The Weather of July 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
|Beginning Time||Ending Time|
|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|
|Colour||Beginning Time||Ending Time|
|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|
|Beginning Time||Ending Time|
|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|
|Beginning Time||Ending Time|
|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|
|Beginning Time||Ending Time|
|20 / 7||1225||20 / 7||1620|
|25 / 7||0110||25 / 7||0325|
|29 / 7||1435||29 / 7||1640|
|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.
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.
§ 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
Extract of Meteorological Observations in Hong Kong for July 2021