Shelf cloud Known to meteorologists as arcus clouds, they form at the leading edge of a thunderstorm when descending air flows outwards from the storm front at the base of the cumulo-nimbus supercell. As the cool air hits the land, it spreads out as the powerful wind preceding the storm and pushes warmer air upwards, where the moisture within the column then condenses into the shelf cloud, that roils with the shearing air currents. As they pass over, the winds shift and change, shortly followed by violent rain or hail. They tend to be very low, highly turbulent, and obstruct the view of the storm in a dark and menacing manner while indicating an approaching squall. They are often confused with wall clouds (which form at the trailing edge of storms), and most usually occur in lines of storms rather than solitary anvils. Shelf clouds often seem to rotate about a horizontal axis, while wall clouds will do so in the vertical one, and the former are associated with rain while the latter are a sign that the storm has passed. Loz Image credit: Thalia Traianou via EPOD http://www.strangephoton.com/ http://www.weather.com/news/shelf-cloud-photos-20130412 http://www.accuweather.com/en/features/trend/what-are-shelf-clouds/5506892 http://www.crh.noaa.gov/lmk/?n=shelfcloudversusawallcloud http://eo.ucar.edu/webweather/cloudpic1.html