Pannus,
or scud clouds,
is a type of
fractus cloud at low height above ground, detached, and of irregular form, found beneath
nimbostratus,
cumulonimbus,
altostratus and
cumulus clouds
Cumulus clouds are clouds which have flat bases and are often described as "puffy", "cotton-like" or "fluffy" in appearance. Their name derives from the Latin ''cumulo-'', meaning ''heap'' or ''pile''. Cumulus clouds are low-level clouds, gener ...
. These clouds are often ragged or wispy in appearance. When caught 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 ...
(
downdraft) beneath a
thunderstorm, scud clouds will often move faster than the storm clouds themselves. If the parent cloud that scud clouds pair with were to suddenly dissipate, the pannus cloud accessory would not be able to be told apart from a
fractus cloud formation.
When in an inflow (
updraft) area, scud clouds tend to rise and may exhibit lateral movement ranging from very little to substantial.
Formation
Pannus clouds are formed as the warmer (and often more moist) updraft of a thunderstorm lifts the relatively warm air near the surface. These clouds condense as the warm, moist air saturates through ascent and is pushed outward from the storm. Scud clouds are very commonly found on the leading edge of a storm front. In this area of a storm, scud are commonly associated with
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 ...
s.
[
Pannus clouds may also form when an updraft ingests precipitation-cooled air from the downdraft. Scud forming in this region of the storm, if moving laterally, will tend to move inward towards the dominant updraft. Rising scud may condense and organize into a wall cloud.][ The key to differentiating between a scud cloud, wall cloud, or funnel cloud is to find signs of rotation or to determine if they are attached to the base of the thunderstorm.
Pannus clouds can often be mistaken for a developing tornado, landspout, or waterspout. The difference is determinable by observing the presence or absence of rotation (not just movement) of the scud clouds. If rotation is present, then a tornado, landspout, or waterspout is possible, and the more intense the rotation, the more likely.
]
Forecast
Viewing scud clouds means that there is a nimbostratus or cumulonimbus parent cloud. Rain, hail
Hail is a form of solid precipitation. It is distinct from ice pellets (American English "sleet"), though the two are often confused. It consists of balls or irregular lumps of ice, each of which is called a hailstone. Ice pellets generally fal ...
, strong wind, and lightning can be expected from the parent cloud depending on its genus type. Other than signifying severe weather, scud clouds are entirely harmless.
References
[Timothy. (n.d.). Scud Clouds - Weather - Creepy Clouds That Form Under Thunderstorms • Tim’s Weird & Wonderful World. Retrieved November 8, 2021, from https://timswww.com.au/scud-clouds-weather-creepy-clouds-that-form-under-thunderstorms/
]
See also
* Scud running
* List of cloud types
Accessory clouds
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