Cap cloud or cloud cap is a stratiform, orographic cloud that hovers above or over an isolated mountain peak, formed by the cooling and condensation of moist air forced up and over the peak and lenticularly shaped by horizontal upper level winds. The cloud appears to remain essentially stationary.
The term is also occasionally used for pileus (Latin for cap) cloud. Unlike the mountain cap cloud the pileus is essentially an accessory cloud, that appears as a smooth cap, or hood above a cumulus or cumulonimbus cloud. The cap forms when a humid layer is lifted to its dew point above a rising thermal. This may later penetrate the pileus, which will eventually be absorbed into the main cloud body. Sometimes several layers of pileus form above one another.