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Wyss Yim

Since the establishment in 1883 of the Royal Observatory Hong Kong (renamed Hong Kong Observatory after the handover), continuous records have been kept at the headquarters station in Tsim Sha Tsui, except from 1940 to 1946.
From 1884 to 2011, the mean annual rainfall at the station was 2,228 mm, ranging from 901.1 to 3,343 mm. The top 10 wet and dry years are indicated in the chart (right),  which shows that eight of the wettest years occurred during the past 60. Five were strong El Nino years, with the two wettest years, 1997 and 1982, ranking the most intense and second-most intense respectively.
Satellite tracking of volcanic clouds has indicated that two wet years, 1982 and 2008, were connected to major volcanic events - the eruption of the El Chichon volcano in Mexico in March and April, 1982, and the Chaiten volcano in Chile in May 2008. 
The El Chichon debris took 12 days to reach Hong Kong, contributing condensation nuclei for torrential rainfall in late April to May. Stratospheric warming of the tropics may have been responsible for the intense El Nino episode. In 2008, the Chaiten debris took 35 days to arrive, and caused the wettest June in Hong Kong's history, including the once-in-1,100 year rainstorm on June 7. Besides severe flooding in various locations, the intense rainstorm caused more than 1,600 landslides on Lantau Island. 
The 10 driest years are relatively evenly distributed from 1898 to 2011. During 1963, Hong Kong's worst drought year, water supply was reduced to just four hours in over four days. The severe drought has been attributed to air circulation changes resulting from the March-May eruption of the Agung volcano on the Indonesian island of Bali. 
The June 1991 eruption of the Pinatubo volcano in the Philippines was a factor in Hong Kong's 11th driest year on record. 
Similarly to volcanic eruptions, large nuclear explosions may also cause drought years. The detonation of the world's largest nuclear bomb in the Soviet Union on October 31, 1961, was followed by the 20th driest year on record in 1962. The atomic bomb exploded by France at Mururoa Atoll in September 24, 1966, was followed by the eighth driest year on record in 1967.
As for the 10 heaviest episodes of hourly rainfall recorded by the Observatory, all except 1926 occurred within the past 50 years. The first, the second and the ninth-ranked have occurred within the past five years, which may be attributed to the influence of the urban heat island effect.
Rainstorms accompanying low-pressure troughs are exacerbated by the heat generated by human activities. Additionally, the air pollutants, including aerosols and particulates, may intensify rainstorms by acting as condensation nuclei.
Hong Kong is particularly sensitive to wind shifts due to its location on the continental margin. Under 'normal' monsoon conditions, summer winds are from the southwest and winter winds are from the northeast.
Drought conditions can be explained by the 'normal' wind pattern changing to predominantly offshore after volcanic eruptions or large nuclear explosions.
Abnormally wet conditions can be explained by the 'normal' wind pattern changing to predominantly onshore. This is supported by satellite tracking of the spread of volcanic debris, as in 1982 and 2008.
Volcanic eruptions are natural atmospheric chemistry experiments that enable us to learn about climatic variability. Nowadays we can study their impacts closely, with Nasa operating five A-train satellites carrying instruments to provide atmospheric observations.
In January last year, the respected Browning Newsletter suggested that volcanic eruptions in the northwestern Pacific were responsible for the La Nina episode of 2010/2011. Their impact on circulation changes through strengthening of the onshore trade winds and the offshore westerly winds helped account for the summer floods in Queensland, Australia, and the winter drought in Shandong province respectively. Locally, 2011 was the seventh driest year on record.
The role of water vapour as a greenhouse gas is underestimated by the Intergovernmental Panel on Climate Change in comparison with carbon dioxide (CO2). While volcanic eruptions are a natural cause of atmospheric water vapour redistribution, the world's seven billion humans are also changing the natural water cycle through their actions, including deforestation, dams and domestic, agricultural and industrial consumption.
A study of the warming power of CO2 and water vapour by Paulo Cesar Soares in 2010 concluded that, unlike CO2, water vapour in the atmosphere is rising in tune with monthly temperature changes.
Volcanic eruptions and nuclear explosions have been overlooked as natural and anthropogenic causes respectively for extreme rainfall variability. My findings support a greater role for water vapour than CO2 in driving climate change. 
Wyss Yim is an earth scientist specialising in environmental change. From 2007 to 2009, he served as the deputy chairman of the Climate Change Science Implementation Team of Unesco's International Year of Planet Earth. The views expressed here are his alone, and are based on his research.




How volcanic activity has influenced our rainfall