The Belgium Institute for Space Aeronomy has posted simulations of the dispersion routes of clouds of radioactive pollution emitted by the explosion in Daiichi Building-3 nuclear power plant in Fukushima in Japan.
“This simulation is one of the first to take into account the leaching by rain as the cloud on the route,” their website notes. “This results in a much faster decrease of the concentration of particulate pollutants. Based on these projections (which extend up to this Saturday), the cloud will disappear before they reach U.S. shores.  This simulation is based on the hypothetical initial cloud between 0 and 1500 m altitude. We tested the sensitivity to this assumption with another simulation where the initial cloud rises up to 3000 m. Thanks to leaching by rainfall, the result is almost the same: the cloud of radioactive pollution disappears over the Pacific Ocean.”

You can watch more cloud dispersal simulations by clicking here. And you can read updates on the nuclear power plants in Japan by checking out the International Atomic Energy Agency's website.