Today, I found a mackerel sky. This is a cloudscape that looks like rows of fish scales, or, as I like to think of it, a popcorn sky.
Technically, these rows of small puffy clouds are altocumulus, mid-level cumulus that form in the lower middle portion of the sky. These clouds are sometimes confused with cirrocumulus, although the two form in very different vertical layers. If you look through the bumpy rows of clouds and the sun looks fuzzy, you've spotted the rarer and higher lying cirrocumulus.
Over on NASA's S'COOL website (Students' Cloud Observations On-line), Lin Chambers suggests holding your hand up at arm's length when trying to differentiate between these two cloud types. If the individual clouds are the size of your thumbnail, you have altocumulus; if they are the size of your pinky fingernail, you have cirrocumulus.
Note: Since I posted this, I've learned that the term mackerel sky can be used to describe either cirrocumulus or altocumulus or both!