Saturday was a humid day throughout the state, and late in the day it cooled off dramatically. This is usually the result of a thunderstorm, but sometimes before the storm front hits, the cooling will cause arcus or 'roll' clouds to form. They don't happen often, but when they do, it's pretty cool.

Here's some footage of a roll cloud over Lake Michigan from Saturday afternoon.

The official definition of an arcus cloud is a low, horizontal cloud formation. Roll clouds and shelf clouds are the two types of arcus clouds. A shelf cloud is usually associated with the leading edge of thunderstorm outflow; roll clouds are usually formed by outflows of cold air from sea breezes or cold fronts in the absence of thunderstorms.

This is a time lapse capture of a roll cloud near Sleeping Bear national Lakeshore north of Arcadia.