What causes a mackerel sky?

Have you ever looked up at a pattern of clouds dotting the sky and noticed that occasionally, they look like the scales of a fish?

“Mackerel sky” is a fairly common term for that pattern and can be a sign of changing weather ahead.

Not only is it beautiful in photos, but the presence of a mackerel sky indicates changing weather ahead and is a useful tool for weather forecasters.

Mackerel sky formation

Ahead of an approaching warm front, warm air pushes up into the high levels of the atmosphere. As the air is forced upward, it cools and condenses to form clouds.

Cirrocumulus and altocumulus clouds tend to form when moisture becomes trapped between dry air at the Earth’s surface and dry, cold air at high levels of the atmosphere. These clouds are puffy and tend to be patchy, with blue sky peeking between the clouds.

Gravity and wind can create waves in the atmosphere, causing the clouds to form in rows or ripples. This pattern resembles the scales on the back of a mackerel, and people commonly refer to it as “mackerel sky.”

Mackerel sky in weather forecasting

The appearance of a mackerel sky usually indicates a change in weather in the near future. As more of these puffy clouds thicken up and invade the sky, weather forecasters can usually expect precipitation to begin falling in the next 6 to 12 hours.

Old rhymes like “mackerel sky, not 24 hours dry” and “mare’s tails and mackerel scales make lofty ships to carry low sails” both refer to the likelihood of changing weather ahead when a mackerel sky is present.