would exceed 3 to 6 roentgens, but when such levels are indicated the shot is, of course postponed. It is the possibility of heavy fallout within 20 miles and undesirable fallout within perhaps 30 or 40 miles which prevents firing when the winds are out of the north or the west-of-north, and sometimes when they are from other directions, \ The prevailing winds in southern Nevada are westerlies and 'south-westerlies, For this reason, the permanent on-site control structures were located to the south of Yucca Flat (the area most used for detonations) and Camp Mercury was located on the southern edge of Nevada Test Site. Camp Desert Rock is in the same general area as Mercury. The location of the Control Point, which contains much film and other sensitive materiel, of troop and other observer areas, of Camp Mercury, and of Camp Desert Rock (all being within range of heavy, early fallout on the more sensitive shots) prevents firing when winds are out of the north. Shots have, of course, been fired many times when the atomic cloud was blown overhead to the south —- but these were shots which did not involve heavy early fallout. : Indian Springs~AF Base is only 37 miles to the southeast. Its location has caused postponements on many shots where stronger winds, without shear, would result in unacceptable fallout across the air base, It is usually the presence of Camp Mercury or of Indian Springs AF Base which has caused postponements of "sensitive" shots on occasion, not the presence of Las Vegas which is about 90 miles southeast of the center of Yucca Flat. For like reasons, close attention is also always paid to Groom Mine, which is about 22 miles northeast from the center of Yucca, and to Lathrop Wells, about 37 miles’ to the southwest, Outside of the four areas mentioned, to the southwest, south, southeast, and northeast, the direction which will be acceptable on such shots depends largely on wind speeds and wind shear, inasmuch as in all other directions there are quite a few miles of Test Site or unoccupied Bombing Range before occupied places are reached. _ Three directions from Yucca are most desirable. If wind speeds are low and if the winds at various altitudes are from different directions, the result will be a short, widely-diffused fallout pattern. Im such instances, the areas immediately east, north, northwest are fully acceptable, because of the open spaces and the distances to occupied places. Most of such shots have been fired while winds were blowing into the east solely because winds in Nevada usually blow in that direction. If winds blew out of the southeast, more shots would be fired with winds blowing into tle northwest. Maps used by the Test Organization place a 10-mile buffer zone ¢@round all occupied places in the region out to 100 to 150 miles. “g- Shots are fired only if it appears that fallout will be very light - 53 - 7 ELBOE toe \) \