constructed. On tnis figure tne scaled radius (on the basis m = 3.4), is plotted-against tne scaled charge depth (on the basis m= 3).* The next step is tne determination of the curve for nuclear charges based on tnis curve for TNT charges. In this procedure con- sideration must be given to the ditference in mechanism of nuclear and TNT bursts, particularly for bursts on une surface or at very low heights above the surface. In the early stages of a nuclear explosion fired at or near the interface between air and eartn, the shock wave velocity is very much higher in tne air than in the earth;** nence, at a time when the nuclear explosion process nas proceeded to the point where the average energy density***within tne ooundary of tne snack wave is equal to the average energy density» at the surface of a spnerical TNT charge which has been detonated at .ts center, the envelope ov tne nuclear explosion is es- sentially hemispherical. If average energy density 1s a good criterion of crater size and shape, then on this oasis the crater formed by a given nuclear energy release on the surface snould be similar to the crater formed by a TNT charge of the same yield fired well above the surface. **** The crater resulting from a auclear surface charge should differ extensively from that produced by a TNT charge whose c.g. is at the surface, both vecause of the different mechanism mentioned above and because a hemispnerical excavation was required before the TNT charge could be placed. Consider a nuclear charge at A- = -).13. Within its shock wave the total energy will be identically the same as that within a sphere of TNT tangent to the surface when both shock waves reach the surface. This argument can oe summarized by saying tnat the crater radius produced by a low aboveground nuclear snot snould be essentially independent of neight, and (if the efficiency were 100 per cent) should have about tne same value as that preduced cy a TNT snot at AG = -0.13. On this basis tne dotted curve in the region AB has been drawn on Fig. 4.3. * Since the range of scaled depths is small in the interval of greatest interest,the distinction between determining scaled depths on tne basis m = 3,0 and on tne basis m= 3.4, :s5 relatively trivial and will not affect tne conclusions reached in this analysis. ** DT. Griggs, 11 predicting tne effects of JANGLE ut computes shock wave velocities i: alr to be approximately 25 times those in soil in tne radius range from approximately A= U.l1to,A= 1.0. Similarly, Porzel, in predicting the effects of IVY Mike ,2/estimates snock velocities in the arr and water soaked sand far high overpressures such that in the early stages of a nuclear explosion tne ratio of velocity in air to velocity in soil may be as nign as 1l30C:1). wee By “averaze energy density” is meant the total energy contained within tne snock wave, divided by tne total volume within it. weee Actually, as Porzel points out,£’at a time when the nuclear shock wave has reacnced tne same radius as tnat of tne TNT sphere of equiva- lent energy release, (and hence wnen average energy densities are equal ) there is still an enormous difference .n tne two situations since the mass enclosed witrin tne shock wave in the case of TNT is some 1500 times that in the nuclear case. Hence, pressures are very much higher and tre + bal ae in tne nuclear situation the durations shorter tnan in the