Here's the sort of thing that you might expect to see on the reboot of "The X-Files." Thousands of snowballs spontaneously form, without any human hands to help, and then roll together like a ghostly invasion force through a field in Idaho.
But as agent Mulder probably would explain to us, all those snowballs actually are a natural, albeit extremely rare, phenomenon known as snow rollers.
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As the National Oceanic and Atmospheric Administration's website explains, snow rollers form when an unusual sequence of events occur. First, a smooth crust of snow that's already on the ground is covered by newer, lighter layer of snow. Then, the weather abruptly changes, warming rapidly, and strong wind kicks up. The wind picks up small pieces of moist, sticky snow and sends them rolling along. Once the pieces are on the move, they collect more snow around them.
While snow rollers often are cylindrical, the ones in Idaho formed spherical shapes that resembled snowballs. Unlike the ones you threw at your neighbors as a kid, snow rollers aren't packed together firmly, and sometimes they're even hollow.
While snow rollers don't occur very often, scientists have known about them for a long time. This 1893 article describes their appearance in a field in Ohio.
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In this case, the snow rollers appeared in and around The Nature Conservancy's Silver Creek Preserve in south-central Idaho. Sunny Healey, the preserve's manager, told Nature's Cool Green Science blog that she had never seen them before in 20 years of living on the preserve. She described the snow rollers as being about 18 inches in diameter.
When Healey first glimpsed the snow rollers from a distance, she thought they were trumpeter swans, which sometimes bunch together in the fields. On closer observation, she realized they were frozen spheres, and noticed the roll marks that they left in the snow.