Volutus is a supplementary cloud feature recognised by the WMO in 2017. It takes the form of a horizontal tube that can extend hundreds of kilometres, slowly rotating about its longitudinal axis. Unlike the arcus cloud (shelf/roll cloud), volutus is completely detached from its parent cloud.
Morning Glory
The most famous example is the Morning Glory of the Gulf of Carpentaria (Australia), where tubes up to 1,000 km long appear at dawn between September and November. They propagate at 40–60 km/h as atmospheric gravity waves and are prized by glider pilots, who can surf them like ocean waves.