Can grow more rapidly, and to a greater extent, in the vertical direction than in the horizontal. The result will be a fireball shaped something like half an egg, rather than a perfect hemisphere. . The appropriate air density is that occurring at the center. of the fireball area, such that half the area is above, and half below, the chosen alti- tude. For a hemisphere the altitude for center-of-area occurs at one-half the radius; we apply the same criterion to the distorted fireball, and use a different value of air density for each frame that we measure. This effect does not alter the validity of the scaling laws; it simply introduces another step that must be taken in the analysis of the data and requires detailed data on the variation of air density with elevation. 6. Ground Reflertion Effect The effect of the rigid water surface must be considered for the Pinon shot, which is to be detonated within a few feet of sea level. The acoustic impedance of water is about 150,000 grams/cm*/sec, whereas that of air is only 44. The result is that very little energy goes into the water; instead it is reflected almost perfectly back into the fireball and reinforces the shock front that was directed upward initially. The fireball becomes a nearly perfect hemisphere (excluding the effects of the inhomogeneous atmosphere which were just discussed), and the hemisphere represents half of a fireball having twice the apparent energy. Thus the factor F = 0.50is introduced into the scaling equation, and the total yield for a surface burst is given by E = 0.5KpD°t™ 7. (4) Measurements Required It has been shown that four quantities must be measured in order to determine the total fireball energy release: a) diameter of the fireball, b) time after burst at which the measured diameter obtains, c) air density (specific gravity) as function of altitude, and d) ground-reflection factor. The diameter of the image of the fireball, formed by the cameralens upon the film, will be measured in the analysis procedure, In order to relate