l hour after the burst would be expected to be 0.1 R/hr after 7 hours and 0.01 R/hr after 49 hours. This rule seems to be valid for about 6 months following an explosion, after which the observed decay is somewhat faster than that predicted by this relationship. The activation products, in general, decay at a faster rate than the fission products. Fission products and the activation products, along with unfissioned uranium or plutonium from the device, are the components of the radioactive material in the fallout cloud, and this cloud is the primary source of po- tential exposure to residual radiation. In a nuclear airburst in which the central core of intensely hot material, or fireball, does not touch the surface, the bomb residues (including the fission products, the activation products resulting from neutron interaction with device materials, and unfissioned uranium and/or plutonium) are vaporized. These vapors condense as the fireball rises and cools, and the particles formed by the condensation are small and smokelike. They are carried up with the cloud to the altitude at which its rise stops, usually called the cloud stabilization altitude. of this material then depends on the winds and weather. The spread If the burst size is small, the cloud stabilization altitude will be in the lower atmosphere and the material will act like dust and return to the Earth's surface ina matter of weeks. Essentially all debris from bursts with yields equiva- lent to kilotons of TNT will be down within 2 months (Reference 2). The areas in which this fallout material will be deposited will appear on maps as bands following the wind's direction. Larger bursts (yields equivalent to megatons of TNT) will have cloud stabilization altitudes in the stratosphere (above about 10 miles [16 km] in the tropics); the radioactive ma- terial from such altitudes will not return to Earth for many months and its distribution will be much wider. Thus, airbursts contribute little potential for radiation exposure to personnel at the testing area, although there may be some residual and short-lived radiation coming from activated surface materials under the burst if the burst altitude is sufficiently low for neutrons to reach the surface. 29