constructed. On this figure the scaled radius (on the basis m = 3. 4), is plotted against the scaled charge depth (on the basis m= 3).* The next step is the determination of the curve for nuclear charges based on this curve for TNT charges. In this procedure con- sideration must be given to the difference in mechanism of nuclear and TNT bursts, particularly for bursts on the surface or at very low heights above! the surface. : a the early stages of a nuclear explosion fired at or near the interface between air and earth, the shock wave velocity is very much higher~ in the air than in the earth;** hence, at a time when the nuclear explosion ‘process has proceeded to the point where the average energy density**within the boundary of the shock wave is equal to the average energy dessity at the surface of a spherical TNT charge which has been detonated at its center, the envelope of the nuclear explosion is essentially hemispherical. If average energy density is a good criterion of crater size and shape, then on this basis the crater formed by a given nuclear energy release on the surface should be similar to the crater formed by a TNT charge of the same yield fired well above the surface.*#*# The crater resulting from a nuclear surface charge should differ extensively from that produced by a TNT charge whose c.g. is at the surface, both because of the different mechanism mentioned above and because & hemispherical excavation was required before the TNT charge could be placed. Consider a nuclear’ ‘charge at A, = -0.13. 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 be summarized.by’ saying that the crater radius pro- duced by a low aboveground nuclear sho} should be essentially independent of height, and (if the officiency.were 100 per cent) should have about the same value as that producéd)by a TNT shot at A, = -0.13. On this basis the dotted curve in the r ond has been drawn on Fig. 4.3. | * Since the range of scaled depths is s in the interval of greatest interest,the distinction between determining scaled depths on the basis m = 3.0 and on the basis m= 3.4, is relatively‘trivial and will not affect the conclusions reached in this analysis... we DT. Griggs, in predicting the effects of JANGLE u2/computes shock wave velocities in air to be approximately 25 times those in soil in the radius range from approximately A= 0.1 tg A= 1.0. Similarly, Porzel, in predicting the effects of IVY Mike ,2/estimates shock veloci- ties in the air and water soaked sand for high overpressures such that in the early stages of a nuclear explosion the ratio of velocity in air to velocity in soil may be as high as 1000:1. pe tet By "average energy density" is meant the total energy| ontained within the shock wave, divided by thg total volume within-it. . eet Actually, as Porzel points out, 2/at a time when the nuclear.shock wave has reached the same radius as that of the TNT sphere of equiva- lent energy release, (and hence when average energy densities are,equal) there is still an enormous difference in the two situations since the Mass enclosed within the shock wave in the case of TNT is some 1500 times that in the nuclear case. Hence, in the nuclear situation the pressures are very much higher and the durations shorter than in the TTsituation. Lb