Fractus clouds, also called fractostratus or fractocumulus, are small, ragged

cloud In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles, suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may ...

fragments that are usually found under an ambient

cloud base A cloud base (or the base of the cloud) is the lowest altitude of the visible portion of a cloud. It is traditionally expressed either in metres or feet above mean sea level or above a planetary surface, or as the pressure level corresponding t ...

. They form or have broken off from a larger cloud, and are generally sheared by strong

wind Wind is the natural movement of atmosphere of Earth, air or other gases relative to a planetary surface, planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heatin ...

s, giving them a jagged, shredded appearance. Fractus have irregular patterns, appearing much like torn pieces of

cotton candy Cotton candy, also known as candy floss (candyfloss) and fairy floss, is a spun sugar confection that resembles cotton. It is made by heating and liquefying sugar, and spinning it centrifugally through minute holes, causing it to rapidly cool ...

. They change constantly, often forming and dissipating rapidly. They do not have clearly defined bases. Sometimes they are persistent and form very near the surface. Common kinds include and .

Forms

Fractus are accessory clouds, 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 Cumulus clouds are clouds that have flat bases and are often described as puffy, cotton-like, or fluffy in appearance. Their name derives from the Latin , meaning "heap" or "pile". Cumulus clouds are low-level clouds, generally less than in a ...

if the ground heats enough to start

convection Convection is single or Multiphase flow, multiphase fluid flow that occurs Spontaneous process, spontaneously through the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoy ...

. Stratus fractus is distinguishable from cumulus fractus by its smaller vertical extent, darker color, and the greater dispersion of its particles. Cumulus fractus clouds actually look like ragged cumulus clouds. They may originate from dissipated cumulus clouds, appearing in this case as white ragged clouds located at significant distances from each other. Cumulus fractus in particular form on the leading and trailing edges of summer storms in warm and humid conditions.NOAA Cloud Types
- NOAA Chart of Different Cloud Types. Observing fractus gives an indication of wind movements under the parent cloud. Masses of multiple fractus clouds, located under a main cloud, are called pannus or scud clouds. Fractonimbus is a 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 A nimbostratus cloud is a multilevel, amorphous, nearly uniform, and often dark-grey cloud that usually produces continuous rain, snow, or sleet, but no lightning or thunder.altostratus Altostratus is a middle-altitude cloud genus made up of water droplets, ice crystals, or a mixture of the two. Altostratus clouds are formed when large masses of warm, moist air rise, causing water vapor to condense. Altostratus clouds are usual ...

cumulonimbus Cumulonimbus () is a dense, towering, vertical cloud, typically forming from water vapor condensing in the lower troposphere that builds upward carried by powerful buoyant air currents. Above the lower portions of the cumulonimbus the water va ...

), and don't produce precipitation themselves. Fractonimbus may eventually merge completely with overlying

nimbostratus A nimbostratus cloud is a multilevel, amorphous, nearly uniform, and often dark-grey cloud that usually produces continuous rain, snow, or sleet, but no lightning or thunder.Stratus silvagenitus clouds also commonly form as scattered stratus fractus silvagenitus. These clouds can be seen as the occasional ragged tuft of cloud seeming to rise from forests, commonly just after rain or during other periods of high humidity.

Significance in thunderstorms

In rainstorms, scud often forms in the

updraft In meteorology, an updraft (British English: ''up-draught'') is a small-scale air current, current of rising air, often within a cloud. Overview Vertical drafts, known as updrafts or downdrafts, are localized regions of warm or cool air that mov ...

area where the air has been cooled by precipitation from the

downdraft In meteorology, an updraft (British English: ''up-draught'') is a small-scale current of rising air, often within a cloud. Overview Vertical drafts, known as updrafts or downdrafts, are localized regions of warm or cool air that move vertically ...

, thus condensation occurs below the ambient cloud deck. If the scud is rising and moving towards the main updraft, sometimes marked by a rain-free base (RFB) or

wall cloud A wall cloud (murus or pedestal cloud) is a large, localized, persistent, and often abrupt lowering of cloud that develops beneath the surrounding base of a cumulonimbus cloud and from which tornadoes sometimes form. It is typically beneath the ...

, then the thunderstorm is still developing from the rising scud. In addition to forming in

inflow Inflow may refer to: * Inflow (hydrology), the water entering a body of water * Inflow (meteorology) Inflow is the flow of a fluid into a large collection of that fluid. Within meteorology, inflow normally refers to the influx of warmth and mo ...

, fractus also forms in the

outflow Outflow may refer to: *Capital outflow, the capital leaving a particular economy *Bipolar outflow, in astronomy, two continuous flows of gas from the poles of a star * Outflow (hydrology), the discharge of a lake or other reservoir system *Outflow ...

. Scud are very common on the leading edge of a thunderstorm where warm, moist air is lifted by the

gust front An outflow boundary, also known as a gust front, is a storm-scale or mesoscale boundary separating thunderstorm-cooled air ( outflow) from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually ...

. Scud are usually found under

shelf cloud An arcus cloud is a low, horizontal cloud formation, usually appearing as an accessory cloud to a cumulonimbus. Roll clouds and shelf clouds are the two main types of arcus clouds. They most frequently form along the leading edge or gust fronts o ...

References

{{Cloud types Accessory clouds

Forms

Significance in thunderstorms

See also

References