The Year's Weather – 2021

Friday, 7th January 2022


Globally, 2021 is likely to be between the fifth and seventh warmest year on record according to the World Meteorological Organization’s preliminary assessment. Global mean sea level reached a new record high in 2021. In 2021, different parts of the world were ravaged by various extreme weather events, including heatwaves in western North America, Mediterranean region and eastern Europe; extreme cold events in many parts of the central United States, northern Mexico, northern Asia and Europe; severe drought in subtropical South America, western North America, southwest Asia and southern Madagascar; extreme rainfall triggered severe flooding in Henan Province of China, western Europe, eastern New South Wales in Australia, Afghanistan, the Mediterranean and Black Sea coasts, northern South America, India, Pakistan, Bangladesh, Nepal, and western Japan; and severe damages and heavy casualties brought by tropical cyclones in the United States, Venezuela, Indonesia, Timor-Leste, Australia, Mozambique, South Africa, Zimbabwe, Eswatini, Madagascar, India, Pakistan, Oman, Iran and the Philippines. Accentuated by widespread drought and high temperature weather, destructive wildfires wreaked havoc in northern California of the United States, Algeria, southern Turkey, Greece, Siberia and parts of Brazil.

A short-lived La Niña event was established in January 2021 and ended in March of that year. Sea surface temperatures of the central and eastern equatorial Pacific remained normal throughout spring and summer, but became below normal again in October 2021. The colder-than-normal sea surface temperatures persisted afterwards, indicating the development of another La Niña event.

In Hong Kong, with record-breaking monthly mean temperatures of 22.0 degrees in March, 29.0 degrees in May and 29.7 degrees in September, 2021 was the warmest year on record with an annual mean temperature of 24.6 degrees, 1.1 degrees above the 1991-2020 normal^[1] (or 1.3 degrees above than the 1981-2010 normal). The annual mean maximum temperature of 27.5 degrees and annual mean minimum temperature of 22.6 degrees were also the highest on record. The highest temperature recorded at the Hong Kong Observatory in the year was 36.1 degrees on 23 May, one of the third highest on record. There were 61 Hot Nights^[2] and 54 Very Hot Days^[3] in Hong Kong in 2021, both ranking the highest on record and breaking the previous records set in 2020.

The lowest temperature recorded at the Observatory in the year was 7.7 degrees on 8 January. The number of Cold Days^[4] in the year was 13 days, 2.2 days less than the 1991-2020 normal (or 4.1 days less than the 1981-2010 normal).




The annual total rainfall in 2021 was 2,307.1 millimetres, 5 percent below the 1991-2020 normal of 2,431.2 millimetres (or 4 percent below the 1981-2010 normal of 2,398.5 millimetres). Six red rainstorm warnings and two black rainstorm warnings were issued by the Observatory in the year. The number of days with thunderstorms reported in Hong Kong was 41 days in 2021, about 1 day less than the 1991-2020 normal (or 2 days more than the 1981-2010 normal).


A total of 27 tropical cyclones occurred over the western North Pacific and the South China Sea in 2021, less than the long-term (1961-2010) average of about 30. There were 10 tropical cyclones reaching typhoon intensity^[5] or above during the year, less than the long-term average of about 15, and 5 of them reached super typhoon intensity (with maximum 10-minute wind speed of 185 km/h or above near the centre). In Hong Kong, 8 tropical cyclones necessitated the issuance of tropical cyclone warning signals, more than the long-term average of about 6 in a year. The No. 8 Gale or Storm Signals were issued during the passages of Lionrock and Kompasu in October.


Detailed description of the weather for individual months is available on the Monthly Weather Summary webpage:
https://www.weather.gov.hk/en/wxinfo/pastwx/mws/mws.htm


Some significant weather events in Hong Kong in 2021 are highlighted below:

Warmest March

March 2021 was exceptionally warm in Hong Kong. The monthly mean maximum temperature of 24.8 degrees, monthly mean temperature of 22.0 degrees and monthly mean minimum temperature of 20.2 degrees were 2.9 degrees, 2.5 degrees and 2.6 degrees above their corresponding 1991-2020 normals (or 3.4 degrees, 2.9 degrees and 3.0 degrees above their corresponding 1981-2010 normals). All of them were the highest on record for March.

Hottest May

May 2021 was the hottest May in Hong Kong on record. The monthly mean temperature of 29.0 degrees and monthly mean minimum temperature of 27.0 degrees were 2.7 degrees and 2.5 degrees above their corresponding 1991-2020 normals (or 3.1 degree and 2.9 degrees above their corresponding 1981-2010 normals). Both were the highest on record for May. The mean maximum temperature of 32.1 degrees was 3.3 degrees above the 1991-2020 normal (or 3.7 degrees above the 1981-2010 normal) and the second highest on record for May.

Hottest September

Hong Kong experienced the hottest September on record in 2021. The monthly mean maximum temperature of 32.8 degrees, monthly mean temperature of 29.7 degrees and monthly mean minimum temperature of 27.8 degrees were 2.3 degrees, 1.8 degrees and 1.7 degrees above their corresponding 1991-2020 normals (or 2.7 degrees, 2.0 degrees and 2.0 degrees above their corresponding 1981-2010 normals). All of them were the highest on record for September.

Warmest spring and first half-year

Hong Kong experienced the warmest spring on record from March to May 2021. The mean maximum temperature of 28.0 degrees, mean temperature of 25.0 degrees and mean minimum temperature of 23.2 degrees were all the highest on record for spring. Moreover, the first half of 2021 from January to June was exceptionally warm. The mean maximum temperature of 26.3 degrees, mean temperature of 23.3 degrees and mean minimum temperature of 21.3 degrees were all the highest on record for the same period.

Record-breaking numbers of Hot Nights and Very Hot Days

With much hotter than usual weather from May to mid-October in the year, 2021 saw the following record-breaking numbers of Hot Nights and Very Hot Days in Hong Kong:

• There were 14 hot nights in May 2021, the highest on record for May;
• The streak of 6 consecutive hot nights that started from 16 May 2021 was the longest on record for May;
• There were 11 hot nights and 15 very hot days in September 2021. Both were the highest on record for September;
• There were 4 hot nights in October 2021, the highest on record for October (on par with October 2017);
• There were 22 hot nights in the first half of the year (January to June), the highest on record for the same period;
• The total numbers of 61 hot nights and 54 very hot days in the year were both the highest annual numbers on record.

Exceptionally dry in the first five months

Mainly attributing to the well below normal rainfall in spring from March to May 2021, the weather in Hong Kong was exceptionally dry in the first five months of the year. The accumulated rainfall recorded in those five months was only 163.1 millimetres, a deficit of 72 percent as compared to the 1991-2020 normal of 590.9 millimetres (or 75 percent below the 1981-2010 normal of 640.8 millimetres) and the second lowest on record for the same period. The very dry condition was alleviated by the abundant rainfall in June 2021 due to tropical cyclone Koguma, troughs of low pressure and the active southwest monsoon.

Rainstorm on 28 June

Affected by an active southwesterly airstream, Hong Kong experienced heavy and persistent rain with squally thunderstorms on the morning of 28 June 2021. The incessant downpour necessitated the issuance of the first Black Rainstorm Warning in the year. More than 150 millimetres of rainfall were recorded over some parts of Hong Kong on that day.


Successive strikes of Lionrock and Kompasu in October

Tropical Storm Lionrock and Typhoon Kompasu successively affected Hong Kong within a week from 8 October to 14 October 2021. Under the combined effect of the tropical cyclones and the northeast monsoon, the No. 8 Gale or Storm Signals were issued for both tropical cyclones with only a break of 60 hours and 40 minutes in between the two No. 8 signals, the shortest record for two different tropical cyclones since 1946.

(a) Lionrock
Lionrock brought strong to gale force winds and heavy squally showers to Hong Kong on 8 – 10 October 2021. Winds over the southwestern part of Hong Kong even occasionally reached storm force on 9 October 2021. Moreover, the rain was particularly heavy and persistent on 8 October 2021 with more than 200 millimetres of rainfall generally recorded over Hong Kong, necessitating the issuance of the second Black Rainstorm Warning in the year. The rainfall recorded at the Observatory on that day even reached 329.7 millimetres, more than two times of October’s monthly total normal figure of 120.3 millimetres and the highest daily rainfall on record for October.

(b) Kompasu
With the approach of Kompasu, local winds strengthened again on 12 October 2021. Winds over Hong Kong were generally strong to gale force with winds reaching storm force offshore and even hurricane force on high ground on 13 October 2021. The outer rainbands of Kompasu brought heavy showers to Hong Kong on 12 – 13 October 2021 with over 100 millimetres of rainfall recorded in some parts of the territory on 13 October 2021. Moreover, the storm surge induced by Kompasu raised the water level in Hong Kong over 1 metre higher than the normal tide levels on the morning of 13 October 2021. Coincided with the astronomical high tide, the aggregated effect resulted in inundation of some low-lying areas in Hong Kong, including Tai O, Shing Mun River and Lei Yue Mun.


Extremely wet October

Attributing to the heavy rain induced by tropical cyclones Lionrock and Kompasu, October 2021 was much wetter than usual in Hong Kong. The monthly rainfall of 631.1 millimetres was more than five times of the 1991-2020 normal figure of 120.3 millimetres (or more than six times of the 1981-2010 normal of 100.9 millimetres) and the second highest on record for October.

Rai – the record-breaking storm in December

After wreaking havoc in the Philippines, Super Typhoon Rai weakened into a severe typhoon and continued to move westwards entering the southern part of the South China Sea on 17 December 2021. Rai re-intensified into a super typhoon on 18 December 2021, making it the first super typhoon occurred in the South China Sea in December since 1961. On 19 December 2021, Rai gradually turned to move north towards the northern part of the South China Sea and weakened progressively. With Rai weakening into a severe tropical storm and tracking northeastwards towards the coast of Guangdong, the Observatory issued the Standby Signal No. 1 on the morning of 20 December 2021. Breaking the record of Irma in 1974, Rai became the storm which necessitated the issuance of the latest tropical cyclone warning signal in a year since 1946.


Notes :
[1] Climatological normals for the reference period of 1961-1990, 1971-2000, 1981-2010 and 1991-2020 are available at: https://www.weather.gov.hk/en/cis/normal.htm.  Climatological normals of 1991-2020 are referenced in the text unless otherwise stated.
[2] 'Hot Night' refers to the condition with the daily minimum temperature equal to or higher than 28.0 degrees.
[3] 'Very Hot Day' refers to the condition with the daily maximum temperature equal to or higher than 33.0 degrees.
[4] 'Cold Day' refers to the condition with the daily minimum temperature equal to or lower than 12.0 degrees.
[5] Information on the classification of Tropical Cyclones is available at: https://www.weather.gov.hk/en/informtc/class.htm