Before each and every nuclear detonation at the Nevada Test Site, a panel of experts weighed carefully all of the factors that insured safety. On the panel were representatives from the fields of public health, medicine, meteorology, fallout phenomenology, blast effects, etc. As-a result of these deliberations, more than 200 delays in firing have been made at a cost of millions of dollars, to insure safety. The principal cause for the delays was weather conditions, i.e., to insure minimum fallout in populated areas. The U. S. Weather Bureau predicted downwind trajectories, precipitation and other factors which could affect levels of fallout. The data from the weather stations were currently available almost up to the exact time of the shot. A detonation could be cancelled at any time up to a few second before shot time. A more complete description of the meteorological program is given below. To insure safety to aircraft, both from the initial flash of light and any radioactivity in the air mass moving off-site from atmospheric tests, a representative of the Federal Aviation Agency was made an integral part of the Test Organization. He prepared flight advisory plans based on the type of event and on the predicted meteorological conditions. The plan delineated flight patterns and areas and recommended alternate routes, if required, by commercial and private aircraft. Fre- quently the FAA closed specific air lanes and rerouted aircraft for specified periods. Blast effects were minimized by predicting blast wave intensities based on the wind and temperature profile expected at shot time. 46.