This beautiful picture was taken by Richard Barnhill in eastern MD on 12 August 2014. The sun is highlighting the tops of growing cumulus congestus clouds, which are capped by another cloud, called a pileus cloud. Pileus is Latin for “cap” and resembles lenticular clouds that are also highlighted in this atlas. Lenticular clouds form when moist stable air encounters a mountain barrier, whereas these pileus clouds form when moist stable air is disrupted by the growing cumulus cloud below.
Strong updrafts occur within these growing cumulus clouds, defined by their well-defined edges and tufted appearance. If the air above is moist and relatively stable, but is forced upward by this strong upward motion from the cloud below, it can cool to its dewpoint, leading to condensation and the formation of this pileus cloud. This happens quite rapidly and the pileus cloud does not last long as typically the cumulus cloud beneath continues to grow through it.
Here is a schematic we created to try to simply explain this process:
The following series of images shows another example of a pileus cloud, where the cumulus below quickly produced this cap cloud and then grow through it to form a mature cumulonimbus.
Here is yet another great example of a pileus cloud, sent to us from North Carolina back in May. Notice how clearly it sits atop the cumulus congestus cloud, resembling a cap cloud hugging the top of a mountain barrier.
The term “pileus” isn’t unique to clouds, however. Another example of this “cap” feature is given for the tops of mushrooms, such as shown in this diagram below. It’s great to see the commonalities in nature!
Given that pileus are quite the fleeting phenomena, we are curious: have you seen a pileus? We would love to see your examples!