Fortunately, because of the earth's atmosphere, only a trace of this dangerous ultraviolet radiation actually reaches the earth. By the time sunlight reaches the top of the stratosphere, at about 30 miles altitude, almost all the radiation shorter than 1,900 A has been absorbed by molecules of nitrogen and oxygen. Within the stratosphere itself, molecular oxygen (02) absorbs the longer wavelengths of ultraviolet, up to 2,420 A; and ozone (O3) is formed as a result of this absorption process. It is this ozone then which absorbs almost all of the remaining ultraviolet wavelengths up to about 3,000 A, so that almost all of the dangerous solar radiation is cut off before it reaches the earth's surface.