A pileus (;

Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...

for "cap"), also called scarf cloud or cap cloud, is a small, horizontal,

lenticular cloud Lenticular clouds ( Latin: ''Lenticularis'' lentil-shaped, from ''lenticula'' lentil) are stationary clouds that form mostly in the troposphere, typically in parallel alignment to the wind direction. They are often comparable in appearance to ...

appearing above a

cumulus 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 ...

cumulonimbus cloud Cumulonimbus (from Latin ''cumulus'', "heaped" and ''nimbus'', "rainstorm") 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. ...

. Pileus clouds are often short-lived, with the main cloud beneath them rising through convection to absorb them. They are formed by strong

updraft In meteorology, an updraft is a small-scale current of rising air, often within a cloud. Overview Localized regions of warm or cool air will exhibit vertical movement. A mass of warm air will typically be less dense than the surrounding region, ...

at lower altitudes, acting upon moist air above, causing the air to cool to its

dew point The dew point is the temperature to which air must be cooled to become saturated with water vapor, assuming constant air pressure and water content. When cooled below the dew point, moisture capacity is reduced and airborne water vapor will ...

. 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. Types of severe weather phenomena vary, depending on the latitude, altitude, topography, and atmo ...

, 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 clouds, and pyrocumulus clouds from erupting

volcano A volcano is a rupture 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 often found where tectonic plates ...

es. Pilei form above some

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

s of high- yield

nuclear detonation A nuclear explosion is an explosion that occurs as a result of the rapid release of energy from a high-speed nuclear reaction. The driving reaction may be nuclear fission or nuclear fusion or a multi-stage cascading combination of the two, tho ...

Appearance

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

Forecast

Pilei 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 (Latin ''incus'', "anvil"), also known as an anvil cloud, is a cumulonimbus cloud which has reached the level of stratospheric stability and has formed the characteristic flat, anvil-top shape. It signifies the thunderstor ...

cloud.

References

External links

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

Appearance

Variants

Forecast

See also

References

External links