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  • The Weather of August 2017

  • 4 September 2017

    August 2017 was hotter than normal and the prolonged heat was relieved by the successive strikes of tropical cyclones Hato and Pakhar within a 5-day period during the latter part of the month. Both cyclones led to the raising of Gale or Storm Signal No.8, with Hato even necessitating the issuance of the Hurricane Signal No.10 on 23 August, the first time since July 2012. The mean temperature recorded in the month was 29.3 degrees, the seventh highest for August on record and 0.7 degree above the August normal of 28.6 degrees. Due to the rain brought by Hato and in particular Pakhar, the monthly total rainfall amounted to 489.1 millimetres, about 13 percent more than the normal figure of 432.2 millimetres. The accumulated rainfall this year up to August was 2248.9 millimetres, a surplus of 18 percent compared to the normal figure of 1905.5 millimetres for the same period.

    Under the influence of an active southwest monsoon, the weather in Hong Kong was generally cloudy with morning showers that affected mostly the southeastern part of the territory on the first two days of the month. The showers got heavier and became more widespread on the morning of 3 August, leading to the issuance of the Red Rainstorm Warning Signal. After another showery morning on 4 August, particularly over the southern and southeastern part of the territory, the weather turned sunny and very hot on 5 August as a ridge of high pressure extended over southeastern China. Generally fine and very hot conditions then persisted for another three days.

    With a freshening of the southwest monsoon, the weather turned cloudy on 9 August and showery activities increased. A southwest-to-northeast corridor of heavier showers extended from Tsuen Wan to the Tolo Harbour on 10 August, and then shifted eastwards the next day running from Hong Kong Island to Sai Kung. As the monsoon winds subsided, convective development became less active on 12 August despite some localized showers over Lantau Island. A spell of fine weather then prevailed for the next ten days, with a ridge of high pressure extending westwards from the Pacific to cover southeastern China on 16 – 18 August. Despite an outbreak of thundery showers due to intense day heating at Tai Po on 18 August, showers during the fine spell were mostly isolated and the territory enjoyed two shower-free days on 20 and 21 August.

    With prolonged sunshine, daytime temperatures at the Hong Kong Observatory reached 33 degrees or higher during the latter part of the fine spell. As Hato headed towards Hong Kong, subsidence effect ahead of its circulation brought hazy skies and oppressive heat on 22 August. The maximum temperature at the Hong Kong Observatory that afternoon soared to an all-time record-breaking high of 36.6 degrees. Squally showers associated with the outer rainbands of Hato then started to affect the territory later that day. The situation deteriorated further overnight as stormy weather battered the city during the passage of Hato on the morning of 23 August. Hurricane force winds affected Cheung Chau and the southeastern part of the territory, and a maximum gust of 193 kilometres per hour was recorded at Waglan Island. As the approach of Hato coincided with the astronomical high tide, storm surge induced by Hato resulted in unusually high water level and serious flooding in many parts of the territory, including Tai O, Heng Fa Chuen, Lei Yue Mun, Sha Tin and Lau Fau Shan. The water level at Quarry Bay rose to a maximum of 3.57 metres that morning, the second highest after the record high of 3.96 metres set by Super Typhoon Wanda in 1962. With Hato making landfall over Zhuhai to the west of Macau in the afternoon and weakening further inland, local winds subsided significantly later in the day.

    The weather was a mixture of sunshine and scattered showers over the next couple of days, with some heavy showers affecting Tuen Mun and Shek Kong on 25 August. Ahead of the visit of Pakhar, oppressively hot and hazy conditions affected Hong Kong once again on 26 August. Rain and squalls associated with the intense rainbands north of Pakhar started to affect the territory later that evening. Stormy weather persisted for most of the day on 27 August as Pakhar skirted past just to the southwest of Hong Kong, with winds persistently reaching storm force over the northeastern and southern parts of the territory and occasionally attaining hurricane force on high ground. Temperature at the Hong Kong Observatory fell to the month’s lowest of 24.0 degrees in rain that day. Heavy showers and some squally thunderstorms continued to affect the territory the next day as winds gradually subsided. As the lingering rainbands associated with Pakhar finally cleared away, the weather turned sunny on 29 August. Hot conditions with a mixture of sunshine, haze and thundery evening showers then persisted towards the end of the month.


Radar imagery of Severe Typhoon Hato skirting to the south of Hong Kong at 9:12 a.m. on 23 August 2017
Radar imagery of Severe Typhoon Hato skirting to the south of Hong Kong at 9:12 a.m. on 23 August 2017


The approach of Hato resulted in fallen trees in many parts of the territory
The approach of Hato resulted in fallen trees in many parts of the territory
(Photos courtesy of Mr. W. Kong, Mr. Derek Li and Mr. Y. W. Ng)



Storm surge induced by Hato resulted in serious flooding in many parts of the territory
Storm surge induced by Hato resulted in serious flooding in many parts of the territory
(Photos courtesy of Drainage Services Department (left) and Mr. Steve Lee (right))



Vessels in distress over the waters southwest of Hong Kong on 23 August 2017
Vessels in distress over the waters southwest of Hong Kong on 23 August 2017
(Photos courtesy of Government Flying Service)



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

    Details of issuance and cancellation of various warnings/signals in the month are summarized in Tables 1.1 to 1.6.  Monthly meteorological figures and departures from normal for August are tabulated in Table 2.
 

Warnings and Signals issued in August 2017

Table 1.1   Tropical Cyclone Warning Signals
Name of
Tropical Cyclone
Signal
Number
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
HATO 1 22 / 8 0840 22 / 8 1820
3 22 / 8 1820 23 / 8 0520
8 NE 23 / 8 0520 23 / 8 0810
9 23 / 8 0810 23 / 8 0910
10 23 / 8 0910 23 / 8 1410
8 SE 23 / 8 1410 23 / 8 1710
3 23 / 8 1710 23 / 8 1820
1 23 / 8 1820 23 / 8 2040
PAKHAR 1 26 / 8 0940 26 / 8 2040
3 26 / 8 2040 27 / 8 0510
8 SE 27 / 8 0510 27 / 8 1340
3 27 / 8 1340 27 / 8 1740
1 27 / 8 1740 27 / 8 2210


Table 1.2   Strong Monsoon Signal
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
28 / 8 0245 28 / 8 0440


Table 1.3   Rainstorm Warning Signals
Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Amber 3 / 8 0505 3 / 8 0530
Red 3 / 8 0530 3 / 8 0705
Amber 3 / 8 0705 3 / 8 0745
Amber 4 / 8 0620 4 / 8 0820
Amber 23 / 8 0800 23 / 8 1210
Amber 27 / 8 0510 27 / 8 1410
Amber 28 / 8 0615 28 / 8 0855


Table 1.4   Landslip Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
27 / 8 1610 27 / 8 2315


Table 1.5   Thunderstorm Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
1 / 8 0630 1 / 8 1000
2 / 8 0250 2 / 8 1400
3 / 8 0030 3 / 8 0130
3 / 8 0325 3 / 8 0815
3 / 8 1555 3 / 8 1700
4 / 8 0515 4 / 8 1115
6 / 8 1210 6 / 8 1245
7 / 8 0715 7 / 8 0830
9 / 8 0750 9 / 8 1000
9 / 8 1215 9 / 8 1400
10 / 8 0305 10 / 8 0340
10 / 8 0555 10 / 8 1230
10 / 8 1355 10 / 8 1515
12 / 8 0345 12 / 8 0430
16 / 8 1035 16 / 8 1340
18 / 8 1415 18 / 8 1500
22 / 8 1235 22 / 8 1845
23 / 8 0700 23 / 8 1500
25 / 8 1235 25 / 8 1500
26 / 8 0225 26 / 8 0330
26 / 8 1850 26 / 8 2000
26 / 8 2120 26 / 8 2330
27 / 8 1155 27 / 8 2000
27 / 8 2300 28 / 8 1730
30 / 8 1550 30 / 8 1700
30 / 8 1825 30 / 8 2045
31 / 8 1730 31 / 8 2100


Table 1.6   Very Hot Weather Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
1 / 8 1200 1 / 8 1830
5 / 8 1215 9 / 8 1100
11 / 8 1145 15 / 8 1945
17 / 8 1500 22 / 8 1800
24 / 8 1145 24 / 8 1930
26 / 8 0645 26 / 8 1800
30 / 8 0645 31 / 8 0130


Table 2   Figures and Departures from Normal - August 2017
Meteorological Element Figure of the Month Departure from Normal*
Mean Daily Maximum Air Temperature 32.1 degrees C 1.0 degree above normal
Mean Air Temperature 29.3 degrees C 0.7 degree above normal
Mean Daily Minimum Air Temperature 27.3 degrees C 0.7 degree above normal
Mean Dew Point Temperature 25.3 degrees C 0.3 degree above normal
Mean Relative Humidity 80 % 1 % below normal
Mean Cloud Amount 70 % 1 % above normal
Total Rainfall 489.1 mm 56.9 mm above normal
Number of hours of Reduced VisibilityΔ 8 hours 40.4 hours below normal§
Total Bright Sunshine Duration 205.4 hours 16.5 hours above normal
Mean Daily Global Solar Radiation 17.92 Megajoule / square metre 2.29 Megajoule above normal
Total Evaporation 127.9 mm 7.0 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 2016

daily values of selected meteorological elements for HK for August 2017

The percentile map of mean temperature of August 2017

  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 August 2017