Clouds Formation

• Supplementary features
One group of supplementary features are not actual cloud formations but precipitation that falls when water droplets that make up visible clouds have grown too heavy to remain aloft. Virga is a feature seen with clouds producing precipitation that evaporates before reaching the ground, these being of the genera cirrocumulus, altocumulus, altostratus, nimbostratus, stratocumulus, cumulus, and cumulonimbus. When the precipitation reaches the ground without completely evaporating, it is designated as the feature praecipitatio. This normally occurs with altostratus opacus which can produce widespread but usually light precipitation, and the thicker low to middle genera. Of the latter, cumulus mediocris produces only isolated light showers, while nimbostratus of the same family D1 is capable of heavier more extensive precipitation. Of the family D2 clouds, cumulus congestus can produce showers of moderately heavy intensity, with cumulonimbus having the capacity to produce very heavy showers. Low stratus clouds usually produce only light precipitation, but this always occurs as the feature praecipitatio due to the fact this cloud genus lies too close to the ground to allow for the formation of virga. The heavier precipitating clouds, nimbostratus, towering cumulus (cumulus congestus), and cumulonimbus, also typically see the formation in precipitation of the pannus feature, low ragged clouds of the genera and species cumulus fractus and/or stratus fractus.
Another group of supplementary features are cloud formations that are associated mainly with cumuliform clouds of free convection. Pileus is a cap cloud that can form over a cumulonimbus or large cumulus cloud, while a velum feature is a thin horizontal sheet that sometime forms around the middle or in front of the parent cloud. A tuba feature is a cloud column that may hang from the bottom of a cumulus or cumulonimbus. An arcus feature is a roll or shelf cloud that forms along the leading edge of a squall line or thunderstorm outflow. Some arcus clouds form as a consequence of interactions with specific geographical features. Perhaps the strangest geographically specific arcus cloud in the world is the Morning Glory, a rolling cylindrical cloud which appears unpredictably over the Gulf of Carpentaria in Northern Australia. Associated with a powerful "ripple" in the atmosphere, the cloud may be "surfed" in glider aircraft. Mamma (sometimes known informally as mammatus) form on the bases of clouds as downward facing bubble-like protuberances caused by localized downdrafts within the cloud. The best known is cumulonimbus with mammatus, but the mamma feature is also seen occasionally with cirrus, cirrocumulus, altocumulus, altostratus, and stratocumulus. Incus is the most type-specific supplementary feature, seen only with cumulonimbus of the species capillatus. A cumulonimbus incus cloud top is one that has spread out into a clear anvil shape as a result of rising air currents hitting the stability layer at the tropopause where the air no longer continues to get colder with increasing altitude.