The Weather of April 2021
Under the influence of a strong easterly airstream, the weather of Hong Kong was mainly cloudy with sunny intervals on 18 - 20 April. With the moderation of the easterly airstream and dominance of an anticyclone aloft, local weather became generally fine on 21 - 23 April. With plenty of sunshine, the temperature at the Observatory soared to a maximum of 32.6 degrees on the afternoon of 23 April, the highest of the month. The onset of an easterly airstream brought cloudier weather and a few rain patches to Hong Kong on 24 April. With a trough of low pressure edging closer to the coastal areas of Guangdong, the weather of Hong Kong was cloudy with some showers on 25 - 28 April. More than 5 millimetres of rainfall were generally recorded over the territory on 27 April. With the departure of the trough of low pressure and setting in of a dry continental airstream, local weather improved with sunny periods on 29 April and became mainly fine and hot during the day on the last day of the month.
One tropical cyclone occurred over the South China Sea and the western North Pacific in April 2021.
Details of issuance and cancellation of various warnings/signals in the month are summarized in Tables 1.1 to 1.2. Monthly meteorological figures and departures from normal for April are tabulated in Table 2.
Warnings and Signals issued in April 2021
|Beginning Time||Ending Time|
|19 / 4||0500||20 / 4||0540|
|Colour||Beginning Time||Ending Time|
|Yellow||2 / 4||1100||2 / 4||1800|
|Yellow||3 / 4||0600||3 / 4||1800|
|Yellow||4 / 4||0600||4 / 4||1645|
|Yellow||5 / 4||1145||5 / 4||2000|
|Yellow||6 / 4||0600||6 / 4||1800|
|Yellow||10 / 4||0600||10 / 4||1945|
|Yellow||11 / 4||0600||11 / 4||1945|
|Yellow||18 / 4||0600||18 / 4||1800|
|Meteorological Element||Figure of the Month||Departure from Normal(1991-2020)||Departure from Normal (1981-2010)|
|Mean Daily Maximum Air Temperature||27.0 degrees C||1.4 degrees above normal||2.0 degrees above normal|
|Mean Air Temperature||24.1 degrees C||1.1 degrees above normal||1.5 degrees above normal|
|Mean Daily Minimum Air Temperature||22.4 degrees C||1.3 degrees above normal||1.6 degrees above normal|
|Mean Dew Point Temperature||20.1 degrees C||0.4 degree above normal||0.7 degree above normal|
|Mean Relative Humidity||79 %||4 % below normal||4 % below normal|
|Mean Cloud Amount||71 %||6 % below normal||10 % below normal|
|Total Rainfall||32.5 mm||120.5 mm below normal||142.2 mm below normal|
|Number of hours of Reduced VisibilityΔ||2 hours||73.7 hours below normal§||73.7 hours below normal§|
|Total Bright Sunshine Duration||131.4 hours||18.2 hours above normal||29.7 hours above normal|
|Mean Daily Global Solar Radiation||14.42 Megajoule / square metre||1.9 Megajoule above normal||2.82 Megajoule above normal|
|Total Evaporation||95.8 mm||8.6 mm above normal||12.0 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 mean value between 1997 and 2020|
|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 1991 to 2020
Extract of Meteorological Observations in Hong Kong for April 2021