from snow以外的其它原因引起。原文紧接着的是：Yet，dark glasses or not，they find themselves suffering from headaches and watering eyes，and even snowblindness，when exposed to several hours of “snow light”.
The United States Army has now determined that glare from snow does not cause snowblindness in troops in a snow-covered country. Rather，a man’s eyes frequently find nothing to focus on in a broad expanse of barren snow-covered terrain. So his gaze continually shifts and jumps back and forth over the entire landscape in search of something to look at. Finding nothing hour after hour，the eyes never stop searching and the eyeballs become sore and the eye muscles ache. Nature offsets this irritation by producing more and more fluid which covers the eyeball. The fluid covers the eyeball in increasing quantity until vision blurs，then is obscured，and the result is total，even though temporary，snowblindness.
Although most universities in the United States are run on a semester system, which offers classes in the fall and spring, some schools observe a quarter system comprised of fall, winter, spring, and summer quarters. The academic year, September to June, is divided into three quarters of eleven weeks each beginning in September, January, and March, the summer quarter, June to August is composed of shorter sessions of varying length. Students may take advantage of the opportunity to study year around by enrolling in all four quarters, Most students begin their programs in the fall quarter, but they may enter at the beginning of any of the other