Altostratus cloud size again. During the sampling of one cloud, the scientists noted a halo while flying near the top of the cloud, which indicated that the ice crystals were hexagonal near the top. However, farther down, the ice crystals became more conglomerated. Altostratus tends to form ahead of a warm or occluded front, the altostratus gradually thickening into rain bearing nimbostratus. However it can also occur together with cumulus congestus or cumulonimbus at a cold front. "Velum" This is considered a type of cumulonimbus cloud so it is not listed. Altostratus cloud Altostratus is a middle altitude cloud genus belonging to
Mesoscale convective system of severe weather. The supercell is the strongest of the thunderstorms, most commonly associated with large hail, high winds, and tornado formation. Precipitable water values of greater than favor the development of organized thunderstorm complexes. Those with heavy rainfall normally have precipitable water values greater than . normally greater than , Upstream values of CAPE of greater than 800 J/kg are usually required for the development of organized convection. A mesoscale convective complex (MCC) is a unique kind of mesoscale convective system which is defined by characteristics observed in infrared satellite imagery. Their area of cold cloud tops exceeds with