Pileus is an accessory cloud of small horizontal extent, shaped like a cap or hood, sitting above or attached to the top of a cumuliform cloud that often pushes up through it. It forms when a vigorously rising convective tower lifts a moist, stable layer of air above it; that air cools as it is forced upward and condenses into a smooth, lens-like cap. Because the cap marks fast updraughts, it often appears and vanishes within minutes as the tower grows.
Worth knowing
Because a pileus only forms over a fast-rising tower, it acts as a visible speedometer for a powerful updraught — spotting one means the convection beneath it is climbing hard, often towards a thunderstorm.
Note
Still unsure it's the same cloud? Our app, Cloud Point— on iOS and Android — reads a photo and returns the likely type with altitude, useful when a formation is fading faster than you can look it up.
