The Portuguese Institute of the Sea and Atmosphere (IPMA) has explained that the rare meteorological phenomenon which was ...

Mackerel skies: a sky filled with rows of cirrocumulus or altocumulus clouds that resemble the scales of a mackerel fish.

Altocumulus: These cumulus clouds are grouped closer together than regular cumulus clouds. They resemble a white or grey patch in the sky and can cover the sun occasionally.