in precipitation. Pileus (species calvus only) Small cap-like cloud over parent cumulonimbus. Velum A thin horizontal sheet that forms around the middle of a cumulonimbus. Genitus mother clouds Cumulonimbus altocumulogenitus Cumulonimbus altostratogenitus Cumulonimbus nimbostratogenitus Cumulonimbus stratocumulogenitus Cumulonimbus flammagenitus Formed by large-scale fires or volcanic eruptions. Mutatus mother cloud Cumulonimbus cumulomutatus Genus cumulus: Towering vertical Abbreviations: Cu con (cumulus congestus) or Tcu (towering cumulus) Species Cumulus congestus (V-59) These large cumulus clouds have flat dark grey bases and very tall tower-like formations with tops mostly in the high level of the troposphere. The International Civil Aviation Organization (ICAO) designates this species as towering cumulus (Tcu). Varieties Opacity-based varieties None (always opaque). Pattern-based variety None (not generally discerned with highly unstable cumulus congestus). Supplementary features Precipitation-based supplementary features Virga Accompanied by precipitation that evaporates before reaching the ground. Praecipitatio Produces precipitation that reaches the ground. Cloud-based supplementary features Mamma Downward facing bubble-like protuberances caused by localized downdrafts within the cloud. Arcus (including roll and shelf clouds) Low horizontal cloud formation associated with the leading edge of a thunderstorm outflow. Tuba Column hanging from the cloud base which can develop into a small funnel cloud. Accessory clouds Pannus Accompanied by a lower layer of fractus species cloud forming in precipitation. Pileus Small cap-like cloud over parent cumulus cloud. Velum A thin horizontal sheet that forms around the middle of a cumulus cloud. Mother clouds Cumulus congestus flammagenitus Other genitus and mutatus types are the same as for small and moderate cumulus. Multi-level stratiform and moderate vertical cumuliform (low to mid-level cloud base) Genus nimbostratus: Multi-level Abbreviation: Ns (V-60) Clouds of the genus nimbostratus tend to bring constant precipitation and low visibility. This cloud type normally forms above from altostratus cloud but tends to thicken into the lower levels during the occurrence of precipitation. The top of a nimbostratus deck is usually in the middle level of the troposphere. Species No differentiated species (always nebulous). Varieties No varieties (always opaque and never forms in patterns). Supplementary features Precipitation-based supplementary features Virga Accompanied by precipitation that evaporates before reaching the ground. Praecipitatio Produces precipitation that reaches the ground. Accessory cloud Pannus Nimbostratus with lower layer of fractus species cloud forming in precipitation. Genitus mother clouds Nimbostratus cumulogenitus Nimbostratus cumulonimbogenitus Mutatus mother clouds Nimbostratus altostratomutatus Nimbostratus altocumulomutatus Nimbostratus stratocumulomutatus Genus cumulus: Moderate vertical Abbreviation: Cu Moderate vertical cumulus is the product of free convective airmass instability.