Shelf Cloud Observed Over Quincy, Massachusetts
December 12, 2014
Shelf clouds are a form of low, horizontal (arcus) cloud that mark the leading edge of a thunderstorm. They're associated with the gust front of a thunderstorm’s downdraft. When the downdraft spreads out radially on the land surface its higher density (cooler air) forces the warmer, moister air on the periphery of the thunderstorm aloft, where it condenses to form shelf-shaped clouds.
In this photograph, taken from Massachusetts’ Quincy Bay on September 6, 2014, the shelf cloud is at right and the thunderstorm itself is centered to the left of the photo. Note the downpour or rain masking the horizon beneath the storm. The flattened sea grass in the left foreground (Spartina alternaflora) shows the direction of the spreading downdraft.