The two things—snow and mountains—which are needed for a ski area are the two ingredients that create avalanches(雪崩), large masses of snow and ice crushing down the side of a mountain. It was the threat of the avalanche and its record as a killer of man in the Western mountains that created the snow ranger. He first started on avalanche control work in the winter of 1937—1938 at Alta, Utah, in Wasatch National Forest. This mountain valley was becoming well known to skiers. It was a valley of er, where more than 120 persons had lost their lives in 1936 as a result of avalanches before it became a major ski area. Thus, development of Alta and other major ski resorts was dependent upon controlling the avalanche. The Forest Service set out to do it, and did, with its snow rangers. It takes many things to make a snow ranger. The snow ranger must be in excellent physical condition. He must be a good skier and a skilled mountain climber. He should have at least a high school education, and the more college courses in geology, physics, and related fields he has, the better. He studies snow, terrain(地形), wind, and weather. He learns the conditions that produce avalanches. He learns to forecast avalanches and to bring them roaring(轰响着快速行进)on down the mountainsides to reduce their strength. The snow ranger learns to do this by using artillery(火炮), by blasting with TNT, and by the difficult and skillful art of skiing avalanches down. The snow ranger, dressed in a green parka which has a bright yellow shoulder patch, means safety for people on ski slopes. He pulls the trigger on a 75 mm, recoilless(无后坐力的)rifle, skis waist deep in powder testing snow stability, or talks with the ski areas operator as he goes about his work to protect the public from the hazards of deep snow on steep mountain slopes. A snow ranger himself must be______.