Pileus Clouds

A cumulus cloud with pileus. This pileus is a small one.

There are multiple pilei on the cloud in this photo.
See if you can find them.

A Pileus, pronounced pie-lee-us, is a small, smooth, silky-looking lenticular (lens-shaped) cloud that can appear above parts of either a cumulus or cumulonimbus cloud. (If you don't know what a cumulus cloud is, click HERE. Cumulonimbus clouds are basically very large cumulus clouds.) A cloud attached to pileus is said to be "with pileus" or to "have pileus".

Clouds with pilei can develop into very large
cumulonimbus clouds.

Pilei form when a strong updraft at a lower altitude acts upon air at a higher altitude, and causes it to cool below its dew point. The result is that as the air cools, the water vapor in it condenses and forms a cloud. Because of the conditions required for it to form, a pileus cloud is usually a sign of severe weather; the cloud that it forms above is likely to develop into a cumulonimbus if it hasn't already.

Early this afternoon, it was bright and sunny, but there were a couple of cumulus clouds with pileus, so I immediately predicted a thunderstorm. Sure enough, only a couple hours after that, it started raining harder than it has in a long time, and I could smell ozone from the lightning that kept flashing outside.

It isn't raining any more, but I can see a huge cumulonimbus far in the distance, and dark clouds overhead.