Volutus is a term that encompasses the various roll clouds. These form as long, horizontal tubes, detached from any other clouds in their vicinity, which are caused by differences in wind speed and direction between the surface and higher up (wind shear).
Roll Clouds over Racine WI, USA. Source:wikicommons (Eazydp), 2007
Roll Clouds over Las Olas Beach, Punta del Este, Uruguay. Source:wikicommons (Daniela Mirner Eberl), 2009