|Occurrence||Worldwide except Antarctica|
|Altitude||6000-9000m, high in the troposphere|
|Formation||Strong convection lifting and then absorbing a layer of air|
|Precipitation||None, but associated cloud may produce precipitation|
|Potential Hazards||Significant turbulence at cloud level; may indicate coming storms|
Pileus (Latin for felt cap) is a thin tropospheric sheet of cloud made of ice crystals on the top of or direcly above another cloud (usually cumulus or cumulonimbus calvus). Pileus looks like a cap-like formation on top of its associated cloud. Pileus clouds form when air in a cloud ascends quickly (usually at speeds of 20 to 50 km/hr.), causing an moist airmass to be thrust upward and causing the water inside it to condense into a pileus cloud as the airmass cools. A pileus will change shape quickly and usually fades away or joins its associated cloud after a short while. For some time after the associated cloud joins the pileus, the pileus may remain draped over the cloud or penetrated by it before the two completely merge. Some pileus clouds may not form above cumulus or cumulonimbus clouds, but rather above volcanic ash clouds or above mountainsides. Clouds associated with pileus clouds may be suffixed with "pileus" or, more accurately, with "with pileus".
The convection which produces pilei is often indicative of coming thunderstorms or severe weather. Cumulus or cumulonimbus calvus clouds that develop pilei may become cumulonimbus incus clouds as the updrafts in the cloud cause it to further expand.