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  • The Weather of April 2019

  • 3 May 2019

    The exceptionally warm weather in the first quarter of 2019 continued in April 2019, mainly attributing to the warmer than normal sea surface temperature and stronger than usual southerly flow in the lower atmosphere over the northern part of the South China Sea. The mean minimum temperature of 22.9 degrees and mean temperature of 24.7 degrees were both 2.1 degrees above the corresponding normal and respectively one of the highest and second highest on record for April. The mean maximum temperature of 27.2 degrees was 2.2 degrees above the normal and the fifth highest on record for April. The monthly rainfall was 185.8 millimetres, about 6 percent above the normal of 174.7 millimetres. The accumulated rainfall recorded in the first four months of the year was 445.7 millimetres, about 33 percent above the normal figure of 336.1 millimetres for the same period.

    Under the influence of a fresh to strong northeast monsoon, it was rather windy with a mixture of sunny periods and a few rain patches on the first four days of April. The northeast monsoon also brought the minimum temperature at the Hong Kong Observatory down to 18.9 degrees on 2 April, the lowest of the month. With the moderation of the northeast monsoon, local weather became generally fine and warm apart from some haze under light wind conditions on 5 April. Affected by a southerly airstream and an anticyclone aloft, it was generally fine and hot on 6 - 10 April.

    With a trough of low pressure over Guangdong moving across the coastal areas on the night of 11 April, local weather became cloudy in the afternoon and there were heavy showers and squally thunderstorms that night. More than 30 millimetres of rainfall were recorded over the northern part of the New Territories, and the rainfall even exceeded 70 millimetres over parts of the area. Together with the setting in of a strong easterly airstream, it was cloudy and windy with a few showers and thunderstorms in Hong Kong on 12 April. The weather remained showery with a few squally thunderstorms in the next four days under the influence of the trough of low pressure lingering over the coastal areas of Guangdong and the northern part of the South China Sea. As the unstable weather moved to the south of Hong Kong, there were sunny periods on 17 April.

    With the onset of an active southerly airstream over the coast of Guangdong, the weather of Hong Kong deteriorated with outbreaks of heavy showers and squally thunderstorms on the night of 18 April. The weather became even more unsettled in the next two days. There were heavy showers on 19 April with 75.8 millimetres of rainfall recorded at the Observatory, making it the wettest Good Friday on record. Rainfall even exceeded 100 millimetres over parts of the New Territories. On 20 April, a band of intense thunderstorm packed frequent lightning, heavy rain and severe squalls swept across Hong Kong in the afternoon and necessitated the issuance of the first Red Rainstorm Warning in the year. More than 40 millimetres of rainfall were generally recorded over the territory, and the rainfall even exceeded 70 millimetres over parts of the New Territories. The severe squalls also induced gusts exceeding 100 kilometres per hour in some places of the Victoria Harbour. The maximum gust recorded at King’s Park was 117 kilometers per hour. Excluding tropical cyclone cases, this is the highest record since the operation of the automatic weather station in King’s Park in 1992. Moreover, 9094 cloud-to-ground lightning strokes were detected in Hong Kong on that day, the highest on record for April since the launch of the lightning location system in 2005. During the inclement weather in that afternoon, a person died after being struck by lightning while hiking in Tai O and two persons were tragically drowned when a sampan capsized in seas off Tung Lung Chau. There were also reports of tree failure and scaffolding collapsing in different parts of the territory.

    Affected by a strengthening anticyclone aloft, apart from a few showers, sunny skies emerged gradually on 21 and 22 April. The weather improved further and became fine and hot on 23 - 26 April. The daily mean temperature of 28.5 degrees on 25 April was one of the highest on record for April and the maximum temperature at the Observatory soared to 31.5 degrees on 26 April, the highest of the month. Under the influence of a strong easterly airstream and an upper-air disturbance, there were heavy showers and squally thunderstorms in Hong Kong on the afternoon of 27 April. As the easterlies abated gradually, it was mainly cloudy with the showers easing off during the day on 28 April. With the easterly airstream replacing by a southerly airstream, there were sunny periods on April 29. Under the influence of a trough of low pressure, local weather became cloudy with occasional showers and squally thunderstorms on the last day of the month.


Radar imagery at 2:18pm, April 20, when a band of intense thunderstorm moved across Hong Kong
Radar imagery at 2:18pm, April 20, when a band of intense thunderstorm moved across Hong Kong


     There was no tropical cyclone 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 1.5.  Monthly meteorological figures and departures from normal for April are tabulated in Table 2.
 

Warnings and Signals issued in April 2019

Table 1.1   Strong Monsoon Signal
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
31 / 3 0715 1 / 4 1500
12 / 4 1050 13 / 4 1140
27 / 4 1945 28 / 4 0140


Table 1.2   Rainstorm Warning Signals
Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Amber 11 / 4 2150 11 / 4 2345
Amber 18 / 4 2135 18 / 4 2235
Amber 19 / 4 1440 19 / 4 1910
Amber 20 / 4 1340 20 / 4 1420
Red 20 / 4 1420 20 / 4 1520
Amber 20 / 4 1520 20 / 4 1645
Amber 27 / 4 1600 27 / 4 1810


Table 1.3   Thunderstorm Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
11 / 4 2120 12 / 4 0210
12 / 4 1105 12 / 4 1600
14 / 4 1310 14 / 4 1645
16 / 4 1010 16 / 4 1400
18 / 4 1705 19 / 4 0815
19 / 4 1105 19 / 4 2000
20 / 4 1245 20 / 4 1800
27 / 4 0055 27 / 4 0140
27 / 4 1430 27 / 4 1930
30 / 4 1000 30 / 4 1400


Table 1.4   Fire Danger Warnings
Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Yellow 5 / 4 0600 5 / 4 1915
Yellow 7 / 4 0600 7 / 4 1800


Table 1.5   Special Announcement on Flooding
in the Northern New Territories
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
11 / 4 2225 12 / 4 0100
20 / 4 1430 20 / 4 1700


Table 2   Figures and Departures from Normal - April 2019
Meteorological Element Figure of the Month Departure from Normal*
Mean Daily Maximum Air Temperature 27.2 degrees C 2.2 degrees above normal
Mean Air Temperature 24.7 degrees C 2.1 degrees above normal
Mean Daily Minimum Air Temperature 22.9 degrees C 2.1 degrees above normal
Mean Dew Point Temperature 21.7 degrees C 2.3 degrees above normal
Mean Relative Humidity 84 % 1 % above normal
Mean Cloud Amount 76 % 5 % below normal
Total Rainfall 185.8 mm 11.1 mm above normal
Number of hours of Reduced VisibilityΔ 18 hours 62.5 hours below normal§
Total Bright Sunshine Duration 123.1 hours 21.4 hours above normal
Mean Daily Global Solar Radiation 13.37 Megajoule / square metre 1.77 Megajoule above normal
Total Evaporation 84.4 mm 0.6 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 1981 - 2010 climatological normal, except for number of hours of reduced visibility

  §   Departure from mean value between 1997 and 2018

daily values of selected meteorological elements for HK for April 2019

The percentile map of mean temperature of April 2019

  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


Extract of Meteorological Observations in Hong Kong for April 2019