Definition - What does Mackerel Sky mean?
Mackerel Sky refers to a sky covered with the altocumulus or cirrocumulus clouds. Mackerel Sky is also known as buttermilk sky. Usually, it indicates the transient levels and presence of moisture, which can result in thunderstorms or rain showers if the lower cumulus clouds transform to the altocumulus form. Rain is usually expected from a Mackerel Sky. In the Mackerel Sky, the arrangement of the clouds is in a wavy pattern which shows the blue sky through the gaps. The word "Mackerel" is used to describe the sky because the cloud pattern is similar to the mackerel fish scales. In France and Germany, Mackerel Sky is also referred to as sheep cloud.
Petropedia explains Mackerel Sky
When an altostratus cloud disintegrates into altocumulus cloud due to the degenerated frontal system, it leads to the formation of Mackerel Sky. In this case, clouds break up to form ripples and the pattern resembles to that of the scales of a mackerel fish. Also, the Mackerel Sky resembles the waves, which may or may not have gaps in between showing the blue sky. A Mackerel Sky may follow up with light rain or thunderstorms. However, the rain is not a must. This is because the altocumulus cloud is largely composed of water droplets. If the altocumulus cloud becomes heavier, it can cause a thunderstorm. The altocumulus clouds can be found at an altitude of 8,000 to 20,000ft.