Unusual cloud formation

This picture was taken by Tim Pederson while on a flight over the Pacific Ocean. He says "I was wondering if you could explain the dome over the cloud. I have never seen that before."

First of all Tim, the large cloud with the cauliflower top is a cumulus (cumulonimbus if there's rain being produced out the bottom...which it probably is.) The little dome on top is called a pileus cloud (pī'lē-əs). Pileus clouds often form on top of cumulus and indicate strong updrafts iwithin the cloud mass. You can almost picture the main cloud rising so rapidly that it is pushing the remaining moisture (pileus) in the clear-air right up against the top of the atmosphere. Tim, I am glad you caught a glimpse of it because pileus clouds don't usually last long. This cloud was probably only around for a few minutes at most.

Thanks for sending it in!