the sun can be seen. Altostratus opacus (V-50) Altostratus that completely blocks out the sun. Pattern-based variety radiatus Bands that appear to converge at the horizon. Altostratus translucidus radiatus (V-51) Altostratus opacus radiatus (V-52) Pattern-based variety duplicatus Altostratus in closely spaced layers, one above the other. Altostratus translucidus duplicatus (V-53) Altostratus opacus duplicatus (V-54) Pattern-based variety undulatus Altostratus with wavy undulating base. Altostratus translucidus undulatus (V-55) Altostratus opacus undulatus (V-56) Supplementary features Precipitation-based supplementary features Virga Accompanied by precipitation that evaporates before reaching the ground. Seen mostly with opacus varieties. Praecipitatio Produces precipitation that reaches the ground; associated with opacus varieties. Cloud-based supplementary feature Mamma Altostratus with downward facing bubble-like protuberances caused by localized downdrafts within the cloud. Accessory cloud Seen mostly with opacus varieties Pannus Accompanied by ragged lower layer of fractus species clouds forming in precipitation. Genitus mother clouds Altostratus altocumulogenitus Altostratus cumulonimbogenitus Mutatus mother clouds Altostratus cirrostratomutatus Altostratus nimbostratomutatus Towering vertical cumulonimbiform and cumuliform (low to mid-level cloud base) Clouds with upward-growing vertical development usually form below , but can be based as high as in temperate climates, and often much higher in arid regions. Genus cumulonimbus: Towering vertical Abbreviation: Cb Clouds of the genus cumulonimbus have very-dark-gray-to-nearly-black flat bases and very high tops that can penetrate the tropopause. They develop from cumulus when the airmass is convectively highly unstable. They generally produce thunderstorms, rain or showers, and sometimes hail, strong outflow winds, and/or tornadoes at ground level. Species Cumulonimbus calvus (V-57) Cumulonimbus with high domed top. Cumulonimbus capillatus (V-58) Towering vertical cloud with high cirriform top. Varieties No varieties (always opaque and does not form in patterns visible from surface level). Supplementary features Precipitation-based supplementary features Associated with calvus and capillatus species. Virga Precipitation that evaporates before reaching the ground. Praecipitatio Precipitation that reaches the ground. Cloud-based supplementary features Incus (species capillatus only) Cumulonimbus with flat anvil-like cirriform top caused by wind shear where the rising air currents hit the inversion layer at the tropopause. Mamma Also sometimes called Mammatus, consisting of bubble-like protrusions on the underside caused by localized downdrafts. Arcus (including roll and shelf clouds) Low, horizontal cloud formation associated with the leading edge of thunderstorm outflow. Tuba Column hanging from the cloud base which can develop into a funnel cloud or tornado. Accessory clouds Seen with species and capillatus except where noted. Pannus Accompanied by a lower layer of fractus species cloud forming