Pileus clouds capping thunderstorms over eastern Maryland. (Photo and caption by wolfpackWX via Flickr)

Around mid-afternoon on Tuesday, the rain had pushed east and the western suburbs of D.C. began to clear out. The sun shined on intensely stormy skies to the east, providing an excellent show for weather watchers. Rainbows appeared in lingering rain shafts, and a special cloud was seen atop the building cumulus.

Madness, indeed! Fortunately (unfortunately?) that is NOT the polar vortex. It is a pileus cloud, also known as a “cap cloud,” because it caps the top of cumulonimbus clouds. It’s related to the lenticular cloud, which you will often see downwind of mountains or large hills.

Pileus clouds form when strong updrafts push drier air (relative to its surroundings) above cumulus clouds. Pileus are usually the sign of strong storms with fast updrafts. The cumulonimbus that is building below them will often grow right up into the pileus, leaving what looks like a skirt around the stormy clouds. Pileus can also be formed over the ash clouds of volcanic eruptions.

In addition to the pileus clouds, many rainbows were captured as the rain moved east. First, a rainbow AND pileus!

And a final rainbow peaking from the clouds over the Washington Monument:

Thanks to everyone who sent us photos on social media during the event!