This shelf cloud approached Hays, Kan., as dusk fell on June 18, 2011. (Credit: iWitness Weather/Brooks97)
Shelf clouds are produced when rain-cooled air rushing out from the base of a cumulonimbus cloud forms a gust front, forcing relatively warm, humid air ahead of it to rise and form a shelf-like cloud formation.
Technically it's the gust front that produces the strong, potentially damaging winds on the ground, and not the shelf cloud itself. Nevertheless, an approaching shelf cloud is your clue that strong straight-line winds may be approaching, and it's a good idea to head indoors for sturdy shelter.
Those untrained in severe weather spotting sometimes confuse shelf clouds for wall clouds, the precursors to tornadoes. Wall clouds tend to form near the rear of a thunderstorm (relative to its direction of movement), are often rotating visibly, and are associated with air flowing into a thunderstorm.
Shelf clouds tend to form at the front edge of a thunderstorm and bulge outward.