Two weeks ago the mountain was wearing a hat. Today it's waving a flag.
Banner clouds are stationary, orographic clouds that only form in high wind on the leeward side of an isolated, steep mountain. The Matterhorn, pictured above, is famous for them.
Banner clouds are so picky that we'll never see them in western Pennsylvania simply because we have no isolated steep mountains.
... except ...
Under the right moisture conditions a banner cloud can form above or just behind an airplane's wings. Click here for an example.
Airplanes form banner clouds because there's lower air pressure on top of their wings (to generate lift). The lower pressure results in lower temperature which results in condensation. Hence a cloud.
My favorite banners are the wing tip clouds that look like streamers.
And for a really weird effect, check out this cloud around a fighter jet on the verge of breaking the sound barrier. The shape is so perfect it's hard to call it a banner.
(photo by Zacharie Grossen on Wikimedia Commons. Click on the photo to see the original)