<Li> Mamma Also sometimes called Mammatus, consisting of bubble - like protrusions on the underside caused by localized downdrafts . </Li> <Li> Arcus (including roll and shelf clouds) Low, horizontal cloud formation associated with the leading edge of thunderstorm outflow . </Li> <Li> Tuba Column hanging from the cloud base which can develop into a funnel cloud or tornado . </Li> <Dl> <Dd> <Dl> <Dd> <Dl> <Dd> <Dl> <Dd> Accessory clouds Seen with species and capillatus except where noted . <Ul> <Li> Pannus Accompanied by a lower layer of fractus species cloud forming in precipitation . </Li> <Li> Pileus (species calvus only) Small cap - like cloud over parent cumulonimbus . </Li> <Li> Velum A thin horizontal sheet that forms around the middle of a cumulonimbus . </Li> </Ul> </Dd> </Dl> </Dd> </Dl> </Dd> </Dl> </Dd> </Dl>

High cirrus cirro stratus alto stratus and stratus