Since I say the phrase so much in these ‘pages’, it seems appropriate to point out a photo of an “iridescent pileus cloud’ which was the Astronomy Picture of the Day a couple days ago:
- via: Astronomy Picture of the Day (August 24, 2011)
Of course, it clearly gets its name from the fact that is a sort of ‘cap’ on another cloud, but if you need a bit more detail, the Wikipedia article: Pileus (meteorology) is as good a source as any. If you’re wondering what an ancient Roman pileus looked like:
rogueclassicism 
That was the first thing that I thought of when I saw the APOD post!