Arcus cloud Arcus cloud An arcus cloud is a low, horizontal cloud formation, usually appearing as an accessory cloud to a cumulonimbus. Roll clouds and shelf clouds are the two main types of arcus. Arcus clouds most frequently form along the leading edge or "gust fronts" of thunderstorm outflow; some of the most dramatic arcus formations mark the gust fronts of derecho-producing convective systems. Roll clouds also may arise in the absence of thunderstorms, forming along the shallow cold air currents of some sea breeze boundaries and cold fronts. A shelf cloud is a low, horizontal, wedge-shaped arcus cloud. A shelf cloud
Cloud describes the occasional arrangements of cloud structures into particular patterns that are discernible by a surface-based observer (cloud fields usually being visible only from a significant altitude above the formations). These varieties are not always present with the genera and species with which they are otherwise associated, but only appear when atmospheric conditions favor their formation. "Intortus" and "vertebratus" varieties occur on occasion with cirrus fibratus. They are respectively filaments twisted into irregular shapes, and those that are arranged in fishbone patterns, usually by uneven wind currents that favor the formation of these varieties. The variety "radiatus" is associated with