> Red sky in morning, Sailors take warning.
Red sky at night, Sailors' delight.
> Rainbow in the morning
gives you fair warning
> Mare's tails and mackerel scales make tall ships take in their sails."
A mackerel sky refers to cirrocumulus clouds, which often precede an approaching warm front, which will eventually bring veering winds (changing from northeast and east over to southwest and west) and precipitation.