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