Pileus (meteorology)

Pileus on a Cumulus cloud

Pileus forming over the ash cloud from an eruption of Sarychev Peak

A pileus (/ˈpaɪliː.əs/; Latin for cap), also called scarf cloud or cap cloud, is a small, horizontal, lenticular cloud that can appear above a cumulus or cumulonimbus cloud, giving the parent cloud a characteristic "hoodlike" appearance. Pilei tend to change shape rapidly. They are formed by strong updraft at lower altitudes, acting upon moist air above, causing the air to cool to its dew point. As such, they are usually indicators of severe weather, and a pileus found atop a cumulus cloud often foreshadows transformation into a cumulonimbus cloud, as it indicates a strong updraft within the cloud.

Clouds that are attached to pilei are often given the suffix "pileus" or "with pileus". For example, a cumulonimbus cloud with a pileus attached to it would be called "cumulonimbus with pileus".

Pilei can also form above ash clouds and pyrocumulus clouds from erupting volcanoes (see the image to the right).

Pilei form above some mushroom clouds of high-yield nuclear detonations.

A sheet of altostratus cloud often is seen lower down in a cumulonimbus cloud, this is known as a velum cloud.

Forecast

Pilei clouds indicate that the parent cloud is growing rapidly, has plenty of moisture, and is highly unstable. This means that the parent cloud could quickly grow to become a cumulonimbus cloud and continue to grow into a cumulonimbus incus cloud. These clouds can cause severe weather and sightings of pilei should be viewed as a warning of an impending storm. Pilei also indicate strong and possibly damaging winds.

References

Wikimedia Commons has media related to Pileus clouds.

Video – Video of pileus clouds.

Cloud genera and selected species, supplementary features, and other airborne hydrometeors - WMO Latin terminology except where indicated

Extreme-level	Polar mesospheric cirriform type Noctilucent

Very high-level	Polar stratospheric cirriform type Nacreous

High-level	Tropospheric cirriform, stratiform, and stratocumuliform genera Cirrus (Ci) Cirrostratus (Cs) Cirrocumulus (Cc) General type (non-WMO terminology) Aviaticus cloud (Contrail)

Medium-level	Tropospheric stratiform and stratocumuliform genera Altostratus (As) Altocumulus (Ac) Stratocumuliform species Altocumulus castellanus (Ac cas)

Low-level	Tropospheric stratiform, stratocumuliform and cumuliform genera Stratus (St) Stratocumulus (Sc) Cumulus (Cu) Stratiform and cumuliform species Fractus Cumulus humilis (Cu hum) Stratocumulus variants (non-WMO terminology) Actinoform cloud Undulatus asperatus

Moderate vertical	Tropospheric stratiform and cumuliform genera Nimbostratus (Ns) Cumulus (Cu) Cumuliform species Cumulus mediocris (Cu med) Cumuliform supplementary features Cumulus Pileus Cumulus Arcus (Roll)

Towering vertical	Tropospheric cumulonimbiform genus Cumulonimbus (Cb) Cumuliform species Cumulus congestus (ICAO term Towering cumulus [Tcu]) Cumuliform and cumulonimbiform supplementary features mamma Tuba (Funnel cloud) Pileus Arcus (Shelf) Cumuliform and cumulonimbiform variants (non-WMO terminology) Wall cloud Pyrocumulus Pyrocumulonimbus Cumulonimbus Overshooting top Hot tower

Surface based	General type Fog

Non-height specific	General types Accessory cloud Kelvin-Helmholtz cloud

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Pileus (meteorology)

Forecast

See also

References