A pileus cloud is a thin, smooth cap-shaped cloud that forms just above a rapidly growing cumulus cloud. As the main cloud surges upward, it lifts a layer of moist air above it. That lifted air can cool enough for water vapor to condense into a separate, delicate cap.
How does a pileus cloud form?
Rising air expands and cools as surrounding pressure decreases. A strong updraft beneath a cumulus congestus cloud can push an overlying moist layer upward. Condensation creates a smooth, thin veil while the bigger cloud below remains billowy. The cap is made of cloud droplets, not a rigid lid.
Why does a pileus cap disappear?
The tower can grow through it or the lifted layer can mix with drier air, so the cap often lasts just minutes. A photographer may see several caps form in succession as the cloud continues rising through new layers.
Is a pileus the same as an anvil cloud?
No. A pileus is generally a relatively small smooth accessory cloud above a growing tower. A cumulonimbus anvil is the broad, spreading upper part of the thunderstorm cloud itself as it reaches an atmospheric layer that resists further vertical growth.
Does a pileus cloud mean severe weather?
Its presence indicates vigorous upward growth, but it does not prove severe weather will develop. Look at the whole atmosphere and follow official weather warnings rather than treating one shape as a forecast.
Frequently asked questions
Can pileus clouds appear iridescent?
Yes. A thin cap with similarly sized droplets can sometimes display delicate colors because of diffraction of sunlight.
Do you need a mountain to make a pileus?
No. The cap forms from uplift above a rapidly developing cloud; it is different from a mountain-related lenticular cloud.
Sources: University of British Columbia: Pileus Clouds and WMO International Cloud Atlas.
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