Are there any seasonal changes in the ambient radiation level in Hong Kong ?

The ambient radiation level in Hong Kong is higher in winter than in summer. In winter, the prevailing wind is northerly which is of the continental origin, hence carrying with it more natural radioactive material for the same reason as question 3 above. For most of the time in summer, Hong Kong is under the influence of a southerly airstream of maritime origin, carrying with it much less natural radioactive material. This accounts for the seasonal changes in the ambient radiation level in Hong Kong (see case 3 given here).