Rolling in the sky and breaking like waves in the sea, these clouds photographed over Palmerston North are simply beautiful.

But the scientific explanation for them is anything but simple.

MetService shared the lovely shot, by Carl Gadsby, with a link to more on the phenomenon.

We don't immediately know when the photo was taken.

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"It occurs when the change in wind across the boundary between two fluids is so much that steady simple 'laminar' flow breaks down and becomes turbulent, and the two fluids mix, with regularly spaced eddies," MetService writes.

The full explanation is far longer and more complex than that, but that's enough for us.

To really impress your friends if you see clouds like this, you can call it a classic example of Kelvin-Helmholtz instability.