"Mackerel scales and mare's tails
Make lofty ships carry low sails"
This is an old bit of weather folklore that refers to an unsettled appearance of the sky before incoming bad weather.
Mare's tails are high level cirrus clouds that do indeed look like a horse's tail.
Mackerel scales or a mackerel sky refers to small altocumulus clouds that form at medium levels in the atmosphere (between 8and 20 thousand feet). The patterns formed by these clouds is similar the patterns found on the King Mackerel (Scomberomorus cavalla).
|A Mackerel Sky|
Both Mare's Tails and Mackerel Scales can indicate incoming bad weather, but they are also common in fair weather. In fact there are conflicting folklore sayings to the one above.
Mackerel in the sky,
Three days dry.
Or you can hedge your bets and use a phrase that predicts both good and bad weather.
Never long wet
And never long dry.