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The Weather of July 2020

4 August 2020

With a stronger than usual subtropical ridge persisting over southern China for most of the time in the month, July 2020 became the hottest month in Hong Kong since records began in 1884. The monthly mean maximum temperature of 33.3 degrees, monthly mean temperature of 30.2 degrees and monthly mean minimum temperature of 28.3 degrees were 1.9 degrees, 1.4 degrees and 1.5 degrees above their corresponding normals and all of them were the highest of the correspondingly monthly mean values on record. With a total of 21 hot nights, July 2020 was the month with the highest number of hot nights on record and the 11 consecutive hot nights that started from 5 July also set a new record for July. Moreover, there were 20 very hot days in the month, the highest number of very hot days in a month on record. With long spell sunny weather, the month was also much drier than usual. The total monthly rainfall was only 125.4 millimetres, about 33 percent of the normal figure of 376.5 millimetres. The accumulated rainfall for the first seven months of the year was 1088.8 millimetres, about 26 percent below the normal figure of 1473.3 millimetres.

Affected by a broad trough of low pressure and the subsequent southerly airstream, the weather of Hong Kong was a mixture of sunshine and showers with occasional thunderstorms on the first three days of the month. The showers were heavier on 3 July with more than 10 millimetres of rainfall over most parts of the territory. Dominated by an anticyclone aloft, apart from a few showers, it was generally fine and very hot during the day on 4 – 8 July. Under the influence of a strong southwesterly airstream, local weather became cloudier with isolated showers in the next two days.

With the subtropical ridge extending westwards from the Pacific to cover southeastern China, apart from a few showers, a spell of generally fine and very hot weather set in on 11 July and persisted till 30 July in Hong Kong. The intense heat also triggered isolated thunderstorms on 17, 21, 22 and 27 - 29 July. With plenty of sunshine and under light wind condition, the maximum temperature at the Observatory soared to 35.3 degrees on 23 July, the highest of the month. The unrelenting heat also necessitated the Very Hot Weather Warning to remain in force for 467 hours from 11 to 30 July, setting the longest record since the introduction of the warning in 2000. Meanwhile, the monsoon depression over the South China Sea gradually developed into a tropical depression on the last day of the month and its outer rain bands brought squally showers and thunderstorms to the territory. More than 20 millimetres of rainfall were recorded over most parts of the territory, and rainfall even exceeded 40 millimetres over Hong Kong Island. The oppressive heat and parched condition that persisted for over half a month in Hong Kong were eventually relieved by the windy and showery weather.

Two tropical cyclones occurred over the South China Sea and the western North Pacific in July 2020.


Details of issuance and cancellation of various warnings/signals in the month are summarized in Tables 1.1 to 1.4.  Monthly meteorological figures and departures from normal for July are tabulated in Table 2.

Warnings and Signals issued in July 2020

Table 1.1   Tropical Cyclone Warning Signals
Name of
Tropical Cyclone
Signal
Number
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
SINLAKU 3 31 / 7 2040 1 / 8 2110


Table 1.2   Strong Monsoon Signal
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
31 / 7 0705 31 / 7 2040


Table 1.3   Thunderstorm Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
1 / 7 1115 1 / 7 1230
2 / 7 1335 2 / 7 1600
3 / 7 0810 3 / 7 1200
3 / 7 1620 3 / 7 1715
3 / 7 1830 3 / 7 2015
17 / 7 0222 17 / 7 0430
21 / 7 1230 21 / 7 1400
22 / 7 0245 22 / 7 0345
22 / 7 0700 22 / 7 1000
27 / 7 0705 27 / 7 0945
27 / 7 1353 27 / 7 1500
28 / 7 0645 28 / 7 0800
29 / 7 1140 29 / 7 1630
30 / 7 0435 30 / 7 0730
30 / 7 1640 30 / 7 1800
31 / 7 0550 31 / 7 0945
31 / 7 1740 31 / 7 2000
31 / 7 2150 1 / 8 0915


Table 1.4   Very Hot Weather Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
29 / 6 1145 1 / 7 1630
3 / 7 1335 3 / 7 1620
4 / 7 1110 8 / 7 1730
11 / 7 0645 30 / 7 1745


Table 2   Figures and Departures from Normal - July 2020
Meteorological Element Figure of the Month Departure from Normal*
Mean Daily Maximum Air Temperature 33.3 degrees C 1.9 degrees above normal
Mean Air Temperature 30.2 degrees C 1.4 degrees above normal
Mean Daily Minimum Air Temperature 28.3 degrees C 1.5 degrees above normal
Mean Dew Point Temperature 25.5 degrees C 0.4 degree above normal
Mean Relative Humidity 76 % 5 % below normal
Mean Cloud Amount 73 % 4 % above normal
Total Rainfall 125.4 mm 251.1 mm below normal
Number of hours of Reduced VisibilityΔ 0 hour 13.0 hours below normal§
Total Bright Sunshine Duration 249.7 hours 37.7 hours above normal
Mean Daily Global Solar Radiation 21.83 Megajoule / square metre 4.66 Megajoule above normal
Total Evaporation 156.0 mm 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 1981 - 2010 climatological normal, except for number of hours of reduced visibility

  §   Departure from mean value between 1997 and 2019

daily values of selected meteorological elements for HK for July 2020

The percentile map of mean temperature of July 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 1981 to 2010


Extract of Meteorological Observations in Hong Kong for July 2020