thememegeneration:

Snow rollers form when the snow has just the right combination of sticky snow that is light enough to be moved by wind. If the wind is strong enough, it picks up snow crystals and begins to tumble them over across “downstream” snow cover. Soon a little mini snowball forms. As it grows and increases surface area, the wind gusts push sheets of snow along.

As it grows and rolls along, it picks up more and more snow just like when you build a snowman in your yard. Since there is no compaction going on by pressure from hands, the center can remain open. (via Snow rollers | Updraft | Minnesota Public Radio)