Cloud PointField Guide

High cloud — genus

Cirrocumulus

Tiny rippled grains — rarest of the high clouds

Written by Laurence WayneEntry No. 21Uncommon

Cirrocumulus: Tiny rippled grains — rarest of the high clouds
Plate XXIPhotograph by King of Hearts, CC BY-SA 3.0.

Cirrocumulus is a high, thin sheet broken into countless tiny white grains or ripples, with no grey shading anywhere. Made almost entirely of ice crystals, it is the least common and most short-lived of the high clouds, often appearing only briefly as cirrus becomes slightly unstable. Its rippled pattern is the classic 'mackerel sky'.

Worth knowing

  • The old sailor's saying 'Mackerel sky, mackerel sky, never long wet, never long dry' refers to these clouds.
  • Individual cloudlets are tiny - typically less than 1° across when viewed from the ground.
  • They're one of the rarest cloud types to observe.
  • At their altitude, temperatures can drop below -40°C.
Note

Still unsure it's the same cloud? Our app, Cloud Point— on iOS and Android — reads a photo and returns the likely type with altitude, useful when a formation is fading faster than you can look it up.

In motion

Test yourselfThree levels

Choose a difficulty

How do you recognise cirrocumulus?

  1. aMany tiny white grains or ripples high in the sky, with no grey shading
  2. bA single dark grey rain cloud
  3. cA bright ring around the sun
  4. dA low, lumpy layer with dark undersides
Show answer

AnswerMany tiny white grains or ripples high in the sky, with no grey shading

Cirrocumulus appears as a high sheet of very small white cloudlets or ripples — the 'mackerel sky' — with no grey shadowed undersides.

What is cirrocumulus mostly made of, and how does its rippled pattern arise?

  1. aWater droplets stirred by strong updraughts
  2. bIce crystals, broken into tiny cloudlets by shallow instability in a high, cold layer
  3. cDust lifted from the ground
  4. dFalling snow caught by the wind
Show answer

AnswerIce crystals, broken into tiny cloudlets by shallow instability in a high, cold layer

Cirrocumulus is almost all ice crystals. Shallow convective instability within a high, cold layer breaks the ice sheet into the many tiny, regularly spaced grains.

What single feature distinguishes cirrocumulus from the lower altocumulus?

  1. aCirrocumulus is grey while altocumulus is white
  2. bCirrocumulus cloudlets show no grey shading (and are tiny), while altocumulus cloudlets have shadowed undersides
  3. cOnly altocumulus contains any ice
  4. dCirrocumulus only ever appears at night
Show answer

AnswerCirrocumulus cloudlets show no grey shading (and are tiny), while altocumulus cloudlets have shadowed undersides

The official test is shading: cirrocumulus grains are tiny (under about a finger-width) and show no grey, whereas altocumulus elements are larger and have visibly shadowed undersides.

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