I stand corrected regarding the avalance situation. Still, don't allow this mis-statement to distract from the issue being discussed. In an attic ventilation sitution, snow over the ridgevent is a none issue. The heat combined with the possitive pressure on warm air rising will go though or melt a way out. My ridge vent (and many others)is covered all winter with no adverse effects. If effects are noticed, the problem is not the vent being plugged, but excessive heat entering the attic, usually through air leakage. Ventilation, and added insulation are mistakenly looked at as the "Holy Grail" of heatloss / melting / moisture problems. It's not. the main reason is not taking control of air leakage by making the house tight (Usually cased by a mis-application of the axiomthat "A house has got to breath").