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The Weather of August 2021

2 September 2021

August 2021 was characterized by cloudier than usual weather with localized heavy rain over parts of the New Territories. The mean amount of cloud in the month was 77 percent, 7 percent above the normal of 70 percent. As for monthly rainfall, while over 600 millimetres of rainfall were recorded in parts of the North District of the New Territories, the monthly rainfall recorded at the Observatory was 350.5 millimetres, about 23 percent below the normal figure of 453.2 millimetres (or 19 percent below the 1981-2010 normal of 432.2 millimetres). The accumulated rainfall recorded in the first eight months of the year was 1521.1 millimetres, about 21 percent below the normal figure of 1921.5 millimetres (or 20 percent below the 1981-2010 normal of 1905.5 millimetres) for the same period. The monthly mean temperature of 28.8 degrees was near the normal figure of 28.7 degrees (or 0.2 degrees above the 1981-2010 normal). Mainly attributing to the exceptionally hot weather in July 2021, the summer of this year from June to August was much hotter than usual. The mean temperature of 29.1 degrees was the sixth highest on record for the same period.

Under the influence of a southwesterly airstream, the weather of Hong Kong was a mixture of sunny periods, showers and thunderstorms on the first two days of the month. The showers were rather heavy in some areas on 1 August with more than 70 millimetres of rainfall recorded over Lamma Island. Meanwhile, an area of low pressure over the northern part of the South China Sea intensified into a tropical depression on the night of 2 August. It moved generally eastwards slowly across the northern part of the South China Sea in the next two days. The tropical depression developed into a tropical storm and was named Lupit on the morning of 4 August. Lupit then moved northeastwards and skirted past the coastal areas of Fujian on 5 – 6 August. It continued to track northeastwards towards Japan in the next two days and evolved into an extratropical cyclone over the seas north of Honshu of Japan on 9 August.

Affected by the rainbands associated with Lupit, there were occasional heavy showers and squally thunderstorms in Hong Kong on 3 – 5 August. More than 100 millimetres of rainfall were recorded over most parts of the territory, and rainfall even exceeded 140 millimetres over the central part of Hong Kong Island, the northern part of New Territories and Lantau Island on these three days. With the departure of Lupit, a southwesterly airstream continued to bring unstable weather to the coastal areas of Guangdong on 6 – 10 August. Locally, it was mainly cloudy with occasional showers and thunderstorms on these few days. The showers were particularly heavy in some areas of the New Territories with daily rainfall over 100 millimetres on 6, 7 and 9 August. The weather became less showery with sunny periods on 11 – 13 August.

Under the influence of an upper-air disturbance, there were some showers and thunderstorms in Hong Kong on 14 – 15 August. The showers were heavy in the North District of the New Territories on 14 August with more than 70 millimetres of rainfall recorded. Affected by a southwesterly airstream, local weather was a mixture of sunshine and showers on 16 August. Dominated by an anticyclone aloft, apart from isolated morning showers, the weather became generally fine and very hot on the afternoon of 17 August.

Against the background of a southerly flow over the South China coast, it was hot with sunny periods in Hong Kong on 18 – 19 August. Under light wind conditions, the day heating also triggered localized heavy showers and thunderstorms on these two days. More than 90 millimetres of rainfall were recorded over San Tin and Ngau Tam Mei on 19 August. Dominated by an anticyclone aloft, apart from a few showers, it was mainly fine and very hot for most of the time in the following week. With plenty of sunshine, the temperature at the Observatory soared to 34.4 degrees on 25 August, the highest of the month. Under the influence of an upper-air disturbance, there were occasional showers and thunderstorms on 27 – 29 August. The temperature at the Observatory dropped to a minimum of 23.4 degrees during the thunderstorms on the morning of 27 August, the lowest of the month. While there were sunny periods on 30 August, the upper-air disturbance again brought some showers and a few thunderstorms to Hong Kong on the last day of the month.

Four tropical cyclones occurred over the South China Sea and the western North Pacific in August 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 August are tabulated in Table 2.
 

Warnings and Signals issued in August 2021

Table 1.1   Tropical Cyclone Warning Signals
Name of
Tropical Cyclone
Signal
Number
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
LUPIT 1 2 / 8 2140 3 / 8 1625
3 3 / 8 1625 4 / 8 0420
1 4 / 8 0420 4 / 8 1820


Table 1.2   Rainstorm Warning Signals
Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Amber 4 / 8 1710 4 / 8 2215
Amber 6 / 8 0035 6 / 8 0130
Red 6 / 8 0130 6 / 8 0355
Amber 6 / 8 0355 6 / 8 0435
Amber 9 / 8 0040 9 / 8 0255
Amber 29 / 8 1005 29 / 8 1215


Table 1.3   Thunderstorm Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
31 / 7 2345 1 / 8 0330
1 / 8 1330 1 / 8 1635
2 / 8 1126 2 / 8 1330
2 / 8 1724 2 / 8 2140
3 / 8 0435 3 / 8 0830
4 / 8 0355 4 / 8 0500
4 / 8 1418 5 / 8 0145
5 / 8 0325 5 / 8 0455
5 / 8 0715 5 / 8 0830
5 / 8 1255 6 / 8 0600
6 / 8 1330 6 / 8 1530
6 / 8 2010 6 / 8 2120
7 / 8 0135 7 / 8 0730
7 / 8 1250 7 / 8 1600
8 / 8 1515 8 / 8 1830
8 / 8 2305 9 / 8 0830
9 / 8 1605 9 / 8 2025
10 / 8 0705 10 / 8 0830
10 / 8 0935 10 / 8 1245
11 / 8 0320 11 / 8 0700
11 / 8 1300 11 / 8 1500
11 / 8 1940 11 / 8 2130
12 / 8 0100 12 / 8 0315
13 / 8 0840 13 / 8 1300
14 / 8 0600 14 / 8 1100
14 / 8 1200 14 / 8 1515
15 / 8 1425 15 / 8 1555
15 / 8 1655 15 / 8 1900
15 / 8 2325 16 / 8 0330
16 / 8 0900 16 / 8 1500
18 / 8 1345 18 / 8 1700
19 / 8 0520 19 / 8 0730
19 / 8 0920 19 / 8 1500
19 / 8 1540 19 / 8 1700
24 / 8 0932 24 / 8 1100
25 / 8 1403 25 / 8 1730
27 / 8 0410 27 / 8 1900
28 / 8 0415 28 / 8 0550
28 / 8 1115 28 / 8 1430
29 / 8 0705 29 / 8 1330
30 / 8 0828 30 / 8 0950
30 / 8 1420 30 / 8 1630
31 / 8 1010 31 / 8 1515


Table 1.4   Very Hot Weather Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
1 / 8 1045 1 / 8 1445
8 / 8 1320 8 / 8 1620
12 / 8 1230 12 / 8 1615
17 / 8 1015 18 / 8 1545
20 / 8 1100 23 / 8 1830
25 / 8 1220 25 / 8 1620
26 / 8 0645 26 / 8 1730


Table 1.5   Special Announcement on Flooding
in the Northern New Territories
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
4 / 8 1945 5 / 8 0115
6 / 8 0130 6 / 8 0655
8 / 8 1625 8 / 8 1940
18 / 8 1605 18 / 8 1820
19 / 8 1200 19 / 8 1540


Table 2   Figures and Departures from Normal - August 2021
Meteorological Element Figure of the Month Departure from Normal(1991-2020) Departure from Normal (1981-2010)
Mean Daily Maximum Air Temperature 31.4 degrees C 0.1 degrees above normal 0.3 degrees above normal
Mean Air Temperature 28.8 degrees C 0.1 degrees above normal 0.2 degrees above normal
Mean Daily Minimum Air Temperature 26.8 degrees C 0.1 degrees above normal 0.2 degrees above normal
Mean Dew Point Temperature 25.4 degrees C 0.3 degrees above normal 0.4 degrees above normal
Mean Relative Humidity 83 % 2 % above normal 2 % above normal
Mean Cloud Amount 77 % 7 % above normal 8 % above normal
Total Rainfall 350.5 mm 102.7 mm below normal 81.7 mm below normal
Number of hours of Reduced VisibilityΔ 2 hours 40.5 hours below normal§ 40.5 hours below normal§
Total Bright Sunshine Duration 160.0 hours 22.1 hours below normal 28.9 hours below normal
Mean Daily Global Solar Radiation 15.38 Megajoule / square metre 0.35 Megajoule below normal 0.25 Megajoule below normal
Total Evaporation 101.1 mm 28.6 mm below normal 33.8 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 August 2021

The percentile map of mean temperature of August 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 August 2021