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  • Elevated ambient radiation level during a rain episode

  • Monday, 1st December 2014

On the night of 22 October 2014, the ambient gamma radiation levels at the radiation monitoring stations in Yuen Ng Fan and King's Park rose significantly, with hourly average dose rates of 0.266 µSv/h and 0.263 µSv/h respectively (Figure 1). The reading at Yuen Ng Fan was more than double its annual average of 0.116 µSv/h in 2013. The one-minute average dose rates at Yuen Ng Fan and King's Park even rose to 0.328 µSv/h and 0.304 µSv/h respectively. Fluctuations in readings were also observed at some other radiation monitoring stations. In the following, we will explain how the precipitation that night would lead to the elevated ambient gamma radiation levels.

Figure 1

Figure 1      Time series of the hourly mean ambient gamma dose rate at Yuen Ng Fan and King's Park; and the
corresponding hourly rainfall from 22 to 23 October 2014.

Humans are continuously exposed to natural radiation in the environment that comes from naturally occurring radioactive materials in our habitats, buildings and structures, as well as cosmic rays from outer space. There are differences in the ambient radiation levels at different parts of the world mainly due to variations in the geographical location and the radioactive contents of local rock and soil. The ambient radiation level also varies under different weather conditions.

On the night of 22 October 2014, localized heavy rain affected Sai Kung, central Kowloon and the western part of Hong Kong Island. Figure 2 shows the radar pictures at 10 p.m. and 11 p.m. that night, indicating a northeast-southwest oriented rain band was affecting the above-mentioned areas. It is observed that the timing for the rise in ambient gamma radiation level readings at Yuen Ng Fan and King's Park were well in line with the rain episode over these areas (Figure 1). The downpour brought natural airborne radionuclides aloft down to the ground, leading to an increase in ambient radiation levels [1, 2].

Figure 2

Figure 2      Radar imageries at 10 p.m. and 11 p.m. on 22 October 2014, showing rain was affecting
Sai Kung and the urban areas.

Furthermore, the prevailing wind condition also helped contribute to the relatively high ambient radiation levels during the episode. On 22 October 2014, southern China was dominated by a continental northeasterly airstream. The localized heavy rain was associated with the convergence between the northeasterlies and the easterlies which set in along the coast that night (Figure 3). Convergence of air masses also facilitated accumulation of natural radionuclides in the atmosphere, which were then brought to the ground by the downpour [3].

Figure 3

Figure 3      Local winds on the night of 22 October 2014, showing the convergence of northeasterlies and easterlies along
the northeastern part of the New Territories and the urban areas.

To assess whether the rise in ambient gamma dose rate could be due to non-natural causes, the rain water samples collected at King's Park during the rain episode on 22 October 2014 were analyzed and see whether artificial radionuclides could be identified. Measurement results show that no artificial radionuclide was detected in the samples. This well demonstrates that the variation of ambient gamma dose rate in this episode is due to changes in local weather.

W.H. Leung and Olivia S.M. Lee


[1] United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) 2000 Report, Volume I, Sources and Effects of Ionizing Radiation, Annex B: Exposures from natural radiation sources.

[2] Wong, M.C., H.Y. Mok, H.K. Lam, 1996: Effects of weather on the ambient gamma radiation levels in Hong Kong, Proceedings of the International Congress on Radiation Protection 1996, Volume 2, p.181-183, International Radiation Protection Association.

[3] Chan, S.W., C.W. Lee, K.C. Tsui, 2010: Atmospheric Radon in Hong Kong, Journal of Environmental Radioactivity 101 (2010) 494-503.

Last revision date: <06 May 2015>