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The Weather of September 2021

5 October 2021

Mainly attributing to the stronger than usual subtropical ridge over southern China, September 2021 was the hottest September in Hong Kong on record. The monthly mean maximum temperature of 32.8 degrees, monthly mean temperature of 29.7 degrees and monthly mean minimum temperature of 27.8 degrees were 2.3 degrees, 1.8 degrees and 1.7 degrees above their corresponding normals (or 2.7 degrees, 2.0 degrees and 2.0 degrees above their corresponding 1981-2010 normals) and all of them were the highest on record for September. There were in total 15 very hot days and 11 hot nights in the month, both breaking the records for September. Moreover, from January to September, the numbers of very hot days and hot nights so far in 2021 already reached 53 days and 57 days respectively, both breaking the previous records set in 2020. September 2021 was also much drier than usual with a total rainfall of 129.6 millimetres, about 40 percent of the normal figure of 321.4 millimetres (or 40 percent of the 1981-2010 normal of 327.6 millimetres). The accumulated rainfall up to September this year was 1650.7 millimetres, a deficit of 26 percent compared with the normal of 2242.8 millimetres (or 26 percent below the 1981-2010 normal of 2233.1 millimetres) for the same period.

Under the dominance of the anticyclone over southern China, apart from a few showers and isolated thunderstorms, the weather of Hong Kong was sunny and very hot for most of the time on 1 – 12 September. Meanwhile, over the western North Pacific, Super Typhoon Chanthu tracked northwards and moved across the Luzon Strait and the sea areas to the east of Taiwan on 10 – 12 September. With plenty of sunshine and under the influence of the outer subsiding air of Chanthu, the temperature at the Observatory soared to 34.5 degrees on 12 September, the highest of the month. The daily mean temperature of 31.2 degrees on that day was also one of the highest on record for September.

On 13 – 17 September, under light wind conditions, high temperatures triggered localized thundery showers in some regions, despite sunny periods in between. The daily minimum temperature of 29.5 degrees on 13 September was one of the highest on record for September. On 16 September, apart from the localized heavy showers and thunderstorms, hail was also reported at Tseung Kwan O at around 1:00 p.m. and more than 100 millimetres of rainfall were recorded in that region on that day. Moreover, a person died after being struck by lightning in Clear Water Bay that afternoon.

Affected by an anticyclone aloft, apart from a few showers and isolated thunderstorms, there were sunny periods on 18 – 19 September. With the return of the light wind conditions, day heating triggered a few thundery showers apart from sunny periods on 20 – 22 September. The showers were particularly heavy in some areas on 21 September. More than 60 millimetres of rainfall were recorded over Tsuen Wan District. With plenty of sunshine, the temperature at the Observatory soared to a maximum of 34.0 degrees on the afternoon of 22 September, making it the highest record for the day following the Mid-Autumn Festival.

With the setting in of a fresh to strong easterly airstream on the night of 22 September, the local weather turned mainly cloudy with occasional showers and squally thunderstorms the next day. Under the rain, the temperature at the Observatory dropped to a minimum of 26.0 degrees, the lowest of the month. This was also the highest monthly absolute minimum temperature on record for September. The weather gradually became sunnier and less showery from 24 to 26 September. Under the influence of an anticyclone aloft and the moderation of the easterly airstream, apart from isolated showers and thunderstorms on 29 – 30 September, the local weather became mainly fine and very hot towards the end of the month.

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


Radar image at 12:48 pm on 16 September 2021Radar image at 12:48 p.m. on 16 September 2021

A hail stone picked up in Tseung Kwan O at around 12:50 pm on 16 September 2021A hail stone picked up in Tseung Kwan O at around 12:50 p.m. on 16 September 2021 (courtesy of Bei Lang Ni)


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 September are tabulated in Table 2.

Warnings and Signals issued in September 2021

Table 1.1   Thunderstorm Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
1 / 9 0445 1 / 9 0600
1 / 9 0840 1 / 9 1200
1 / 9 1338 1 / 9 1600
1 / 9 1845 1 / 9 2135
4 / 9 0725 4 / 9 1130
5 / 9 1230 5 / 9 1400
5 / 9 1510 5 / 9 1615
6 / 9 1250 6 / 9 1500
7 / 9 1325 7 / 9 1430
9 / 9 1445 9 / 9 1615
12 / 9 1355 12 / 9 1600
13 / 9 1110 13 / 9 1400
14 / 9 0935 14 / 9 1600
15 / 9 1730 15 / 9 1845
15 / 9 2127 15 / 9 2230
16 / 9 1156 16 / 9 2100
17 / 9 1206 17 / 9 1530
18 / 9 0614 18 / 9 0850
19 / 9 1120 19 / 9 1800
19 / 9 1848 19 / 9 2100
20 / 9 0905 20 / 9 1630
21 / 9 1211 21 / 9 1600
22 / 9 1318 22 / 9 1530
23 / 9 0030 23 / 9 0530
23 / 9 0555 23 / 9 2300
24 / 9 0235 24 / 9 0830
24 / 9 0915 24 / 9 1100
25 / 9 0255 25 / 9 0430
29 / 9 1435 29 / 9 1730
30 / 9 1235 30 / 9 1630


Table 1.2   Very Hot Weather Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
2 / 9 1145 6 / 9 1620
7 / 9 1145 14 / 9 1015
15 / 9 1115 15 / 9 1800
17 / 9 0745 18 / 9 1730
20 / 9 1115 20 / 9 1620
21 / 9 0745 21 / 9 1415
22 / 9 1120 22 / 9 1730
27 / 9 1100 30 / 9 1745


Table 2   Figures and Departures from Normal - September 2021
Meteorological Element Figure of the Month Departure from Normal(1991-2020) Departure from Normal (1981-2010)
Mean Daily Maximum Air Temperature 32.8 degrees C 2.3 degrees above normal 2.7 degrees above normal
Mean Air Temperature 29.7 degrees C 1.8 degrees above normal 2.0 degrees above normal
Mean Daily Minimum Air Temperature 27.8 degrees C 1.7 degrees above normal 2.0 degrees above normal
Mean Dew Point Temperature 25.4 degrees C 1.8 degrees above normal 2.0 degrees above normal
Mean Relative Humidity 78 % normal normal
Mean Cloud Amount 71 % 5 % above normal 5 % above normal
Total Rainfall 129.6 mm 191.8 mm below normal 198.0 mm below normal
Number of hours of Reduced VisibilityΔ 2 hours 68.8 hours below normal§ 68.8 hours below normal§
Total Bright Sunshine Duration 218.5 hours 44.1 hours above normal 46.2 hours above normal
Mean Daily Global Solar Radiation 18.06 Megajoule / square metre 3.07 Megajoule above normal 3.45 Megajoule above normal
Total Evaporation 135.7 mm 12.9 mm above normal 9.8 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

Daily values of selected meteorological elements for September 2021

The percentile map of mean temperature for September 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 September 2021