Really high cumulus.
Mackerel sky, mackerel sky!
Not long wet, not long dry.
Don’t these minute-looking and shiny cumulus look like fish scales? But they are not necessarily small, only distant, three and a half of four miles up there instead of the half-mile to mile of fair-weather cumulus. So you have to multiply their diameters by at least three, and maybe by seven or eight.
And the definite cumulus shape of individual elements means that there is active convection in that part of the sky. Of course the shiny look comes because the sunshine has plenty of opportunity to get through and under them.
We didn’t have rain (here) when we had these clouds, two days ago, but the weather did blow up for a while, and there may have been rain not far away. In any case, there’s another sailor’s verse:
Mare’s tails and mackerel scales
Make tall ships carry low sails.
So look for a brief period of windy storm when you see these elements.
Notice the Universe 