Pileus (from the Latin pileus, meaning cap) is a smooth, rounded accessory cloud that appears as a hood or cap atop a rapidly developing cumulus congestus or cumulonimbus tower. It is a transient and delicate formation that typically lasts only a few minutes before being absorbed by the ascending convective tower. It forms when the updraft within the convective cloud is so vigorous that it pushes the stable air above it, forcing it to rise rapidly over the tower's summit. This forced ascent cools the moist air to its dew point, causing condensation and forming a separate, smooth cloud layer above the main updraft. Pileus is distinguished from the cloud itself by its smooth texture and sheet-like or veil-like appearance, contrasting with the bumpy surface of the cumulus. The presence of pileus is a significant indicator in operational meteorology: it signals an exceptionally strong updraft, which frequently precedes the development of intense thunderstorms or supercells. Pilots and meteorologists recognise it as a sign of vigorous convective activity. Occasionally, multiple stacked pileus can form if updrafts are pulsating. The phenomenon is analogous to lenticular clouds that form over mountains through forced ascent, but here the "obstacle" is the growing cloud itself.