Pannus, or scud clouds, is a type of

fractus cloud Fractus clouds, also called fractostratus or fractocumulus, are small, ragged cloud fragments that are usually found under an ambient cloud base. They form or have broken off from a larger cloud, and are generally sheared by strong winds, givin ...

at low height above ground, detached, and of irregular form, found beneath

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.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 ...

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 ...

and

cumulus clouds Cumulus clouds are clouds that have flat cloud base, 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 ...

. These clouds are often ragged or wispy in appearance. When caught in the outflow (

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 ...

) beneath a

thunderstorm A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustics, acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak 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

formation. When in an inflow (

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, 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 clouds. Pannus clouds may also form when an updraft ingests

precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, rain and snow mixed ("sleet" in Commonwe ...

-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 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 ...

. The key to differentiating between a scud cloud,

, or

funnel cloud A funnel cloud is a funnel-shaped cloud of condensed water droplets, associated with a rotating column of wind and extending from the base of a cloud (usually a cumulonimbus or towering cumulus cloud) but not reaching the ground or a water su ...

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.

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/

Formation

References

See also