In this connection it is worth noting that radiation above normal background levels sometimes has been detected when no atomic tests have occurred. This is due to "cosmic showers," those periodic phenomena of nature which have been occurring for years without any apparent danger to the human race, While it is true that there are radioactive particles in bomb clouds produced by the Nevada tests, this does not mean that dangerous radioactivity 1s scattered over off-site areas, In the first place, the quantities of radioactive particles in these clouds are not comparable to those produced by an H-bomb explosion, and secondly, the bulk of the particles lose most of thelr radioactive strength within a matter of hours, Rigid precautions are taken to hold the fallout from Nevada test shots to an absolute minimum. Tests are conducted only when weather conditions are suitable, thereby explaining the frequent postponements in the current series. Suitable weather conditions cre selected in order to minimize the fallout and off-site blast effects. Methods of forecasting wind speeds