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The Weather of July 2021

3 August 2021

Owing to the stronger than normal upper-air anticyclone over southern China, July 2021 was unusually hot in Hong Kong. The monthly mean minimum temperature of 27.7 degrees, monthly mean maximum temperature of 32.6 degrees and monthly mean temperature of 29.7 degrees were 0.8 degrees, 1.0 degree and 0.8 degrees above their corresponding normals (or 0.9 degrees, 1.2 degrees and 0.9 degrees above their corresponding 1981-2010 normals) and respectively the second, third and fourth highest on record for July. The monthly rainfall was 379.5 millimetres, slightly below the normal figure of 385.8 millimetres (or slightly above the 1981-2010 normal of 376.5 millimetres). The accumulated rainfall recorded in the first seven months of the year was 1170.6 millimetres, about 20 percent below the normal figure of 1468.2 millimetres (or 21 percent below the 1981-2010 normal of 1473.3 millimetres) for the same period.

Under the influence of a southwesterly airstream, the weather of Hong Kong was a mixture of sunshine and showers on the first day of the month. Dominated by an anticyclone aloft, apart from a few showers, it was generally fine and very hot on 2 – 5 July.

Meanwhile, the area of low pressure near Luzon intensified into a tropical depression on the morning of 5 July. It skirted past the southwestern coast of Taiwan on the morning of 6 July and weakened into an area of low pressure near the Taiwan Strait that afternoon. Another area of low pressure over the central part of the South China Sea intensified into a tropical depression on the night of 5 July. It tracked northwestwards towards Hainan Island on 6 July. The tropical depression moved across the southwestern part of Hainan Island and entered Beibu Wan on 7 July. It made landfall over the northern part of Vietnam and then weakened into an area of low pressure on 8 July. Affected by the outer rainbands of these two tropical depressions, there were occasional squally showers and thunderstorms in Hong Kong on 6 July and the early part of 7 July. With showers abating gradually during the day, local weather became very hot with sunny intervals on 7 July.

Under the influence of the subtropical ridge and subsequent anticyclone aloft, apart from a few isolated showers, generally fine and very hot weather persisted during 8 – 15 July. With plenty of sunshine, the temperature at the Observatory soared to 35.4 degrees on 15 July, the highest of the month. It was also one of the third highest temperature in July since records began in 1884. Moreover, dust devils were reported near the third runway construction site of the Hong Kong International Airport on 14 July and 15 July.

Under the influence of a broad area of low pressure over the northeastern part of the South China Sea, the weather of Hong Kong was mainly cloudy with a few showers and isolated thunderstorms on 16 – 17 July. More than 30 millimetres of rainfall were recorded over parts of the territory and rainfall even exceeded 70 millimetres over Tai Po District on these two days.

The area of low pressure over the northern part of the South China Sea intensified into a tropical depression later on 18 July. It lingered over the northern part of the South China Sea and further developed into a tropical storm and was named Cempaka on the morning of 19 July. Cempaka moved northwestwards slowly towards the coast of western Guangdong and rapidly intensified into a typhoon on the early morning of 20 July. It made landfall near Yangjiang and weakened into a severe tropical storm that night. Cempaka then moved across the coastal areas of western Guangdong and Guangxi and weakened gradually into a tropical depression on 21 – 22 July. It entered Beibu Wan on the morning of 23 July and weakened into an area of low pressure over Beibu Wan on 25 July. Affected by the rainbands associated with Cempaka, there were occasional heavy showers and squally thunderstorms in Hong Kong on 18 – 21 July. More than 200 millimetres of rainfall were generally recorded over the territory, and rainfall even exceeded 400 millimetres over Sai Kung District on these four days. A man died after being washed away by floods while hiking in Ma On Shan on 20 July. Under the rain, the temperature at the Observatory dropped to a minimum of 24.9 degrees on 18 July, the lowest of the month.

Tropical Cyclone In-fa lingered over the seas east of Taiwan on 22 July. It then moved towards eastern China and made landfall over the coast of the northern part of Zhejiang four days later. Affected by the subsiding air outside the circulation of In-fa and the subsequent light wind conditions in Hong Kong, apart from a few showers, local weather was sunny and very hot on 22 – 28 July. The oppressive heat also triggered heavy showers and squally thunderstorms on 23 – 28 July. The showers were particularly heavy on 24 – 25 July, more than 50 millimetres of rainfall were recorded over parts of the New Territories and Hong Kong Island on these two days. Under the influence of an active southwesterly airstream, the weather became unsettled with occasional showers and thunderstorms on the last three days of the month. More than 40 millimetres of rainfall were recorded over most parts of the territory and rainfall even exceeded 100 millimetres over Sheung Shui, Tseung Kwan O and the northern part of Lantau Island on these three days.

Five tropical cyclones occurred over the South China Sea and the western North Pacific in July 2021.

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 July are tabulated in Table 2.
 

Warnings and Signals issued in July 2021

Table 1.1   Tropical Cyclone Warning Signals
Name of
Tropical Cyclone
Signal
Number
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
no name 1 6 / 7 0415 7 / 7 1410
CEMPAKA 1 18 / 7 2140 19 / 7 1610
3 19 / 7 1610 20 / 7 1320
1 20 / 7 1320 20 / 7 1940


Table 1.2   Rainstorm Warning Signals
Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Amber 18 / 7 0620 18 / 7 0745
Amber 19 / 7 0235 19 / 7 0350
Amber 19 / 7 1230 19 / 7 1405
Amber 20 / 7 1140 20 / 7 1540
Amber 25 / 7 0005 25 / 7 0300
Amber 30 / 7 0715 30 / 7 0840


Table 1.3   Thunderstorm Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
5 / 7 1100 5 / 7 1230
5 / 7 1350 5 / 7 1530
6 / 7 1352 6 / 7 1930
6 / 7 2300 7 / 7 0900
14 / 7 1322 14 / 7 1600
16 / 7 0735 16 / 7 1800
17 / 7 0520 17 / 7 1300
18 / 7 0315 18 / 7 1700
18 / 7 2055 19 / 7 1900
20 / 7 0215 20 / 7 1640
21 / 7 0255 21 / 7 0730
21 / 7 0955 21 / 7 1200
21 / 7 1340 21 / 7 1630
23 / 7 1625 23 / 7 1800
23 / 7 2250 24 / 7 0400
24 / 7 0440 24 / 7 0830
24 / 7 1325 24 / 7 1930
24 / 7 2140 25 / 7 0430
25 / 7 1525 25 / 7 1800
26 / 7 1350 26 / 7 1730
27 / 7 1355 27 / 7 1730
27 / 7 1920 27 / 7 2330
28 / 7 0615 28 / 7 0715
28 / 7 1015 28 / 7 2000
29 / 7 0735 29 / 7 0945
29 / 7 1030 29 / 7 1630
30 / 7 0125 30 / 7 1030
31 / 7 0010 31 / 7 0930
31 / 7 1445 31 / 7 1715
31 / 7 2345 1 / 8 0330


Table 1.4   Very Hot Weather Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
2 / 7 1115 5 / 7 1900
7 / 7 1425 16 / 7 0945
22 / 7 1330 23 / 7 2330
24 / 7 1000 24 / 7 1900
25 / 7 0745 27 / 7 2010
28 / 7 0645 28 / 7 1615
31 / 7 1145 31 / 7 1620


Table 1.5   Special Announcement on Flooding
in the Northern New Territories
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
20 / 7 1225 20 / 7 1620
25 / 7 0110 25 / 7 0325
29 / 7 1435 29 / 7 1640


Table 2   Figures and Departures from Normal - July 2021
Meteorological Element Figure of the Month Departure from Normal(1991-2020) Departure from Normal (1981-2010)
Mean Daily Maximum Air Temperature 32.6 degrees C 1.0 degree above normal 1.2 degrees above normal
Mean Air Temperature 29.7 degrees C 0.8 degrees above normal 0.9 degrees above normal
Mean Daily Minimum Air Temperature 27.7 degrees C 0.8 degrees above normal 0.9 degrees above normal
Mean Dew Point Temperature 25.8 degrees C 0.6 degrees above normal 0.7 degrees above normal
Mean Relative Humidity 80 % 1 % below normal 1 % below normal
Mean Cloud Amount 71 % 1 % below normal 2 % above normal
Total Rainfall 379.5 mm 6.3 mm below normal 3.0 mm above normal
Number of hours of Reduced VisibilityΔ 0 hour 12.5 hours below normal§ 12.5 hours below normal§
Total Bright Sunshine Duration 203.5 hours 6.2 hours above normal 8.5 hours below normal
Mean Daily Global Solar Radiation 18.43 Megajoule / square metre 1.21 Megajoule above normal 1.26 Megajoule above normal
Total Evaporation 130.5 mm 11.5 mm below normal 15.7 mm below 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

daily values of selected meteorological elements for HK for July 2021

The percentile map of mean temperature of July 2021

  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 July 2021