Ive always use the silver poly tarps from menards, and know that poly tarps will degrade from sun exposure...sometimes less than a year in full sunlight. My tarp covered blowmolds were stored in an area of my property which is very shaded. One of the reasons i stored them there was because there was never enough sunlight to even grow grass, and i didnt want to kill the grass anywhere else in my yard. The poly tarps lasted quite a while, which might indicate that the amount of sunlight getting through the trees wasnt all that much if the tarps lasted 4-5 years. In that regard, the tarps were as much an effort to keep the blowmolds clean from leaves and tree sap etc. I don t disagree that the problem is mostly uv sunlight...and i agree that tarps are not the best uv protection...but some of my point is...that i had quite a few blowmolds under tarps, and the brittle crumbling effect im seeing is mostly on just 2 types of blowmolds. Maybe its a combination of blowmolds with 'thin' material areas combined with the effects of uv light, drying those this areas out even faster than other areas.
That snowcover was an accumulation. We had a couple of big storms that season, and i think these photos were during or just after the snow stopped. Before the snow settles down i few inches lower. The season of 2013-14 was a bigger year for us at 70 inches. I think upstate NY gets signifificantly more than minneapolis does.