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
Tornado outbreak of May 15–17, 2013 severe thunderstorms early that morning over northwestern Texas, for a threat of large hail and damaging winds. A low-pressure area associated with the trough moved over Oklahoma that day, producing light to moderate rainfall and non-severe thunderstorms across that state into parts of North Texas. Later forecasts expanded the slight risk further into northern and central Texas, and later into far southern Oklahoma, and indicated an enhanced risk of a few isolated tornadoes in north Texas. The atmosphere began to destabilize due to a decrease in cloud cover over western and central Texas; the sunshine and heating, combined with sufficient