Cloud PointField Guide

Optical phenomenon — refraction

Fogbow

A ghostly white rainbow in fog

Written by Laurence WayneEntry No. 24Uncommon

Fogbow: A ghostly white rainbow in fog
Plate XXIVPhotograph by Oakley413, CC BY-SA 4.0.

A fogbow is a pale, broad arc — like a rainbow drained of its colour — that appears in fog or thin cloud when the sun is low and behind you. It forms by the same refraction and reflection as a rainbow, but the fog droplets are so tiny that the colours smear together into near-white. Pilots see the same effect as a 'cloudbow' encircling their aircraft's shadow.

Worth knowing

  • A fogbow looks white not because it lacks colour, but because its water droplets are so tiny that diffraction smears the colour bands together — make the droplets bigger and the same arc sharpens back into an ordinary rainbow.
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

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What does a fogbow look like?

  1. aA broad, faint, almost white arc in fog
  2. bA bright, vividly coloured arc in heavy rain
  3. cA ring hugging the sun
  4. dA cloud that glows at night
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AnswerA broad, faint, almost white arc in fog

A fogbow is a wide, pale 'ghost rainbow' seen in fog, much fainter and more washed-out than a normal rainbow.

How does a fogbow form?

  1. aLight reflecting off a wet road
  2. bLight refracting and reflecting inside tiny fog droplets, like a rainbow but in much smaller drops
  3. cIce crystals splitting the light
  4. dSmoke scattering the sunlight
Show answer

AnswerLight refracting and reflecting inside tiny fog droplets, like a rainbow but in much smaller drops

Just like a rainbow, light refracts into each fog droplet, reflects off the back, and exits toward you — but it happens in the very small droplets of fog rather than large raindrops.

Why is a fogbow almost white instead of brightly coloured like a rainbow?

  1. aFog contains no actual water
  2. bThe fog droplets are so tiny that diffraction smears the colour bands together into white
  3. cFogbows only ever form at night
  4. dThe sun is too weak to show colour in fog
Show answer

AnswerThe fog droplets are so tiny that diffraction smears the colour bands together into white

The droplets in fog are so small that diffraction spreads and overlaps the colour bands, blending them into white. Larger raindrops separate the colours cleanly, which is why a true rainbow is vivid.

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