Fractus clouds are small, ragged cloud fragments which, usually found under an ambient cloud base, form or have broken off a larger cloud, and are generally sheared by and shredded-looking due to strong winds. Fractus have irregular patterns, appearing much like torn pieces of cotton candy, change constantly, often forming and dissipating rapidly. They do not have defined base. Sometimes they are persistent and form very near the surface. Common kinds are scud and "cloud tags".

Forms

Fractus are an accessory cloud named for the type of cloud from which they were sheared. The two principal forms are cumulus fractus (formerly, fractocumulus) and stratus fractus (formerly, fractostratus). Fractus clouds may develop into cumulus if ground heats enough to start convection. Stratus fractus is distinguishable from cumulus fractus by its smaller vertical extent, darker color, and by the greater dispersion of its particles.

Cumulus fractus actually looks like a ragged cumulus cloud. Observing fractus gives indications of wind movements under the parent cloud.

Mass of multiple fractus clouds, located under main cloud is called pannus.

Fractonimbus is form of stratus fractus, developing under precipitation clouds due to turbulent air movement. They are dark-gray and ragged in appearance. Fractonimbus exist only under precipitation clouds (such as nimbostratus, altostratus or cumulonimbus) and don't bring precipitation themselves. Fractonimbus can with time completely merge with overlying nimbostratus clouds.

Significance in thunderstorms

In thunderstorms, scud often form in the updraft area where the air has been cooled by precipitation from the downdraft, thus condensation occurs below ambient cloud deck. If scud are rising and moving towards the main updraft, sometimes marked by a rain-free base (RFB) or wall cloud, then the thunderstorm is still in a process of growth. Scud are common and are usually insignificant, however, scud precedes wall clouds, which often form from rising scud.

In addition to forming in inflow, fractus also forms in outflow. Scud is very common on the leading edge of a thunderstorm where warm, moist air is lifted by the gust front. Scud are usually found under shelf clouds.

References

See also

• List of cloud types