Cloud PointField Guide

Optical phenomenon — ice crystals

Sun dogs

Twin 'false suns' flanking the real one

Written by Laurence WayneEntry No. 17Common

Sun dogs: Twin 'false suns' flanking the real one
Plate XVIIPhotograph by John Hardin, CC BY 4.0.

Sun dogs, or parhelia, are bright spots that appear on either side of the sun at the same height above the horizon, often tinged with rainbow colour. They form when sunlight is refracted through flat, plate-shaped ice crystals that drift with their faces roughly horizontal, high in cirrus cloud. They sit about 22° from the sun — roughly a hand-span at arm's length — and are most vivid when the sun is low.

Worth knowing

  • Sun dogs always appear about 22° to each side of the sun, with their red edge facing inward — because they are the locally brightened parts of the same 22° halo, lit up where flat ice crystals refract the sunlight.
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

What does a sun dog look like?

  1. aA bright, often rainbow-tinged spot beside the sun, level with it
  2. bA ring directly overhead
  3. cA beam of light beneath the sun
  4. dA cloud that glows at night
Show answer

AnswerA bright, often rainbow-tinged spot beside the sun, level with it

Sun dogs are bright spots — usually a matching pair — that flank the sun at the same height above the horizon, often with a touch of rainbow colour.

What causes sun dogs?

  1. aSunlight reflecting off a lake
  2. bSunlight refracting through flat, plate-shaped ice crystals drifting horizontally
  3. cPollution scattering the light
  4. dWater droplets diffracting the light
Show answer

AnswerSunlight refracting through flat, plate-shaped ice crystals drifting horizontally

Plate-shaped ice crystals drift with their large faces roughly horizontal; sunlight refracts through their side faces by about 22°, concentrating into bright spots beside the sun.

As the sun climbs higher in the sky, what happens to a sun dog?

  1. aIt stays fixed at exactly 22° from the sun at every sun height
  2. bIt moves outward, beyond 22° from the sun, and grows a whitish tail
  3. cIt slides closer to the sun and brightens
  4. dIt expands into a complete halo ring
Show answer

AnswerIt moves outward, beyond 22° from the sun, and grows a whitish tail

With a higher sun the light no longer passes through the plate crystals at minimum deviation, so the parhelion shifts outside 22° and develops a pale tail pointing away from the sun.

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