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 physics increasing instability promotes upward vertical cloud growth and raises the potential for severe weather. On comparatively rare occasions, convective lift can be powerful enough to penetrate the tropopause and push the cloud top into the stratosphere. A third source of lift is wind circulation forcing air over a physical barrier such as a mountain (orographic lift). If the air is generally stable, nothing more than lenticular cap clouds will form. However, if the air becomes sufficiently moist and unstable, orographic showers or thunderstorms may appear. Along with adiabatic cooling that requires a lifting agent, there are three other main mechanisms