As confirmed in the Chernobyl nuclear plant accident, it is not always the case that contamination levels decline in a gradual linear way as the geographical distance gets farther from the accident site, and the levels in some spots are found high.
If an emergency situation arises at a nuclear facility and radioactive materials leak in the form of gas or in particles, they could travel together with the air like clouds. This is called a radioactive plume. When a radioactive plume is passing high up in the sky, the air dose rate temporarily rises in this spot. Also, the plume may descend, being affected by geographical features such as a land depressions, direction of the wind, rainfall and snowfall. If radioactive substances deposit on the ground surface, the air dose will rise. It is believed that such phenomena cause some places far away from the plant to be observed as having a high dose rate.