"Mackerel sky, mackerel sky. Never long wet and never long dry"

So the saying goes.

In simple terms it means that things are a bit unstable at between 8000 and 20000 feet up.  If the air beneath this level stays dry then the weather will stay good.  If moist air moves in then it'll probably rain.  These clouds are altocumulus which means medium-high (alto) and heaped (cumulus).  They get their name as they resemble the markings on a mackerel as the images below will show.

There's a mackerel sky out tonight and I'm playing golf in the morning so I'm hoping that this means that it'll stay nice and dry for me.