of 172.5 millimetres on April 23 was the second highest for April.
The accumulated rainfall in the first four months of the year amounted to 686.5 millimetres, more than double the normal figure of 299.8 millimetres and ranking the third highest for the same period.
The Observatory issued four Red and one Black Rainstorm Warning Signals this month, the highest on record for April. The Red and Black Rainstorm Warning Signals on April 3 and April 23 respectively were also the earliest for the two Signals in a year since the introduction of the Rainstorm Warning System in 1992.
This April was warmer than usual with the monthly mean temperature of 23.1 degrees, 0.9 degrees higher than normal. The monthly mean pressure of 1 011.1 hectoPascals was the third lowest for April.
An extensive rainband brought thunderstorms to Hong Kong on the morning of April 1. Clouds thinned out in the afternoon and there were sunny intervals.
Under the influence of a trough of low pressure over southern China, there were squally thunderstorms and heavy rain on the late evening of April 2. More than 50 millimetres of rainfall were recorded over the northern part of the New Territories. There were 38 reports of flooding.
Unsettled weather persisted the next day. Thundery showers brought more than 100 millimetres of rainfall to urban areas.
As the trough of low pressure moved into the northern part of the South China Sea and dissipated, the weather became foggy with drizzle on April 4. Visibility inside the harbour fell to around 300 metres in the morning.
A weak surge of the northeast monsoon reached the coastal areas the next day. There were sunny intervals as well as a few rain patches.
A ridge of high pressure over southeastern China brought fresh easterly winds to the coast of Guangdong on April 6. Under its influence, the weather remained cloudy with a few light rain patches. These conditions persisted for the next few days.
Winds became light variable on April 10. There were drizzle and mist with visibility falling to 1 100 metres over the western part of the territory. Visibility improved as winds strengthened from the east in the evening. Winds then moderated gradually and there were sunny intervals in the next couple of days.
Another extensive rainband affected the coastal areas of Guangdong and it became cloudy with rain on April 13.
Under the influence of strong southwesterly winds, heavy rain and thunderstorms affected Hong Kong on April 14. Over 500 millimetres of rainfall were recorded in the western part of the New Territories. Villages were half-submerged in flash floods and water was about a metre deep. There were 128 reports of flooding and 28 landslides. More than 200 hectares of farmland and 35 hectares of fish pond were inundated.
A cold front crossed the coast of Guangdong on the morning of April 15 and the associated northerly winds were strong at first. The weather improved gradually in the afternoon and it turned mainly fine the next day. It was slightly cooler with a minimum temperature of 19.0 degrees on the morning of April 16, the lowest in the month. Generally fine weather prevailed for the next few days.
A trough of low pressure brought showers and isolated squally thunderstorms to the territory on April 21. Weather remained unsettled for the next couple of days. Heavy rain and thunderstorms brought more than 100 millimetres of rainfall to many areas on April 23, with Peng Chau and parts of Lantau Island recording over 200 millimetres. There were altogether 64 reports of flooding and two reports of landslide.
As the trough of low pressure dissipated, rain eased off during the day of April 24 though cloudy to overcast conditions continued.
Under the influence of a southerly airstream, it was hot on April 25 with sunny periods in the afternoon apart from a few showers. Temperatures rose to a maximum of 30.2 degrees in the afternoon, the highest in the month.
Another trough of low pressure crossed the coast of Guangdong on April 26 and again brought thundery showers to Hong Kong in the evening. Rain eased off overnight as the trough moved further southwards. However, heavy showers returned on April 28. Weather remained cloudy with some showers till the end of the month.
No tropical cyclone 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 summarized in Table 1.1. Monthly meteorological figures and departures from normal of April are tabulated in Table 1.2.
Table 1.1 Warnings and Signals in April 2000
Beginning Time Ending Time
Type of Warnings day/month hour day/month hour
/Signals
Strong Monsoon Signal 10/4 2100 11/4 1145
14/4 0620 14/4 1630
15/4 0420 15/4 0900
Rainstorm Warning Signals
Amber 2/4 2215 2/4 2400
Red 3/4 0000 3/4 0550
Amber 3/4 0550 3/4 0730
Amber 3/4 2005 3/4 2230
Amber 14/4 0300 14/4 0535
Red 14/4 0535 14/4 1055
Amber 14/4 1055 14/4 1405
Red 14/4 1405 14/4 1620
Amber 22/4 2340 23/4 0110
Red 23/4 0110 23/4 0150
Black 23/4 0150 23/4 0555
Amber 23/4 0555 23/4 1015
Amber 26/4 1655 26/4 2030
Amber 28/4 1525 28/4 2100
Landslip Warning 3/4 2015 4/4 0215
14/4 0640 15/4 0550
23/4 0225 23/4 1040
Thunderstorm Warning 1/4 0015 1/4 1050
2/4 2140 3/4 0900
3/4 1810 4/4 0500
13/4 1455 14/4 1630
14/4 2345 15/4 0420
21/4 1030 21/4 1730
22/4 0730 22/4 0930
22/4 2150 23/4 2100
26/4 0855 26/4 2200
27/4 2300 28/4 0700
28/4 1045 28/4 2145
29/4 0745 29/4 1345
30/4 2330 1/5 0545
Fire Danger Warnings
Red 16/4 1100 16/4 1800
Table 1.2 Figures and Departures from Normal - April 2000
Meteorological Element Figure of Departure from normal
the month above normal below normal
Mean Daily Maximum Air 25.1 0.2 ----
Temperature (Degree Celsius)
Mean Air Temperature 23.1 0.9 ----
(Degree Celsius)
Mean Daily Minimum Air 21.4 1.2 ----
Temperature (Degree Celsius)
Mean Dew Point 20.4 1.4 ----
(Degree Celsius)
Mean Relative Humidity (%) 85 2 ----
Mean Cloud Amount (%) 85 7 ----
Total Rainfall (mm) 547.7 386.2 ----
Total Bright Sunshine 77.3 ---- 31.6
Duration(hours)
Mean Daily Global Solar 10.35 ---- 2.79
Radiation (MJ/Square Metre)
Total Evaporation (mm) 79.9 ---- 27.0
Remarks : All measurements were made at the Hong Kong Observatory except sunshine, solar radiation and evaporation which were recorded at King's Park Meteorological Station.
End/Tuesday, May 9, 2000
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