Fallout is a grim and long lasting feature of ground burst atomic munitions. Essentially, fallout is the irradiated dust and debris particles resulting from a nuclear blast. The half life of these particles can range from an hour, to a week, to many years depending on the intensity of the blast. All radiation is toxic in varying degrees and doses.

The mixture of radioactive elements formed in a nuclear explosion is so complex, with both short- and long-lasting isotopes, that radioactive decay can only be estimated. During the first hour after a nuclear explosion, radioactivity levels drop precipitously. Radioactivity levels are further reduced by about 90% after another 7 hours and by about 99% after 2 days. That is for short term radioactive material. Long term, such as Plutonium, is a different matter. For more information, see here....

The immediate threat is direct exposure to skin from rainfall and dust particles riding on low breezes. The result is external radiation burns and ARS (Acute Radiation Syndrome). In Jericho, the direct exposure from the rainfall was addressed by seeking shelter. It appears that all Jericho citizens save Stanley Richmond were not directly exposed to the radioactive rainfall.

Precipitation clears the irradiated particles from the air, but that is not the end of the problem. As things dry up, normal, every day dust is kicked up as well. Much of that will carry the contaminants. You breath them in, they embed in your lung tissue. Depending on the level of exposure, this can result in death from radiation poisoning in a number of hours or days or can lead to long term health risks such as cancer and leukemia.

The solution to the radioactive particles on the ground is by removing the top 18 inches of soil and disposing of them in an isolated and safe area. The definition of an "isolated and safe area" is subject to debate, and while some suggest burying the radioactive material underneath a mountain, others suggest disposal in the ocean (salt water is an excellent buffer of radiation). However, these solutions are environmentally controversial.

Note: Nuclear fallout particles will not "re-evaporate" from water sources. They are heavier than water and will sink.