A pileus is like a hat on top of a cloud. It usually can be seen on top of a vigorous growing congests or Cb. The colud composed by super-cooled water of ice crystals. When the cloud is vigorous developing upward, the moist air current is lifted up by the upward motion. When it encounters the horizontal air current at the upper level, it will form a ridged cloud. The lifespan of the pileus is very short. When the cumulus or cumulonimbus changes the pileus cloud may disappear.