Really cool clouds this evening - what causes this?

Almost the entire sky is filled with very uniform cloudlets. What causes this phenomenon?

Photo:
CloudApp

Those look like they might be mammatus clouds - though an unusually extensive display of them.

Mammatus clouds are generated when cold air descends into warmer air.

I don’t think those are mammatus; those are very round and project from the bottom of a cloud, often one associated with severe thunderstorms or even tornadoes. The ones the OP pictured look like something we see all the time in the upper Midwest, where one can safely assume s/he is not because of the palm trees.

Ho high were they? It looks like Altocumulus floccus to me.

Mackerel sky

Yes, that’s it.
Pretty unusual out here…