9th September 2005

Cumulonimbus Pileus

Source: Excerpt from The Book " Weather "

Cumulonimbus with Pileus

  • Distribution: Worldwide,
    except Antarctica.

  • Height: 20,000 to 30,000 feet.

  • Cause: Strong convection lifting and eventually absorbing a layer of air.

  • Associated Weather: None from pileus, but calvus cloud will produce precipitation.

  • Hazard Warning: Significant turbulence at cloud level.

    Once a cloud has reached the calvus stage, and if convection is still occurring and is enhanced by instability in the surrounding air mass, the cloud will continue to grow vertically. When the air is ascending quite rapidly -- speeds of 20 to 30 miles per hour directly upward are possible -- a rather curious phenomenon may take place.

    The strong updraft associated with the calvus cloud picks up a slab of air and thrusts it upward. This causes the water vapor in the slab to condense, and a smooth, elongated, cap-like formation, known as a pileus cloud (pileus is the Latin word for felt cap) appears above the rising mass of the calvus. As the calvus cloud continues to rise, it gradually catches up with the pileus cloud. When the two clouds meet, some of the pileus cloud topples off the sides of the rising calvus. Eventually, the ascending cloud overtakes the pileus, which remains draped over the peak of the calvus until the two clouds merge completely.

    Pileus clouds can help weather-watchers anticipate thunderstorms, as the clouds that generate pileus formations are those most likely to develop into full-blown cumulonimbus clouds.

    • Acknowledgement due: John W. Zillman, William J. Burroughs,
    Bob Crowder, Ted Robertson, Eleanor Vallier-Talbot and Richard Whitaker.

    Check out Skyscapes for cloud photos taken from the aircraft.

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