Thunderstorm in Summer 2010, Waconia, MN. Backlit by the sun, note the shelf or “arcus” cloud.
From Wikipedia: An arcus cloud is a low, horizontal cloud formation associated with the leading edge of thunderstorm outflow, or outflows of cold air from sea breezes or cold fronts in the absence of thunderstorms.
A shelf cloud is attached to the base of the parent cloud (usually a thunderstorm). Rising cloud motion often can be seen in the leading (outer) part of the shelf cloud, while the underside often appears turbulent and wind-torn. Cool, sinking air from a storm cloud’s downdraft spreads out across the surface with the leading edge called a gust front. This outflow undercuts warm air being drawn into the storm’s updraft. As the cool air lifts the warm moist air, water condenses creating a cloud which often rolls with the different winds above and below