9th September 2005

Cumulus Mediocris

Source: Excerpt from The Book " Weather "

Cumulus Mediocris

  • Distribution: Worldwide,
    except Antarctica.

  • Height: 2000 to 4000 feet.

  • Cause: Weak to moderate convection.

  • Associated Weather: None

    Cumulus mediocris is generated by slightly stronger convection than that which gives rise to cumulus humilis. This produces a cumulus cloud as tall as it is wide, as estimated by an observer on the ground.

    The base of this medium-sized cloud (mediocris means moderate in Latin) can begin to form from 2,000 feet upward, depending on the surrounding humidity. Cumulus mediocris is normally white or light gray, and has a comparatively flat base.

    This cloud is often a transitional stage between the lesser humilis and the more developed congestus phase. Mediocris clouds are more common in the late morning or early afternoon, after the ground has warmed enough to generate convection. They are not large enough to produce precipitation. During aircraft penetration of the cloud, they can produce slight and short-lived turbulence.

    If mediocris clouds occur at levels where strong winds are blowing, the winds may shred the clouds into horizontal fragments, which then speed across the sky. This variation is known as cumulus mediocris fractus.

    Mediocris occurs over most oceans and landmasses when conditions are right, with the exception of Antarctica, where the cold surface temperatures generally inhibit convection.

  • 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.

    The Airline Pilots Forum and Resource