seer ee em eee mk - - ae woe eee DNA 1240H-2 Theoretically, radiant exposures at a distance from water-surface bursts (considered as point sources) would be calculated by use of the expression gq = kY 10 EAS 12 Lande = cat/em? Q7-1) Where Y is the weapon yield in kilotons. k, a fraction modifying Y, is a function of (1) the fraction of the total energy appearing as thermal radiation (2) the angle of elevation of the receiver, (3) the shape of the fireball. J is the atmospheric transmissivity (the ratio of the energy incident per unit area on a target in a real atmosphere to that which would be incident on the target in a vacuum). D is the distance from surface zero to the target (in cm). However, there are so many unknown factors in Eq.17-1 that calculated results are unreliable. The value of k may lie between 1/7 and 1/3. Furthermore, there is little reliable verification of the graphical values of T given in Ref. 2. Atmospheric transmissivity is a complex function of several unpredictable variables, such as water-vapor and carbon-dioxide absorption of infra-red radiation, and miltiple scat- tering of all radiation. Furthermore, reflection from partial or total cloud cover, a factor unaccounted for in theoretical calculations, can increase the effective exposure by a factor of as much as 2. Finally values of Q calculated with the values of k and T given in Ref. 2, are not in agreement with available field-test data (some values differ by as mch as a factor of 3). Since theoretically calculated radiant exposures do not agree with empirical data, the curves of Fig. 17-2, which are in good agreement (within + 25%) with data, are considered the most reliable current method for estimating radiant exposures. Rate of EnergyDelivery Analysis of thermal data from weapons tests has resulted in establishment of a relationship between weapon yield and the time required for emission of the thermal energy that is effective in burning. A reevaluation> of the data for the time to final maximm (tq) as a function of weapon yield has provided an expression that is in excellent agreement with field-test data. Water-surface-burst data indicate a cutoff of radiant exposures after 10 t,. This cutoff is apparently caused by the formation of a Wilson Cloud (which, however, may not form under atmospheric conditions different from those at the Pacific Proving Grounds where all the tests were held). Furthermore, 17-8 ree mw — te ee ew ee ge re eee ee ee