A pileus (; ), also called scarf cloud or cap cloud, is a small, horizontal,

lenticular cloud Lenticular clouds (, ) are stationary clouds that form mostly in the troposphere, typically in parallel alignment to the wind direction. They are often comparable in appearance to a lens or saucer. polar stratospheric cloud, Nacreous clouds tha ...

appearing above a cumulus or

cumulonimbus cloud 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 ...

. Pileus clouds are often short-lived, appearing for typically only a few minutes, with the main cloud beneath them rising through convection to absorb them. Furthermore, the clouds are typically formed by drier air with a higher lifting condensation level, which often prevents vertical growth and leads to the smooth horizontal cap shape that the cloud is named for. They are formed by strong

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 ...

at lower

altitude Altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum (geodesy), datum and a point or object. The exact definition and reference datum varies according to the context (e.g., aviation, geometr ...

s, acting upon moist air above, causing the air to cool to its

dew point The dew point is the temperature the air needs to be cooled to (at constant pressure) in order to produce a relative humidity of 100%. This temperature depends on the pressure and water content of the air. When the air at a temperature above the ...

as it rises. As such, they are usually indicators of

severe weather Severe weather is any dangerous meteorological phenomenon with the potential to cause damage, serious social disruption, or loss of human life. These vary depending on the latitude, altitude, topography, and atmospheric conditions. High ...

, and a pileus found atop a cumulus cloud often foreshadows transformation into a cumulonimbus cloud, as it indicates a strong updraft within the cloud. Pilei can also form above mountains,

ash cloud Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, produced during volcano, volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used ...

s, and

pyrocumulus cloud A flammagenitus cloud, also known as a flammagenitus, pyrocumulus cloud, or fire cloud, is a dense cumuliform cloud associated with fire or volcanic eruptions. A flammagenitus is similar dynamically in some ways to a firestorm, and the two ph ...

s from erupting

volcano A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most oft ...

es. Pilei form above some

mushroom cloud A mushroom cloud is a distinctive mushroom-shaped flammagenitus cloud of debris, smoke, and usually condensed water vapour resulting from a large explosion. The effect is most commonly associated with a nuclear explosion, but any sufficiently e ...

s of high- yield nuclear detonations.

Appearance

Though the pileus cloud does not tend to grow vertically, sometimes several pileus clouds are observed above each other. The bright iridescent colors seen in pileus are sunlight diffracted in water vapor. Iridescent colors are strongest when the diffracting droplets are small and similar in size. The newly formed pileus droplet all of similar provenance are ideal for iridescence.

Variants

When sheet of

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 ...

cloud is seen lower down and skirting a cumulonimbus cloud, it is classified as a velum cloud.

Forecast

Pileus clouds indicate the parent cloud is growing rapidly, has plenty of moisture, and is highly unstable. This means the parent cloud could quickly grow to become a cumulonimbus cloud and continue to grow into a

cumulonimbus incus A cumulonimbus incus (), also called an anvil cloud, is a cumulonimbus cloud that has reached the level of stratospheric stability and has formed the characteristic flat, anvil-shaped top. It signifies a thunderstorm in its mature stage, suc ...

cloud.

References

External links

Video – Video of pileus clouds
Cumulus {{Cloud-stub

Appearance

Variants

Forecast

See also

References

External links