9th September 2005

Altocumulus Castellanus

Source: Excerpt from The Book " Weather "

Altocumulus Castellanus

  • Distribution: Worldwide.

  • Height: 6500 to 16,500 feet.

  • Cause: Lifting of a large air mass; followed by condensation combined with instability.

  • Associated Weather:
    May point to thunderstorm development later in the day.

    This cloud is named for the turret-like protruberances that grow from its main deck, resembling the battlements of a medieval castle. While not a spectacular formation, it is significant in that it indicates instability in the middle layers of the atmosphere and may therefore point to thunderstorm development later in the day.

    Castellanus generally occurs when a layer of colder air slides across an area of altocumulus cloud. This creates instability, and localized bubbles of air start to rise from the cloud deck. Condensation within these pockets of air creates the castellanus effect.

    Any subsequent convection from the ground will be enhanced by this middle-level instability. Cumulus clouds forming in these areas are therefore more likely to develop into cumulonimbus clouds.

    For this reason, meteorologists are always alert to reports of castellanus. If several ground observers have noted castellanus development by the middle of the day, then there is a greater likelihood of thunderstorm activity later in the afternoon, following further ground heating.

    Since the formation of altocumulus castellanus involves vertical air currents, pilots can expect to experience slight to moderate turbulence as they pass through these clouds, but this will pose no danger to aircraft safety.

  • 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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