Roll Clouds

Shown here washing over the sky and rolling up to the Seattle shore, roll clouds are a rare treat usually awarded to those living in coastal areas. When a large pool of cooler, sinking air near the ground sweeps under moister, warmer air, the contrast can give the air a spin along a long horizontal axis, much like a rolling pin. The cool air may come from a sea breeze or a decaying thunderstorm. A roll cloud is often detached from all other nearby clouds, giving it extra prominence.