Astronomy Picture of the Day

Explanation: Perhaps it's time to go inside. Such thoughts might occur to people witnessing the approach of an impressive shelf cloud. Shelf clouds are typically seen leading thunderstorms, although they may precede any well defined front of relatively cold air. Shelf clouds differ from roll clouds because shelf clouds are attached to a larger cloud system lurking above. Similarly, shelf clouds differ from wall clouds because wall clouds typically trail storm systems. The above pictured shelf cloud was photographed toward the southwest during a trip crossing the prairies of Saskatchewan, Canada on the Trans-Canada Highway in 2001 August. A rising Sun illuminated the impressive cloud from the east as it advanced from the west.