According to Dr. Jeff Masters (from Weather Underground),

"Radiation from Japan's stricken ***ushima Daiichi nuclear plant has been detected 100 miles to the northeast, over the Pacific Ocean, by the U.S. military. Westerly to southwesterly winds have predominated over Japan the past few days, carrying most of the radiation eastwards out to sea. The latest forecast for Sendai, Japan, located about 40 miles north of the ***ushima nuclear plant, calls for winds with a westerly component to dominate for the remainder of the week, with the exception of a 6-hour period on Tuesday.

Thus, any radiation released by the nuclear plant will primarily affect Japan or blow out to sea. A good tool to predict the radiation cloud's path is NOAA's HYSPLIT trajectory model. The model uses the GFS model's winds to track the movement of a hypothetical release of a substance into the atmosphere. One can specify the altitude of the release as well as the location, and follow the trajectory for up to two weeks. However, given the highly chaotic nature of the atmosphere's winds, trajectories beyond about 3 days have huge uncertainties.

One can get only a general idea of where a plume is headed beyond 3 days. I've been performing a number of runs of HYSPLIT over past few days, and so far great majority of these runs have taken plumes of radioactivity emitted from Japan's east coast eastwards over the Pacific, with the plumes staying over water for at least 5 days. Some of the plumes move over eastern Siberia, Alaska, Canada, the U.S., and Mexico in 5 - 7 days.

Such a long time spent over water will mean that the vast majority of the radioactive particles will settle out of the atmosphere or get caught up in precipitation and rained out. It is highly unlikely that any radiation capable of causing harm to people will be left in atmosphere after seven days and 2000+ miles of travel distance. Even the Chernobyl nuclear disaster, which had a far more serious release of radioactivity, was unable to spread significant contamination more than about 1000 miles." - Dr. Jeff Masters.

For the original article please follow the original article on Dr. Masters' official page.

For more videos, maps and "How To Protect Yourself From Radiation Exposure" visit: