the ground. Layered forms of altocumulus are generally an indicator of limited convective instability, and are therefore mainly stratocumuliform in structure. Mid-level stratocumuliform species Altocumulus stratiformis Always dividable into opacity-based varieties. Sheets or relatively flat patches of altocumulus. Altocumulus lenticularis (V-26) Lens-shaped middle cloud. Includes informal variant altocumulus Kelvin–Helmholtz cloud, lenticular spiral indicative of severe turbulence. Altocumulus volutus (V-27) Elongated, tube shaped, horizontal stratocumuliform cloud. Altocumulus castellanus (V-28) Turreted layer cloud. Altocumulus floccus (V-29) Tufted stratocumuliform clouds with ragged bases. Varieties Opacity-based varieties Altocumulus stratiformis translucidus (V-30) Translucent altocumulus through which the sun or moon can be seen. Altocumulus stratiformis perlucidus (V-31) Opaque middle clouds with translucent breaks. Altocumulus stratiformis opacus (V-32) Opaque altocumulus that obscures the sun or moon. Pattern-based varieties Radiatus Rows of altocumulus that appear to converge at the horizon; normally associated with stratiformis species. Altocumulus stratiformis translucidus radiatus (V-33) Altocumulus stratiformis perlucidus radiatus (V-34) Altocumulus stratiformis opacus radiatus (V-35) Duplicatus Altocumulus in closely spaced layers, one above the other; normally associated with stratiformis and lenticularis species. Altocumulus stratiformis translucidus duplicatus (V-36) Altocumulus stratiformis perlucidus duplicatus (V-37) Altocumulus stratiformis opacus duplicatus (V-38) Altocumulus lenticularis duplicatus (V-39) Undulatus Altocumulus with wavy undulating base; normally associated with stratiformis and lenticularis species. Altocumulus stratiformis translucidus undulatus (V-40) Altocumulus stratiformis perlucidus undulatus (V-41) Altocumulus stratiformis opacus undulatus (V-42) Altocumulus lenticularis undulatus (V-43) Lacunosus Altocumulus with circular holes caused by localized downdrafts; normally associated with stratiformis and castellanus species (also with cumuliform floccus species). Altocumulus stratiformis translucidus lacunosus (V-44) Altocumulus stratiformis perlucidus lacunosus (V-45) Altocumulus stratiformis opacus lacunosus (V-46) Altocumulus castellanus lacunosus (V-47) Altocumulus floccus lacunosus (V-48) Supplementary features Precipitation-based supplementary feature Virga Altocumulus producing precipitation that evaporates before reaching the ground; usually associated with species stratiformis, castellanus, and floccus. Cloud-based supplementary feature Mamma Altocumulus (usually species castellanus) with downward facing bubble-like protuberances caused by localized downdrafts within the cloud. Genitus mother clouds Altocumulus cumulogenitus Altocumulus cumulonimbogenitus Mutatus mother clouds Altocumulus cirrocumulomutatus Altocumulus altostratomutatus Altocumulus nimbostratomutatus Altocumulus stratocumulomutatus Genus altostratus Abbreviation: As Stratiform clouds of the genus altostratus form when a large convectively stable airmass is lifted to condensation in the middle level of the troposphere, usually along a frontal system. Altostratus can bring light rain or snow. If the precipitation becomes continuous, it may thicken into nimbostratus which can bring precipitation of moderate to heavy intensity. Species No differentiated species (always nebulous). Varieties Opacity-based varieties Altostratus translucidus (V-49) Altostratus through which