The Weather of September 2019

3 October 2019

   With the dominance of upper-air anticyclone over southern China for most of the time in the month, Hong Kong experienced a hot, sunny and dry September in 2019. The monthly mean temperature was 28.7 degrees, 1.0 degree above the normal figure of 27.7 degrees and the seventh highest on record for September. There were seven consecutive hot nights from 8 to 14 September, the longest on record for September. Moreover, up to September, the annual number of hot nights in 2019 already reached 45, which is 27.2 days above the annual normal and the highest on record since 1884. September 2019 was marked by sunny weather with the monthly total sunshine duration amounting to 216.3 hours, about 26 percent above the normal of 172.3 hours. The month was also drier than usual with a total rainfall of 198.9 millimetres, about 61 percent of the normal figure of 327.6 millimetres. The accumulated rainfall this year up to September was 2233.2 millimetres, on par with the normal figure of 2233.1 millimetres for the same period.

   An area of low pressure intensified into a tropical depression over the northern part of the South China Sea on the first day of September and later named as Kajiki. It moved generally westward across the northern part of the South China Sea and made landfall over the southeastern coast of Hainan Island on the morning of 2 September. Kajiki entered the seas south of Hainan Island in the afternoon and moved slowly towards central Vietnam. Kajiki then lingered over the vicinity of the coast of central Vietnam on 3 September and weakened into an area of low pressure over there on the morning of 4 September.

   Under the combined effect of Kajiki and the continental anticyclone, local weather became windy with occasional squally showers and thunderstorms in the afternoon of 1 September and remained so on the next two days. The showers were particularly heavy on 2 September with more than 50 millimetres of rainfall over most parts of the territory and rainfall even exceeding 100 millimetres over Sai Kung, Sha Tin and Tai Po. Affected by a broad trough of low pressure over the northern part of the South China Sea, the weather in Hong Kong remained unstable with occasional heavy showers and thunderstorms on 4 – 5 September. More than 40 millimetres of rainfall were recorded in many places and the rainfall recorded at Hong Kong Observatory even exceeded 90 millimetres in these two days.

   Under the influence of a continental airstream, local weather became generally fine and very hot on 6 – 9 September. The high temperature also triggered thundery showers on the afternoons of 7 - 9 September. With the anticyclone aloft over southeastern China strengthening gradually, the generally fine and very hot weather continued on 10 – 13 September. With plenty of sunshine, the maximum temperature at the Observatory soared to 33.5 degrees on 12 September, the highest of the month. Moreover, the maximum temperature at the Observatory reached 33.0 degrees on 13 September, making it the hottest Mid-Autumn Festival on record.

   With the weakening of the anticyclone aloft and under light wind conditions, it was hot with a mixture of sunshine and thundery showers in Hong Kong on 14 – 17 September. The showers were rather heavy and localized on 17 September. More than 50 millimetres of rainfall were recorded over Tuen Mun and Yuen Long and the rainfall even exceeded 100 millimetres over the western part of Lantau Island. With the anticyclone aloft strengthening again, apart from a few showers and thunderstorms in the morning and at night, the weather improved gradually on 18 September. While there were still a few showers in the morning, the weather turned fine and dry in the afternoon of 19 September. The temperature of the Observatory dropped to 24.9 degrees under the morning showers on 19 September, the lowest of the month. With the prevalence of the dry northeast monsoon over South China, the weather remained generally fine and dry for the rest of the month. The weather was very dry during the day on 20 – 22 September with the relative humidity over parts of the territory fell below 40 percent. It was also hazy on the last three days of the month with the visibility in the harbour fell below 4000 metres.

   Six 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 September are tabulated in Table 2.

Warnings and Signals issued in September 2019

this table lists tropical cyclone warning signals issued during the month

Table 1.1 Tropical Cyclone Warning Signals

Name of Tropical Cyclone	Signal Number	Beginning Time		Ending Time
		Day/Month	HKT	Day/Month	HKT
KAJIKI	1	1 / 9	0840	1 / 9	1620
	3	1 / 9	1620	2 / 9	1040
	1	2 / 9	1040	3 / 9	0920

this table lists strong monsoon signals issued during the month

Table 1.2 Strong Monsoon Signal

Beginning Time		Ending Time
Day/Month	HKT	Day/Month	HKT
21 / 9	0905	21 / 9	1200
22 / 9	0530	22 / 9	1330

this table lists rainstorm warning signals issued during the month

Table 1.3 Rainstorm Warning Signals

Colour	Beginning Time		Ending Time
	Day/Month	HKT	Day/Month	HKT
Amber	2 / 9	1225	2 / 9	1400

this table lists thunderstorm warnings issued during the month

Table 1.4 Thunderstorm Warning

Beginning Time		Ending Time
Day/Month	HKT	Day/Month	HKT
1 / 9	0945	1 / 9	1045
1 / 9	1345	1 / 9	2100
1 / 9	2320	2 / 9	0300
2 / 9	0435	2 / 9	1930
2 / 9	2025	2 / 9	2230
3 / 9	0547	3 / 9	0715
3 / 9	2255	4 / 9	1900
5 / 9	0215	5 / 9	0700
7 / 9	1535	7 / 9	1720
8 / 9	1330	8 / 9	1500
9 / 9	1350	9 / 9	1500
14 / 9	1405	14 / 9	1545
14 / 9	1755	14 / 9	1900
14 / 9	2110	14 / 9	2300
15 / 9	0525	15 / 9	0930
16 / 9	0115	16 / 9	0700
16 / 9	1725	16 / 9	1845
17 / 9	0245	17 / 9	0345
17 / 9	1305	17 / 9	2000
18 / 9	2030	19 / 9	0300

this table lists fire danger warnings issued during the month

Table 1.5 Fire Danger Warnings

Colour	Beginning Time		Ending Time
	Day/Month	HKT	Day/Month	HKT
Yellow	13 / 9	0600	14 / 9	1100
Red	20 / 9	0600	23 / 9	1945
Yellow	28 / 9	0600	28 / 9	1845
Yellow	29 / 9	0600	29 / 9	1800

this table lists very hot weather warnings issued during the month

Table 1.6 Very Hot Weather Warning

Beginning Time		Ending Time
Day/Month	HKT	Day/Month	HKT
7 / 9	1045	7 / 9	1800
8 / 9	0645	8 / 9	1730
9 / 9	1145	10 / 9	1800
11 / 9	1145	13 / 9	1800
20 / 9	0745	20 / 9	1800
30 / 9	1100	30 / 9	1900

this table lists the month's figures and departures from normal

Table 2 Figures and Departures from Normal - September 2019

Meteorological Element	Figure of the Month	Departure from Normal*
Mean Daily Maximum Air Temperature	31.8 degrees C	1.7 degrees above normal
Mean Air Temperature	28.7 degrees C	1.0 degree above normal
Mean Daily Minimum Air Temperature	26.6 degrees C	0.8 degree above normal
Mean Dew Point Temperature	23.1 degrees C	0.3 degree below normal
Mean Relative Humidity	73 %	5 % below normal
Mean Cloud Amount	50 %	16 % below normal
Total Rainfall	198.9 mm	128.7 mm below normal
Number of hours of Reduced VisibilityΔ	30 hours	45.2 hours below normal§
Total Bright Sunshine Duration	216.3 hours	44.0 hours above normal
Mean Daily Global Solar Radiation	17.95 Megajoule / square metre	3.34 Megajoule above normal
Total Evaporation	129.9 mm	4.0 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 2018