The Air Force needs additional area to conduct supersonic flight training for aircraft associated with Hill Air Force Base, Utah. The preferred alternative to meet this requirement is to designate an area of existing military airspace which is adjacent to the area currently used for supersonic flight for this purpose. The resulting supersonic area, a combination of the existing plus the proposed areas, would accommodate up to about 768 supersonic sorties per month. The existing area now accommodates a maximum of about 614 such sorties per month. Thus, the preferred alternative could result in an increase of about 150 supersonic sorties in an area about double the size of the area currently accommodating the 612 sorties. The new land area beneath the airspace is predominately BLM land, but does have an estimated 50 residents. If all 768 supersonic sorties were flown into this new area (a worst case which will not occur), it is estimated that any one spot on the ground would be subjected to one or more sonic booms per day only 35 percent of the time, and two or more sonic booms per day only seven percent of the time. As a result of public comments on the draft EIS, the Air Force substantially reduced the number of proposed additional sorties from about 400 to 150 per month, the number of people potentially affected from 350 to 50, and the land area from 2478 to 1360 square miles. The environmental impacts are summarized on the following pages.