Shelf clouds, also known as arcus clouds, occur at the leading edge of thunderstorms. They announce the arrival of storms and, when combined with cumulonimbus thunderheads, can look like giant alien motherships. Shelf clouds form when the cooler air of a thunderstorm encounters warm, humid air and lifts it. As the shelf cloud passes over you, you’ll notice a drop in temperature, and winds pick up as the gust front moves through. Rain follows soon behind.
Shelf cloud photo gallery
Bottom line: A shelf cloud, also known as an arcus cloud, forms at the leading edge of thunderstorms and signals that gusty winds are coming.
Claudia Crowley proofs and helps edit all EarthSky website material. She says working for EarthSky is the most exciting job she's had except one other - which was editing space shuttle documentation at NASA JSC. After writing and editing manuals for Dell and other major companies, she moved to the technical support side during the wild early days of the Internet, and served as general manager at a small wireless ISP. Claudia is a space enthusiast and fan of science.
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