Mechanisms of radioactive fall-outs
The deposits of radioactive isotopes on the ground are the cause of external exposure as well as internal exposure if they find their way into the food chain. In dry, windy weather the radioactive particles may reach the ground, but it is mainly rain that can accelerate the deposits, forcing the particles to the ground. This explains the patches of radioactivity observed on the map of Europe shortly after Chernobyl. These patches correspond either to regions where it had been raining or to regions of mountains which had stopped the radioactive cloud.
IRSN/dessin : Martine Beugin