SECTIONIl. OTHER ASPECTS A. BLAST— DIRECT AND REFLECTED Background Information — Direct blast waves that are potentially damaging are confined to the immediate testing site areas. Under certain meteorological conditions, however, blast waves may be refracted (bent) from an upper atmospheric level back to the earth and thus create higher air pressures than would be expected at those distances. One layer in which this may happen is between 25,000 and 50,000 feet altitude where winds may cause a focusing effect ile hi at some 20-50 miles from the point of detonation. In turn, the blast wave may be repeatedly reflected from the ground and bent back from the atmosphere creating a series of regular i spaced points of focus at the earth’s surface with intervening “silent” spaces. Such an effect has resulted in minor structural damage, such as breaking of windows, 75 to 100 miles from the point of detonation at the Nevada Test Site “4 (fig. 7). A similar effect is obtained when blast waves are bent from a layer of relatively warm air, called the ozonosphere, at a haba it eeeell ee ete height of 20 to 30 miles. The point of first return to the earth in this case is 70 to 150 miles from the burst. There may be a return of sound waves from an altitude above 60 miles (ionosphere). Most of this blast energy is absorbed, however, resulting in no recorded structural damage. In some cases audible sharp cracks and pops have been heard. Procedures and equipment have now been developed to . predict with greater accuracy the magnitude and direction of these refracted blast waves. The Data Although the blast wave decreases in energy with each suc- ceeding refraction back to the earth’s surface, there has been breakage of windows on a second “strike” at 285 miles from only a 17 thousand ton (TNT equivalent) nuclear explosion.*# (Altogether about $50,000 has been paid for structuraldamage claims from all tests at the Nevada Test Site.) There have 21 50 0e 9 io