Cirrocumulus

How to recognise it

• A thin patch or layer made of tiny grains, ripples or granules, usually arranged in regular rows.
• Each individual element is very small — under one degree wide, roughly the width of your little finger held at arm's length.
• Entirely white with no grey shading whatsoever. This is the single most reliable way to separate it from altocumulus.
• The rippled pattern across a wide area is what gives the "mackerel sky" its name.

What weather it signals

• Cirrocumulus itself brings no precipitation. It is a sign of instability in the high atmosphere.
• It is usually short-lived, forming from cirrus or cirrostratus and dissolving back into them within minutes to hours.
• In temperate latitudes a mackerel sky often accompanies an approaching front and a change in the weather. In the tropics it can indicate an approaching tropical cyclone.

How to photograph it

• Use the longest focal length you have. The elements are small, and a wide shot reduces the whole pattern to noise.
• Photograph it quickly — cirrocumulus frequently dissolves within a quarter of an hour.
• Include a fixed reference such as a rooftop or tree line so the scale of the ripples is readable.

Fun facts

• Cirrocumulus is the rarest of the ten cloud genera and almost never covers the whole sky on its own.
• The old sailors' rhyme "mackerel sky, mackerel sky — never long wet, never long dry" refers to this cloud and its association with changeable weather.
• Although the cloud is made of ice crystals, the ripples form because supercooled water droplets are freezing — which is why the pattern collapses so quickly.

Easily confused with

Frequently asked questions