- Arcus clouds are dramatic, low-level, horizontal cloud formations that often appear as a dense, shelf-like or roll-like structure along the leading edge of thunderstorm outflows or cold fronts. They are not a separate cloud genus but are classified as an accessory cloud—typically associated with cumulonimbus or, less commonly, cumulus clouds. Arcus clouds are mainly of two types: shelf clouds (arcus) and roll clouds.
- A shelf cloud is the more commonly observed form of arcus and presents as a low, wedge-shaped cloud that juts out ahead of a thunderstorm or squall line. Its appearance can be ominous, often resembling a giant, rolling wave. Shelf clouds form as a result of warm, moist air being forced upward by the dense, cold air outflowing from a thunderstorm. As the warm air is lifted, it cools and condenses, forming a horizontal cloud that marks the boundary between warm inflow and the cooler outflow below. Strong winds are typically present beneath shelf clouds, and their arrival often signals the onset of gusty winds, heavy rain, and sometimes severe weather like hail or tornadoes.
- Roll clouds, by contrast, are rarer and more isolated from their parent storm systems. They appear as long, horizontal tubes that seem to roll about a horizontal axis. Unlike shelf clouds, roll clouds are completely detached from the thunderstorm base and usually form along gust fronts or in areas with strong wind shear. They can stretch for kilometers across the sky and move independently of the prevailing wind, creating a striking visual phenomenon. Despite their menacing appearance, roll clouds are generally not associated with severe weather.
- Both types of arcus clouds are indicators of strong atmospheric dynamics and often form in environments with significant instability, wind shear, and moisture. They are especially notable to meteorologists and storm chasers as precursors to severe convective activity. While visually intimidating, arcus clouds themselves do not produce precipitation, but their presence marks the boundary of turbulent air masses and can precede intense weather events.
