Mini Cloud Atlas
Pileus caps are made of ice crystals high in the troposphere. They form as a slab of air is shoved upward, in the shape of a dome or cap, just above a rapidly rising convective tower. Moisture in the dome condenses directly into an ice fog as the air rises and cools, forming the pileus. Next, the convection shoots right through the pileus layer. The lifted layer above the convective tower can't be too dry (must have high humidity), or the pileus cap won't develop.
5 NW Wanette OK (21 Apr 4) Looking SE