Cloud PointField Guide

Special — contrail cirrus

Homomutatus

Aircraft contrails spread into man-made cirrus

Written by Laurence WayneEntry No. 46Common

Homomutatus: Aircraft contrails spread into man-made cirrus
Plate XLVIPhotograph by Willow2012, CC0.

Homomutatus is the WMO term for a persistent aircraft contrail that has, over time, been spread and reshaped by strong upper-level winds until it takes on the appearance of natural cirrus. The trail grows, spreads sideways and transforms internally, eventually mimicking ordinary high cloud. The name combines the appropriate genus with the suffix homomutatus, marking a man-made cloud that has changed form since it appeared.

Worth knowing

  • Homomutatus is genuinely new to cloud science: it was added to the WMO International Cloud Atlas only in 2017, alongside homogenitus — the first time human-made clouds were given formal classification names.
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

What does a homomutatus cloud start out as?

  1. aA thunderstorm anvil
  2. bAn aircraft contrail
  3. cSea fog
  4. dVolcanic ash
Show answer

AnswerAn aircraft contrail

It begins as a persistent contrail (cirrus homogenitus) that later transforms into natural-looking cirrus.

What turns a thin contrail into a broad homomutatus sheet?

  1. aRain falling through it
  2. bStrong upper-level winds spreading and reshaping it over time
  3. cThe aircraft circling back over it
  4. dSunlight burning its edges
Show answer

AnswerStrong upper-level winds spreading and reshaping it over time

Strong winds aloft spread the persistent trail and reshape it until it resembles natural cirrus.

Why do some contrails vanish within seconds while others persist and spread into homomutatus cirrus?

  1. aIt depends on the colour or size of the aircraft
  2. bPersistent ones form in ice-supersaturated air, so their ice crystals keep growing instead of evaporating
  3. cPersistent ones are made of smoke rather than ice
  4. dThe engine type alone decides whether a trail lasts
Show answer

AnswerPersistent ones form in ice-supersaturated air, so their ice crystals keep growing instead of evaporating

When the surrounding upper air is supersaturated with respect to ice, contrail crystals grow and spread rather than sublimating away — the condition for a trail to persist into homomutatus.

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