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The Weather of October 2020

3 November 2020

The mean temperature for October 2020 was 25.6 degrees, close to the normal figure of 25.5 degrees. Mainly attributing to the heavy downpour on 5 October, the month was wetter than usual with the monthly rainfall of 142.4 millimeters, about 41 percent above the normal of 100.9 millimetres. The accumulated rainfall this year up to October was 2388.4 millimetres, about 2 percent above the normal figure of 2334.0 millimetres for the same period.

Under the influence of a strong easterly airstream, the weather of Hong Kong was mainly cloudy with sunny periods on the first day of the month. Affected by an anticyclone aloft, it was generally fine on 2 – 4 October. With plenty of sunshine, the maximum temperature at the Observatory rose to 31.9 degrees on 3 October, the highest of the month. A cold front moved across the coastal areas of Guangdong and brought heavy rain and squally thunderstorms to Hong Kong on the morning of 5 October. The heavy downpour also necessitated the issuance of the Red Rainstorm Warning in that morning. More than 50 millimetres of rainfall were recorded over many places in Kowloon and the rainfall recorded at the Hong Kong Observatory even exceeded 100 millimetres.

Under the influence of a fresh to strong northeast monsoon, the weather of Hong Kong was mainly cloudy and slightly cooler on 6 – 7 October. Dominated by a dry northeast monsoon, the weather became generally fine and dry during the day on 8 – 12 October. Meanwhile, an area of low pressure gradually developed into a tropical depression over the central part of the South China Sea on the afternoon of 11 October and later named as Nangka. It moved generally west-northwestward across the northern part of the South China Sea and intensified into a tropical storm on 12 October. Nangka moved across Hainan Island on the night of 13 October and entered Beibu Wan in the small hours the next day. Nangka made landfall over Vietnam on the evening of 14 October and weakened gradually into an area of low pressure over Indochina Peninsula the next morning.

Affected by the outer rainbands of Nangka, the weather of Hong Kong turned cloudy with a few rain patches on the night of 12 October. Rainy weather persisted on 13 October. Under the combined effect of Nangka and the northeast monsoon, local winds strengthened during the day with gale force winds over offshore waters and on high ground. The Gale or Storm Signal No. 8 was issued on the early morning of that day. With the departure of Nangka, the strong northeast monsoon continued to bring windy weather with a few rain patches to Hong Kong the next day.

Dominated by a dry northeast monsoon, apart from a few rain patches at night on 16 – 18 October, the weather was generally fine and dry during the day on 15 – 22 October with slightly cooler weather on 17 – 21 October. The temperature at the Observatory dropped to a minimum of 21.7 degrees on 21 October, the lowest of the month. Meanwhile, an area of low pressure gradually developed into a tropical depression over the seas east of the Philippines on 19 October and later named as Saudel. It moved across Luzon on the night of 20 October and entered the central part of the South China Sea the next morning. Saudel moved across the central part of the South China Sea and intensified gradually into a typhoon on the afternoon of 22 October. It continued to move generally westwards across the central part of the South China Sea and weakened into an area of low pressure over the seas east of the central part of Vietnam on morning of 26 October.

Affected by the cloud bands associated with Saudel and a dry northeast monsoon, local weather turned cloudier and slightly cooler but remained dry on 23 – 24 October. After the weakening of Saudel, the dry northeast monsoon brought generally fine, windy and dry weather to the territory in the next few days. Meanwhile, tropical cyclone Molave moved across the southern and central parts of the South China Sea on 26 - 28 October. Under the influence of the moisture spreading from Molave and a strong northeast monsoon, the weather of Hong Kong turned cloudier again with a few rain patches but remained windy on 28 – 29 October. With the dissipation of the rain band over south China coastal areas, local weather became mainly fine towards the end of the month.

Seven tropical cyclones occurred over the South China Sea and the western North Pacific in October 2020.


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 October are tabulated in Table 2.

Warnings and Signals issued in October 2020

Table 1.1   Tropical Cyclone Warning Signals
Name of
Tropical Cyclone
Signal
Number
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
NANGKA 1 11 / 10 2040 12 / 10 1710
3 12 / 10 1710 13 / 10 0540
8 NE 13 / 10 0540 13 / 10 1940
3 13 / 10 1940 14 / 10 0240
SAUDEL 1 22 / 10 1740 23 / 10 0020
3 23 / 10 0020 24 / 10 0910


Table 1.2   Strong Monsoon Signal
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
1 / 10 0045 2 / 10 1340
5 / 10 2135 6 / 10 0740
14 / 10 0240 15 / 10 1015
15 / 10 2205 16 / 10 0510
24 / 10 0911 25 / 10 1300
27 / 10 2355 29 / 10 1330


Table 1.3   Rainstorm Warning Signals
Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Amber 5 / 10 0725 5 / 10 0805
Red 5 / 10 0805 5 / 10 0905
Amber 5 / 10 0905 5 / 10 0930


Table 1.4   Thunderstorm Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
30 / 9 1300 1 / 10 0130
5 / 10 0632 5 / 10 0945


Table 1.5   Fire Danger Warnings
Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Yellow 1 / 10 1100 2 / 10 1900
Yellow 4 / 10 0600 4 / 10 1800
Yellow 10 / 10 0600 10 / 10 1915
Yellow 11 / 10 0600 11 / 10 1900
Yellow 17 / 10 0600 17 / 10 1800
Yellow 18 / 10 0600 18 / 10 1800
Red 21 / 10 0600 24 / 10 0600
Yellow 24 / 10 0600 26 / 10 1830
Yellow 31 / 10 1200 31 / 10 2000


Table 2   Figures and Departures from Normal - October 2020
Meteorological Element Figure of the Month Departure from Normal*
Mean Daily Maximum Air Temperature 28.5 degrees C 0.7 degree above normal
Mean Air Temperature 25.6 degrees C 0.1 degree above normal
Mean Daily Minimum Air Temperature 23.7 degrees C normal
Mean Dew Point Temperature 19.9 degrees C 0.3 degree below normal
Mean Relative Humidity 72 % 1 % below normal
Mean Cloud Amount 68 % 10 % above normal
Total Rainfall 142.4 mm 41.5 mm above normal
Number of hours of Reduced VisibilityΔ 1 hour 120.0 hours below normal§
Total Bright Sunshine Duration 190.9 hours 3.0 hours below normal
Mean Daily Global Solar Radiation 15.27 Megajoule / square metre 1.22 Megajoule above normal
Total Evaporation 123.7 mm 0.2 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 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 October 2020

The percentile map of mean temperature of October 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 October 2020