With relatively less cold air intrusion from the north, April 2021 continued to be much warmer than usual in Hong Kong. The monthly mean minimum temperature of 22.4 degrees, monthly mean maximum temperature of 27.0 degrees and monthly mean temperature of 24.1 degrees were 1.3 degrees, 1.4 degrees and 1.1 degrees above their corresponding normals (or 1.6 degrees, 2.0 degrees and 1.5 degrees above their corresponding 1981-2010 normals) and respectively the fifth, seventh and ninth highest on record for April. With the dominance of upper-air anticyclone over southern China for most of the time in the month, April 2021 was also much drier than usual with a total rainfall of only 32.5 millimetres, about 21 percent of the normal figure of 153.0 millimetres (or 19 percent of the 1981-2010 normal of 174.7 millimetres). The accumulated rainfall recorded in the first four months of the year was 98.1 millimetres, a deficit of 67 percent when compared to the normal of 300.4 millimetres (or 71 percent below the 1981-2010 normal of 336.1 millimetres) and the tenth lowest on record for the same period.

With the establishment of an anticyclone aloft and the departure of the cloud band from the South China coast, the weather of Hong Kong was generally fine with hot afternoons on 1 - 3 April. Under the influence of a fresh to strong easterly airstream, local weather became mainly cloudy and slightly cooler with a few rain patches on 4 - 5 April.

Affected by an anticyclone aloft southern China, local weather improved with sunny periods on 6 - 7 April. Under the influence of an easterly airstream, the weather became cloudier on 8 April. With the strengthening of the easterly airstream and affected by the upper-air disturbance, it was cloudy with a few showers and cooler on 9 April. More than 5 millimetres of rainfall were recorded over most parts of the territory. The temperature at the Observatory dropped to 19.7 degrees under the rain in that morning, the lowest of the month.

With the setting in of a relatively dry easterly airstream, the weather of Hong Kong turned generally fine and dry during the day on 10 - 11 April. Under the influence of an anticyclone aloft, the weather remained generally fine and hot during the day on 12 - 13 April. Affected by a fresh to strong easterly airstream, local weather became cloudier with a few rain patches on 14 April. With a trough of low pressure edging towards the coast of Guangdong, it was cloudy with some showers in Hong Kong on 15 - 17 April.

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

Table 1.1   Strong Monsoon Signal

Beginning Time		Ending Time
Day/Month	HKT	Day/Month	HKT
19 / 4	0500	20 / 4	0540

Table 1.2   Fire Danger Warnings

Colour	Beginning Time		Ending Time
	Day/Month	HKT	Day/Month	HKT
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

Table 2   Figures and Departures from Normal - April 2021

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.
Δ	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. Before 10 October 2007, the number of hours of reduced visibility at the Hong Kong International Airport in 2005 and thereafter displayed in this web page was based on hourly visibility observations by professional meteorological observers. Since 10 October 2007, the data have been revised using the average visibility readings over the 10-minute period before the clock hour, as recorded by the visibility meter near the middle of the south runway.
§   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