An arcus cloud (from the Latin arcus, "arch") is a low, dense, and spectacular accessory cloud that forms at the leading edge of intense thunderstorms, squall lines, or cold fronts. It appears as a dark horizontal arch or roll extending along the storm front, heralding the imminent arrival of strong winds, heavy showers, and possible hail.
Types: shelf cloud and roll cloud
Two main varieties are distinguished: the shelf cloud, which remains attached to the base of the parent cumulonimbus and has a wedge or shelf shape with a turbulent underside; and the roll cloud, which detaches completely from the storm and rotates about its horizontal axis as it advances. The roll cloud is rarer and more spectacular.
They form through the interaction between the gust front (outflow boundary) — cold air descending from the storm and spreading along the surface — and the warm, moist environmental air. The warm air is forced to rise over the cold air wedge, condensing rapidly to form the arcus cloud. The visible turbulence on the underside reflects the active mixing between the two air masses.
In Spain they are common in summer and autumn, especially in the Mediterranean and the Ebro valley, where storms tend to be more organised. The appearance of an arcus cloud indicates that wind gusts of 60-100 km/h should be expected within minutes. See also: cumulonimbus incus, thunderstorm.