photo #5828754

The sky darkened, and a low ridge of ominous cloud pushed in over Lexington, Massachusetts, US and Gary Geissler (Member 51,111). This was an arcus cloud feature, the low front bumper of a storm system. Gary would have felt a sudden cool wind as the arcus cloud approached, and as it moved overhead, the first dollops of rain would begin. Also known as a shelf cloud, the arcus cloud feature juts out from the storm’s base just ahead of its showers. It forms as cold air pulled down by all the precipitation splays out as it reaches the ground. This cold wind, called a gust front, flows out ahead of the storm. It burrows beneath the warmer, less dense air near the ground up ahead, lifting it up as it’s drawn in to help feed the growth of the approaching storm. Forced up in this way, the warmer air is made to cool on its way in, and some of its moisture forms into cloud that appears as the looming wedge feature jutting out ahead of the deluge. An arcus like Gary’s is the 10-minute warning of the cloud world.