Altocumulus

How to recognise it

• Rounded clumps, rolls or flattened tiles arranged in groups, lines or waves.
• Each element is roughly one to five degrees wide — between the width of your little finger and three fingers at arm's length.
• Grey shading on the underside of the elements. This is the decisive difference from the pure white cirrocumulus.
• Gaps of blue sky are often visible between the elements, though a dense layer can close over completely.

What weather it signals

• A single flat layer of altocumulus on an otherwise calm morning usually means the weather is unchanged.
• Altocumulus with turret-like tops (Altocumulus castellanus) on a summer morning is a strong warning: the mid-atmosphere is unstable and thunderstorms are likely later that day.
• Altocumulus thickening and lowering ahead of a front often precedes steady rain.

How to photograph it

• Shoot when the sun is low. Side lighting exaggerates the shading that defines the element structure.
• A wide angle captures the wave pattern across the sky, which is the most striking thing about this cloud.
• Look for the iridescent colours near the thinner edges when the sun is just outside the frame.

Fun facts

• Altocumulus is the cloud most often responsible for iridescence — the pale pink and green bands that appear at thin cloud edges near the sun.
• Lens-shaped altocumulus lenticularis forms in standing waves downwind of mountains, hangs motionless for hours, and is a routine source of UFO reports.
• Because it sits between the water-droplet clouds below and the ice clouds above, altocumulus often contains both at once.

Easily confused with

Frequently asked questions