The Weather of September 2006

September 2006 was cooler and wetter than usual. The mean temperature of 26.6 degrees was 1 degree lower than the normal. The monthly rainfall of 420.2 millimetres was about 40 percent above normal. The accumulated rainfall since 1 January amounted to 2467.1 millimetres, about 23 percent above the normal of 2007.1 millimetres for the same period.

A southwesterly airstream brought fine and hot weather to Hong Kong on the first two days of the month. Under the influence of an upper air disturbance over the coast of southeastern China, the weather became showery on 3 and 4 September. As the upper air disturbance moved west away from Hong Kong, the weather turned mainly fine in the next two days.

A trough of low pressure over southern China moved south across coastal areas on 7 September and brought scattered showers and squally thunderstorms to Hong Kong. The weather remained cloudy with a few showers on the next day. Local weather deteriorated with heavy rain on 9 September when a cold front crossed the south China coast. More than 100 millimetres of rainfall was recorded at Sha Tin, Tai Po and the western part of Hong Kong Island and Kowloon. Under the influence of the northeast monsoon behind the cold front, it was cooler with some rain in the ensuing three days.

An area of low pressure over the northern part of the South China Sea developed into a tropical depression on 12 September. It moved north initially and then took on a west-northwesterly track. The tropical depression weakened into an area of low pressure over coastal waters of western Guangdong the following night. The outer rain band of the tropical depression brought torrential rain to the territory on 13 September. Over 200 millimetres of rainfall was recorded in most places of Hong Kong. There were over 30 reports of flooding and 9 reports of landslide. The weather stayed mainly cloudy with a few showers in the following three days.

Weather improved on 17 September as a continental airstream prevailing over southern China. Sunny weather persisted in the next five days. It was windy on 23 and 24 September under the influence of an intense northeast monsoon. A ridge of high pressure brought generally fine weather to southeastern China from 25 to 29 September.

After crossing the Philippines, Typhoon Xangsane entered the South China Sea on 29 September and continued to move westward towards Vietnam. The outer cloud bands of Xangsane brought some rain patches to the territory on 30 September.

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

A big banyan tree at the Hong Kong Observatory Headquarters fell down during the inclement weather on 13 September 2006.

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 2006

Table 1.1 Tropical Cyclone Warning Signals

Name of Tropical Cyclone	Signal Number	Beginning Time		Ending Time
Name of Tropical Cyclone	Signal Number	Day/Month	HKT	Day/Month	HKT
no name	1	12 / 9	1340	13 / 9	1035
	3	13 / 9	1035	13 / 9	1440
	1	13 / 9	1440	13 / 9	1610

Table 1.2 Strong Monsoon Signal

Beginning Time		Ending Time
Day/Month	HKT	Day/Month	HKT
23 / 9	1930	25 / 9	1500
30 / 9	0215	30 / 9	1055

Table 1.3 Rainstorm Warning Signals

Colour	Beginning Time		Ending Time
Colour	Day/Month	HKT	Day/Month	HKT
Amber	9 / 9	1305	9 / 9	1345
Red	9 / 9	1345	9 / 9	1530
Amber	9 / 9	1530	9 / 9	1615
Amber	13 / 9	0750	13 / 9	0930
Red	13 / 9	0930	13 / 9	1215
Amber	13 / 9	1215	13 / 9	1345
Amber	13 / 9	1740	13 / 9	2315

Table 1.4 Landslip Warning

Beginning Time		Ending Time
Day/Month	HKT	Day/Month	HKT
13 / 9	1145	14 / 9	0415

Table 1.5 Thunderstorm Warning

Beginning Time		Ending Time
Day/Month	HKT	Day/Month	HKT
2 / 9	1215	2 / 9	1500
3 / 9	0735	3 / 9	1100
3 / 9	1130	3 / 9	1530
4 / 9	0415	4 / 9	1330
6 / 9	0150	6 / 9	0600
6 / 9	1330	6 / 9	1530
6 / 9	1930	6 / 9	2330
7 / 9	0720	7 / 9	1355
8 / 9	1020	8 / 9	1400
9 / 9	0400	9 / 9	0600
9 / 9	1050	9 / 9	2000
9 / 9	2215	10 / 9	0145
13 / 9	0335	13 / 9	2345
14 / 9	1205	14 / 9	1430
15 / 9	1442	15 / 9	2245

Table 1.6 Fire Danger Warnings

Colour	Beginning Time		Ending Time
Colour	Day/Month	HKT	Day/Month	HKT
Yellow	30 / 9	0600	30 / 9	1530

Table 2 Figures and Departures from Normal - September 2006

Meteorological Element	Figure of the Month	Departure from Normal*
Mean Daily Maximum Air Temperature	29.1 degrees C	1.2 degrees below normal
Mean Air Temperature	26.6 degrees C	1.0 degree below normal
Mean Daily Minimum Air Temperature	24.8 degrees C	0.7 degree below normal
Mean Dew Point Temperature	22.4 degrees C	0.9 degree below normal
Mean Relative Humidity	78 %	normal
Mean Cloud Amount	71 %	8 % above normal
Total Rainfall	420.2 mm	120.5 mm above normal
Number of hours of Reduced VisibilityΔ	81 hours	17.4 hours below normal§
Total Bright Sunshine Duration	170.3 hours	11.4 hours below normal
Mean Daily Global Solar Radiation	14.86 Megajoule / square metre	1.63 Megajoule below normal
Total Evaporation	122.3 mm	28.0 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.
	The calibration factors of the pyranometer at King's Park in 2006 and 2007 have been re-computed based on the results of a regional instrument comparison in Asia in 2007. The readings of total global solar radiation at King's Park in 2006 and 2007 displayed in this web page before 13 June 2008 have been revised on 13 June 2008 using the new calibration factors.
* Departure from 1961-1990 climatological normal, except for number of hours of reduced visibility.
§ Departure from mean value between 1997 and 2005.

daily values of selected meteorological elements for HK for September 2006

Extract of Meteorological Observations in Hong Kong for September 2006