April 2018 was warmer than usual with a mean temperature of 23.6 degrees, 1.0 degree above the normal of 22.6 degrees. The month was also much drier than usual with only 28.1 millimetres of rainfall recorded in the month, about one-sixth of the normal of 174.7 millimetres and the eighth lowest on record for April. The accumulated rainfall recorded in the first four months of the year was 117.5 millimetres, a deficit of 65 percent compared to the normal of 336.1 millimetres for the same period.
Under the influence of a ridge of high pressure, the weather in Hong Kong was generally fine and warm on the first five days of the month. A cold front moved across the coast of Guangdong on 6 April. Local weather turned cloudy with a few showers and became significantly cooler when winds strengthened from the north in that afternoon. The weather remained generally cloudy on 7 April with temperature at the Hong Kong Observatory dropping to a minimum of 16.1 degrees in the morning, the lowest of the month. Under the influence of the dry northeast monsoon, it was fine and dry in Hong Kong on 8 April.
With the moderation of the northeast monsoon and the onset of a southerly airstream, the temperatures rose gradually on 9 - 13 April. With plenty of sunshine, the temperature at the Hong Kong Observatory reached a maximum of 30.6 degrees on 13 April, the highest of the month.
A cold front moved across the coast of Guangdong on the night of 14 April and the associated northeast monsoon brought cooler weather to Hong Kong in next two days. Under the influence of an upper-air disturbance, there were also showers and squally thunderstorms on 15 April with more than 10 millimetres of rainfall recorded generally over the territory.
With a broad area of cloud band covering the coastal areas of Guangdong, it was mainly cloudy with a mixture of isolated showers and sunny intervals in Hong Kong on 17 – 21 April. Under the influence of a southerly airstream, apart from coastal fog on the morning of 23 April, the weather was hot with sunny periods on 22 - 23 April.
A trough of low pressure moved across the coast of Guangdong on 24 April. The weather in Hong Kong turned cloudy with showers and a few thunderstorms during the day. More than 20 millimetres of rainfall were recorded over Hong Kong Island, Tuen Mun and Lantau Island. Under the influence of a broad cloud band over the south China coast, local weather remained cloudy with isolated showers on 25 – 27 April.
With the onset of an anticyclone aloft, the weather gradually became mainly fine and hot towards the end of the month apart from isolated showers.
One tropical cyclone occurred over the South China Sea and the western North Pacific in the month.
Details of issuance and cancellation of various warnings/signals in the month are summarized in Tables 1.1 to 1.3. Monthly meteorological figures and departures from normal for April are tabulated in Table 2.
|Beginning Time||Ending Time|
|6 / 4||1250||7 / 4||1445|
|Beginning Time||Ending Time|
|15 / 4||0910||15 / 4||1300|
|24 / 4||0915||24 / 4||1300|
|Colour||Beginning Time||Ending Time|
|Yellow||1 / 4||0600||2 / 4||2100|
|Yellow||5 / 4||0600||5 / 4||1930|
|Red||7 / 4||0930||8 / 4||2045|
|Meteorological Element||Figure of the Month||Departure from Normal*|
|Mean Daily Maximum Air Temperature||26.9 degrees C||1.9 degrees above normal|
|Mean Air Temperature||23.6 degrees C||1.0 degree above normal|
|Mean Daily Minimum Air Temperature||21.7 degrees C||0.9 degree above normal|
|Mean Dew Point Temperature||19.4 degrees C||normal|
|Mean Relative Humidity||78 %||5 % below normal|
|Mean Cloud Amount||71 %||10 % below normal|
|Total Rainfall||28.1 mm||146.6 mm below normal|
|Number of hours of Reduced VisibilityΔ||26 hours||57.1 hours below normal§|
|Total Bright Sunshine Duration||143.5 hours||41.8 hours above normal|
|Mean Daily Global Solar Radiation||14.71 Megajoule / square metre||3.11 Megajoule above normal|
|Total Evaporation||98.2 mm||14.4 mm above normal|
|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 and visibility which was observed at the Hong
Kong International Airport.
The visibility readings at the Hong Kong International Airport are based on hourly observations by professional meteorological observers in 2004 and before, and average readings over the 10-minute period before the clock hour of the visibility meter near the middle of the south runway from 2005 onwards. The change of the data source in 2005 is an improvement of the visibility assessment using instrumented observations following the international trend.
* Departure from 1981 - 2010 climatological normal, except for number of hours of reduced visibility
§ Departure from mean value between 1997 and 2017
|Remarks :||Extremely high: above 95th percentile
Above normal: between 75th and 95th percentile
Normal: between 25th and 75th percentile
Below normal: between 5th and 25th percentile
Extremely low: below 5th percentile
Percentile and 5-day running average values are
computed based on the data from 1981 to 2010