April 2008, and a strong gust-front surges south (R-L) across Cardigan Bay. Technically, what's going on is that cold air, chilled from the downpour (mostly out-of-view R) is descending from the upper part of the thundercloud and fanning-out away from the storm upon hitting the surface. The cold air's the clear slot higher than the normal cloudbase R. As it travels along in a wedge-shape, it lifts the warmer ambient air around the storm: that air is then itself chilled and its moisture condenses to form the rather scalloped wedge of cloud whose leading edge is almost dragging the ground. Product number 2000-2009615. Email me to enquire about this image: click HERE.