"Mackerel sky, mackerel sky. Never long wet, never long dry"
"Mare's tails and Mackerel scales, make tall ships carry low sails"
The phrase 'mackerel sky' came from the fact that it looks like
the markings of an adult king mackerel.
Altocumulus clouds form when there is moisture present at middle levels,
and when temperatures rang from colder than freezing to a little warmer than freezing.
If the altocumulus are growing in area and thickness with time, and are
approaching from the wind direction at the cloud's level, then a change toward moister weather is indicated.