Robin Pintar (Member 57,049) lay down at the water’s edge of Little Traverse Lake, Michigan, US to enjoy the approach of a storm. From her front-row seat, she watched the storm build slowly from the west, with an arcus cloud feature at its front. The arcus is the ridge, sometimes known as a shelf cloud, that can form at the front base of an advancing Cumulonimbus storm cloud. Robin’s one had a layered appearance to it. Once the Cumulonimbus loomed overhead, it covered most of the sky.

Arcus features form when air dragged down by the storm’s precipitation spreads across at the Earth’s surface, flowing out from the storm, and burrowing beneath the warmer, moister air ahead of the storm. It causes the warmer air to lift, cool, and form a distinct edge of cloud that juts out from the base. It is like the front lip of the storm. In fact, the turbulent underside of the storm just behind the advancing arcus is known by some as the ‘whale’s mouth’. It is as if the storm’s lip has passed over as it swallows you whole. Thankfully, Robin survived to tell the tale of her encounter with this monster from the deep of the sky.