Mackerel sky

Mackerel sky is a popular term for a sky covered with extensive cirrocumulus or altocumulus clouds arranged in regular waves.
The pattern resembles the scales on a mackerel, hence, the name. Small, white and fluffy cirrocumulus clouds typically consist of ice crystals and form at altitudes around 6,000 to 10,000m, often well ahead of depressions and their associated fronts.
There are weather lores associated with a mackerel sky. For example: 'Mackerel sky, mackerel sky - never long wet, never long dry.'
In the days of sailing ships they have been rightly viewed as likely forerunners of stormy weather:
'Mare's tails and mackerel scales make lofty ships take in their sails.'