A nicely nerdy post! A lovely sky we had recently, known as a mackerel or buttermilk sky due to the resemblance to the mackerels distinctive patterning or curdled milk.

These are made up of ripples of cirrocumulus and altocumulus clouds caused by high altitude atmospheric waves ahead of a warm front.

Basically it means that we’re into changeable weather – usually rain within the next six to twelve hours. It’s a pretty reliable forecaster and has been known for a long time, giving rise to various sayings which I quite like:

“Mackerel sky, not twenty-four hours dry”

“Mares’ tails and mackerel scales make lofty ships to carry low sails”

“Mackerel sky, mackerel sky. Never long wet and never long dry”