Cloud Type: Shelf Cloud
Features: A long, horizontal, wedge-shaped cloud on the leading edge of a thunderstorm complex
They form when: Cold air from a thunderstorm's downdraft crashes into the Earth and spreads outward, forcing the surrounding warm air to rise. The front-face is smooth; the back-face is turbulent.
Portends: A cool rush of wind. In fact, farmers used to call these "wind clouds."
Chance of spotting in a year: StrongShare this on Facebook?