DNA 1240H-2 —= ee are discussed in 17.2 2 and 17.2.3, along with the reliability of topside radiant-exposure predictions. ‘ 17.2.2 Free~Field Data Experimental findings have established that the free-field data required to assess the damage produced by thermal radiation are given by two quantities---the radiant exposure, or the amount of incident thermal energy per unit area of the target, and the rate at which this energy is delivered. The total amount of incident thermal energy del- ivered to a target, measured in cal/em*, varies directly with the amount of thermal energy emitted at the fireball. The amount of emitted energy increases linearly with increasing weapon yield, attenuates with distance from the energy source, and varies with atmospheric conditions. The rate at which the energy is delivered ie determined by the duration of the thermal pulse, which lengthens with increasing yield. As oe result, thermal energy from large-yield weapons is delivered more siowly than that from small-yield weapons. The significance of the delivery rate lies in the fact that since a target rapidly dissipates the heat it receives, it will not overheat if the delivery rate is sufficiently slow. Thus, for a given amount of thermal energy per unit target area, damage to a target will be greater when the energy is delivered so rapidly that little heat loss can occur during delivery, than when the energy is delivered more siowly. For instance, the fireball of a 1-KT detonation can deliver 4 cal/ in less than 1 second, resulting in an incapacitating burn on bare skin. A 4-cal/cm® radiant exposure from a 10 Mf burst, which is delivered eat a slower rate (it will take more than 30 sec), may cause no more than a Ist-degree burn on the same bare skin. Radiant Exposures The ranges from surface zero at which water-surface detonations of various yields will cause specified radiant exposures have been estimated through analysis of data taken at weapons tests.1 This analysis is summarized in the lower curve of Figure 17-2, Radiant Exposure Normalized to 1 KT vs Range. From this curve, etary given range, values of the radiant exposure from any yield can be scaled for the atmospheric conditions prevailing during weapons tests at the Pacific Proving Grounds, where visibility was only about 10 miles. The upper curve of the figure was fitted to data obtained at land-surfece bursts in Nevada, including data for tower surface-intersecting shots. Visibility was excellent and atmospheric transmission was high during these tests. Since water-surface bursts may occur in regions such as the North Pacific, where visibility and atmospheric transmission are generally higher than they were in the test area, the Nevada curve is included and represents upper limiting values of radiant exposures from surface bursts. Data points to which both curves were fitted are indicated on the plots. 17-6 Ro cae ence ees ga pee aeere ee ee ee