SECTION II. - OTHER ASPECTS A. Blast - Direct and Reflected Background Information- Direct blast waves that are potentially damaging are confined to the immediate testing site areas. ditions, however,blast waves Under certain meteorological con- may be refracted (bent) from an upper atmospheric level back to the earth and thus create relatively high blast pressures. One level at which this may happen is between 25,000 and 50,000 feet altitude where winds may cause a focusing effect 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 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, 75-100 miles from the point of detonation at the Nevada Test Site2!., A similar effect is obtained when blast waves are bent from a layer of relatively warm air, called the ozonosphere, at a height of 20 to 30 miles. The point of first return to the earth is in this case 70-150 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 but in some cases audible sharp cracks and pops. 27.