Appearance: Resembles the top of a mushroom or a smooth cap.
Occurrence: Occasional. Found above rapidly growing cumulus or cumulonimbus clouds as a result of warm updrafts formed during a thunderstorm.
Conditions: Caused by rapid, relatively strong, upward movements of warm moist air, which acts as a barrier. Air approaching the top of the cloud is forced to move up and around it. The air forced upward, cools and forms the cloud cap.
Location: Pileus clouds form in conjunction with strong convective clouds and are associated with the cumulus cloud that lies beneath. These occur in areas with strong heating and lots of moisture close to the ground. This means that they commonly occur over land in areas like the eastern United States and Australia (more likely over the Great Dividing Range and in the northern parts during monsoon season) during spring and summer.