The Weather of October 2023
2 November 2023
With more than usual moisture in the lower atmosphere over southern China, October 2023 was much cloudier than usual in Hong Kong. The mean amount of cloud in the month was 79 percent, 21 percent above the normal of 58 percent and the second highest on record for October. The duration of bright sunshine in the month was only 138.9 hours, about 30 percent lower than the normal figure of 197.8 hours and the fourth lowest on record for October. Mainly attributing to the record-breaking rainfall associated with tropical cyclone Koinu on 8 – 9 October, the month was also much wetter than usual. The monthly total rainfall was 546.0 millimetres, more than four times of the normal figure of 120.3 millimetres and the fifth highest on record for October. The accumulated rainfall this year up to October was 2770.3 millimetres, about 17 percent more than the normal figure of 2363.1 millimetres for the same period. Despite the below normal sunshine, the month was warmer than usual. The mean temperature of 26.4 degrees was 0.7 degrees above the normal and one of the fourth highest for October on record. There were 3 very hot days in the early part of the month, the highest on record for October.
Under the influence of an anticyclone aloft, apart from a few showers and isolated thunderstorms, it was very hot with sunny periods during the day on the first three days of the month. Meanwhile, severe typhoon Koinu over the western North Pacific tracked northwestwards towards the Luzon Strait. It turned to move westwards on 4 October and skirted past the southern part of Taiwan on the morning of 5 October. Koinu moved slowly across the northern part of the South China Sea and edged towards the Pearl River Estuary in the next two days. Koinu weakened into a typhoon on 8 October and skirted past about 70 kilometres to the south of Hong Kong that night. It continued to move across the coastal waters of Guangdong the next day and weakened progressively into an area of low pressure over the coastal waters of Yangjiang on that night.
Under the influence of the outer subsiding air of Koinu, it was generally fine and very hot during the day in Hong Kong on 4 – 5 October. The maximum temperature at the Observatory soared to 34.6 degrees on the afternoon of 4 October, the highest of the month and the highest monthly absolute maximum temperature on record for October. Moreover, the daily mean temperature on that day reached 30.8 degrees, the highest on record for October. With Koinu edging closer to the coast of Guangdong, while there were sunny periods during the day, winds over Hong Kong picked up gradually and there were a few showers on the night of 6 October. Local wind strength increased further on 7 October with strong winds offshore and occasional gales on high ground. The weather also turned cloudy with a few squally showers on that day.
With Koinu moving towards the vicinity of the Pearl River Estuary, the weather of Hong Kong deteriorated significantly on 8 October and the Gale or Storm Signal No. 8 was issued on that afternoon. Local winds strengthened significantly and reached storm force offshore and on high ground that night. The Increasing Gale or Storm Signal No. 9 was also issued that night when the compact and hurricane force wind bearing eyewall of Koinu skirted past closely to the south of Hong Kong. With Koinu weakening and departing from Hong Kong, local winds moderated later on 9 October. Koinu brought squally heavy showers to Hong Kong on 8 – 9 October and necessitated the issuance of the Black Rainstorm Warning on the morning of 9 October. The rainfall recorded at the Observatory on 9 October reached 369.7 millimetres, more than three times of October’s monthly total normal figure of 120.3 millimetres and the highest daily rainfall on record for October. Moreover, the 24-hour rainfall from 3 p.m. on 8 October to 3 p.m. next day reached 439.8 millimetres, breaking the highest record for October. Overall, more than 300 millimetres of rainfall were recorded over most parts of the territory and rainfall even exceeded 600 millimetres in Central and Western, Wan Chai, Wong Tai Sin and Kwun Tong Districts on 8 – 9 October.
Under the influence of the outer rainbands associated with the remnant of Koinu, it was cloudy with a few showers on 10 October. With the thinning out of the cloud bands, there were sunny periods in Hong Kong on 11 – 15 October. It was also dry during the day on 13 – 15 October. Affected by a fresh to strong northeast monsoon, there were sunny intervals and a few showers on 16 – 17 October. Meanwhile, an area of low pressure over the central part of the South China Sea intensified into a tropical depression on 17 October and was later named as Sanba. It moved northwards slowly towards Beibu Wan and intensified into a tropical storm on 18 October. Sanba lingered over the vicinity of Beibu Wan and Hainan Island on 19 October and dissipated gradually over there the next day.
Affected by the rainbands associated with Sanba, the weather of Hong Kong was cloudy with occasional showers on 18 – 19 October. More than 20 millimetres of rainfall were recorded over most parts of the territory and rainfall even exceeded 80 millimetres over Sha Tin and Causeway Bay on these two days. Local weather remained cloudy with a few showers on 20 October. Meanwhile, a cold front moved across the coast of Guangdong later on that day. Under the influence of the associated northeast monsoon, the weather of Hong Kong became slightly cooler on the next two days. The temperatures at the Observatory fell to a minimum of 22.0 degrees on 21 October under rain patches, the lowest of the month. The weather became dry with sunny intervals on 22 October.
With the prevalence of the anticyclone aloft, apart from a few light rain patches on the morning of 23 October, it was mainly fine on 23 – 27 October. Affected by a fresh to strong northeast monsoon and a broad band of clouds, local weather turned mainly cloudy with a few showers on 28 – 29 October. With the clouds thinning out, it was mainly fine and dry on the last two days of the month.
Three tropical cyclones occurred over the South China Sea and the western North Pacific in October 2023.
With more than usual moisture in the lower atmosphere over southern China, October 2023 was much cloudier than usual in Hong Kong. The mean amount of cloud in the month was 79 percent, 21 percent above the normal of 58 percent and the second highest on record for October. The duration of bright sunshine in the month was only 138.9 hours, about 30 percent lower than the normal figure of 197.8 hours and the fourth lowest on record for October. Mainly attributing to the record-breaking rainfall associated with tropical cyclone Koinu on 8 – 9 October, the month was also much wetter than usual. The monthly total rainfall was 546.0 millimetres, more than four times of the normal figure of 120.3 millimetres and the fifth highest on record for October. The accumulated rainfall this year up to October was 2770.3 millimetres, about 17 percent more than the normal figure of 2363.1 millimetres for the same period. Despite the below normal sunshine, the month was warmer than usual. The mean temperature of 26.4 degrees was 0.7 degrees above the normal and one of the fourth highest for October on record. There were 3 very hot days in the early part of the month, the highest on record for October.
Under the influence of an anticyclone aloft, apart from a few showers and isolated thunderstorms, it was very hot with sunny periods during the day on the first three days of the month. Meanwhile, severe typhoon Koinu over the western North Pacific tracked northwestwards towards the Luzon Strait. It turned to move westwards on 4 October and skirted past the southern part of Taiwan on the morning of 5 October. Koinu moved slowly across the northern part of the South China Sea and edged towards the Pearl River Estuary in the next two days. Koinu weakened into a typhoon on 8 October and skirted past about 70 kilometres to the south of Hong Kong that night. It continued to move across the coastal waters of Guangdong the next day and weakened progressively into an area of low pressure over the coastal waters of Yangjiang on that night.
Under the influence of the outer subsiding air of Koinu, it was generally fine and very hot during the day in Hong Kong on 4 – 5 October. The maximum temperature at the Observatory soared to 34.6 degrees on the afternoon of 4 October, the highest of the month and the highest monthly absolute maximum temperature on record for October. Moreover, the daily mean temperature on that day reached 30.8 degrees, the highest on record for October. With Koinu edging closer to the coast of Guangdong, while there were sunny periods during the day, winds over Hong Kong picked up gradually and there were a few showers on the night of 6 October. Local wind strength increased further on 7 October with strong winds offshore and occasional gales on high ground. The weather also turned cloudy with a few squally showers on that day.
With Koinu moving towards the vicinity of the Pearl River Estuary, the weather of Hong Kong deteriorated significantly on 8 October and the Gale or Storm Signal No. 8 was issued on that afternoon. Local winds strengthened significantly and reached storm force offshore and on high ground that night. The Increasing Gale or Storm Signal No. 9 was also issued that night when the compact and hurricane force wind bearing eyewall of Koinu skirted past closely to the south of Hong Kong. With Koinu weakening and departing from Hong Kong, local winds moderated later on 9 October. Koinu brought squally heavy showers to Hong Kong on 8 – 9 October and necessitated the issuance of the Black Rainstorm Warning on the morning of 9 October. The rainfall recorded at the Observatory on 9 October reached 369.7 millimetres, more than three times of October’s monthly total normal figure of 120.3 millimetres and the highest daily rainfall on record for October. Moreover, the 24-hour rainfall from 3 p.m. on 8 October to 3 p.m. next day reached 439.8 millimetres, breaking the highest record for October. Overall, more than 300 millimetres of rainfall were recorded over most parts of the territory and rainfall even exceeded 600 millimetres in Central and Western, Wan Chai, Wong Tai Sin and Kwun Tong Districts on 8 – 9 October.
Under the influence of the outer rainbands associated with the remnant of Koinu, it was cloudy with a few showers on 10 October. With the thinning out of the cloud bands, there were sunny periods in Hong Kong on 11 – 15 October. It was also dry during the day on 13 – 15 October. Affected by a fresh to strong northeast monsoon, there were sunny intervals and a few showers on 16 – 17 October. Meanwhile, an area of low pressure over the central part of the South China Sea intensified into a tropical depression on 17 October and was later named as Sanba. It moved northwards slowly towards Beibu Wan and intensified into a tropical storm on 18 October. Sanba lingered over the vicinity of Beibu Wan and Hainan Island on 19 October and dissipated gradually over there the next day.
Affected by the rainbands associated with Sanba, the weather of Hong Kong was cloudy with occasional showers on 18 – 19 October. More than 20 millimetres of rainfall were recorded over most parts of the territory and rainfall even exceeded 80 millimetres over Sha Tin and Causeway Bay on these two days. Local weather remained cloudy with a few showers on 20 October. Meanwhile, a cold front moved across the coast of Guangdong later on that day. Under the influence of the associated northeast monsoon, the weather of Hong Kong became slightly cooler on the next two days. The temperatures at the Observatory fell to a minimum of 22.0 degrees on 21 October under rain patches, the lowest of the month. The weather became dry with sunny intervals on 22 October.
With the prevalence of the anticyclone aloft, apart from a few light rain patches on the morning of 23 October, it was mainly fine on 23 – 27 October. Affected by a fresh to strong northeast monsoon and a broad band of clouds, local weather turned mainly cloudy with a few showers on 28 – 29 October. With the clouds thinning out, it was mainly fine and dry on the last two days of the month.
Three tropical cyclones occurred over the South China Sea and the western North Pacific in October 2023.
Radar imagery of Koinu skirting to the south of Hong Kong at 8 p.m. on 8 October 2023
24-hour rainfall distribution map (3 p.m. on 8 October 2023 to 3 p.m. on 9 October 2023)
Details of issuance and cancellation of various warnings/signals in the month are summarised in Tables 1.1 to 1.8. Monthly meteorological figures and departures from normal for October are tabulated in Table 2.
Warnings and Signals issued in October 2023
| Name of Tropical Cyclone |
Signal Number |
Beginning Time | Ending Time | ||
|---|---|---|---|---|---|
| Day/Month | HKT | Day/Month | HKT | ||
| KOINU | 1 | 4 / 10 | 2140 | 6 / 10 | 1740 |
| 3 | 6 / 10 | 1740 | 8 / 10 | 1240 | |
| 8 NE | 8 / 10 | 1240 | 8 / 10 | 1900 | |
| 9 | 8 / 10 | 1900 | 8 / 10 | 2350 | |
| 8 NE | 8 / 10 | 2350 | 9 / 10 | 1140 | |
| 3 | 9 / 10 | 1140 | 9 / 10 | 1440 | |
| 1 | 9 / 10 | 1440 | 9 / 10 | 1620 | |
| Beginning Time | Ending Time | ||
|---|---|---|---|
| Day/Month | HKT | Day/Month | HKT |
| 16 / 10 | 2245 | 19 / 10 | 0430 |
| Colour | Beginning Time | Ending Time | ||
|---|---|---|---|---|
| Day/Month | HKT | Day/Month | HKT | |
| Amber | 8 / 10 | 2025 | 9 / 10 | 0155 |
| Red | 9 / 10 | 0155 | 9 / 10 | 0400 |
| Black | 9 / 10 | 0400 | 9 / 10 | 1030 |
| Red | 9 / 10 | 1030 | 9 / 10 | 1430 |
| Amber | 9 / 10 | 1430 | 9 / 10 | 1745 |
| Beginning Time | Ending Time | ||
|---|---|---|---|
| Day/Month | HKT | Day/Month | HKT |
| 9 / 10 | 0200 | 10 / 10 | 0630 |
| Beginning Time | Ending Time | ||
|---|---|---|---|
| Day/Month | HKT | Day/Month | HKT |
| 1 / 10 | 0715 | 1 / 10 | 1000 |
| 9 / 10 | 0035 | 9 / 10 | 1830 |
| Colour | Beginning Time | Ending Time | ||
|---|---|---|---|---|
| Day/Month | HKT | Day/Month | HKT | |
| Yellow | 1 / 10 | 1030 | 1 / 10 | 1900 |
| Yellow | 2 / 10 | 0800 | 2 / 10 | 1800 |
| Red | 5 / 10 | 0800 | 6 / 10 | 1830 |
| Yellow | 14 / 10 | 0600 | 14 / 10 | 1900 |
| Yellow | 15 / 10 | 0600 | 15 / 10 | 1845 |
| Yellow | 22 / 10 | 0600 | 22 / 10 | 1945 |
| Yellow | 23 / 10 | 0600 | 23 / 10 | 1900 |
| Yellow | 29 / 10 | 0945 | 29 / 10 | 1700 |
| Beginning Time | Ending Time | ||
|---|---|---|---|
| Day/Month | HKT | Day/Month | HKT |
| 29 / 9 | 1000 | 5 / 10 | 1800 |
| 6 / 10 | 0645 | 6 / 10 | 1600 |
| Beginning Time | Ending Time | ||
|---|---|---|---|
| Day/Month | HKT | Day/Month | HKT |
| 9 / 10 | 0315 | 9 / 10 | 1615 |
| Meteorological Element | Figure of the Month | Departure from Normal* |
|---|---|---|
| Mean Daily Maximum Air Temperature | 29.1 degrees C | 1.0 degree above normal |
| Mean Air Temperature | 26.4 degrees C | 0.7 degrees above normal |
| Mean Daily Minimum Air Temperature | 24.8 degrees C | 0.9 degrees above normal |
| Mean Dew Point Temperature | 21.7 degrees C | 1.5 degrees above normal |
| Mean Relative Humidity | 76 % | 3 % above normal |
| Mean Cloud Amount | 79 % | 21 % above normal |
| Total Rainfall | 546.0 mm | 425.7 mm above normal |
| Number of hours of Reduced VisibilityΔ | 0 hours | 107.2 hours below normal§ |
| Total Bright Sunshine Duration | 138.9 hours | 58.9 hours below normal |
| Mean Daily Global Solar Radiation | 12.25 Megajoule / square metre | 2.27 Megajoule below normal |
| Total Evaporation | 92.7 mm | 29.9 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. |
* Departure from 1991 - 2020 climatological normal, except for number of hours of reduced visibility |
|
§ Departure from mean value between 1997 and 2022 |
|
| 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 1991 to 2020 |
