Cloud PointField Guide

High cloud — genus

Cirrostratus

The thin ice veil that haloes the sun

Written by Laurence WayneEntry No. 48Common

Cirrostratus: The thin ice veil that haloes the sun
Plate XLVIIIPhotograph by No machine-readable author provided. LivingShadow assumed (based on copyright claims)., CC BY-SA 2.5.

Cirrostratus is a transparent, whitish veil of high cloud, fibrous or smooth in texture, that partly or wholly covers the sky and generally produces halos. It forms from slowly, gently rising moist air in the high troposphere, and can also grow from spreading contrails. It frequently appears ahead of a warm front, so a sky that whitens and thickens with cirrostratus is a classic early sign of an approaching weather system.

Worth knowing

  • The 22° halo is the span of an outstretched hand at arm's length from the Sun.
  • Halos occur because cirrostratus ice crystals are hexagonal, bending light at specific angles.
  • Ancient sailors used cirrostratus halos to predict storms - 'Ring around the Moon means rain soon!'
  • These clouds can cover the entire sky yet be so thin that stars remain visible at night.
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.

Test yourselfThree levels

Choose a difficulty

Cirrostratus is famous for producing what around the sun or moon?

  1. aA rainbow
  2. bA halo ring
  3. cA green flash
  4. dA dark shadow
Show answer

AnswerA halo ring

Its ice crystals refract light into a halo, most often the 22° ring.

What is cirrostratus made of, and why?

  1. aWater droplets, because it is warm up there
  2. bIce crystals, because it sits in the freezing high troposphere
  3. cDust, because it forms over deserts
  4. dSmoke particles from fires
Show answer

AnswerIce crystals, because it sits in the freezing high troposphere

At its altitude temperatures are well below freezing, so cirrostratus is composed entirely of ice crystals.

How do you tell cirrostratus from ordinary cirrus?

  1. aCirrus is grey while cirrostratus is nearly black
  2. bCirrostratus is a continuous, halo-producing veil; cirrus is detached white filaments or patches
  3. cCirrus always sits lower than cirrostratus
  4. dOnly cirrostratus contains any ice at all
Show answer

AnswerCirrostratus is a continuous, halo-producing veil; cirrus is detached white filaments or patches

Cirrostratus spreads as an unbroken, transparent veil that typically throws a halo, whereas cirrus appears as separate wispy filaments, tufts or banks.

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