This Timelapse by Stephen Locke shows a shelf cloud on the leading edge of a thunderstorm rolling through Kansas City on 29 August 2014.
A shelf cloud is a low, horizontal, wedge-shaped arcus cloud. Shelf clouds are typically attached to the base of the parent cloud, which is usually a thunderstorm, but could form on any type of convective clouds.
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 land surface, with the leading edge called a gust front. This outflow cuts under warm air being drawn into the storm’s updraft. As the lower cooler air lifts the warm moist air, its water condenses, creating a cloud which often rolls with the different winds above and below (wind shear).