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Pileus Cloud

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Educational Resources

Pileus Cloud

November 2015

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Figure 1   Photo by Chang Wen Lan

If you are a frequent cloud observer, have you ever noticed that cumulus clouds may sometimes possess atypical "hairstyle" on its top with a smooth and shining appearance? This type of accessory cloud is called "pileus cloud".

Persistent strong updrafts usually occur within growing cumulus or cumulonimbus. When there is a stable and moist airstream flowing above the cloud top, the airstream will be pushed upward leading to condensation and formation of a cap cloud aloft called "pileus cloud". However, this atypical "hairstyle" does not last long. The cap will eventually be cut through when the cumulus cloud beneath continues to grow upward.

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Figure 2   The schematic diagram above shows the formation of pileus cloud.