A appears as a shaft of light standing vertically above the sun, most often at sunrise or sundown. It looks like a searchlight pointing up out of the horizon. Nothing in the sky is emitting a beam, and the explanation is the opposite of the one that produces a rainbow.
It is , not refraction. Sun pillars develop as ice crystals fall slowly through the air and reflect the sun’s rays off their surfaces. Reflection bounces all wavelengths away at equal angles without bending them, which is why a pillar carries no separated colors the way a halo or a sundog does.
Sun pillars appear as a shaft of light extending vertically above the sun, most often at sunrise or sundown. They develop as a result of ice crystals slowly , reflecting the sun’s rays off them.
That word reflecting is the whole difference. Reflection occurs when waves meet a surface that does not absorb their energy and bounces them away, with all wavelengths leaving at equal angles and nothing being bent.
Refraction, the other half of atmospheric optics, works the opposite way. It is a change in direction caused by a change in speed. Inside an ice crystal or water droplet it breaks light into its component colors, which is what gives halos and sundogs their rainbow edges.
So the same cirrus cloud can produce two different kinds of display. Look for sun pillars when the sun is low on the horizon and are present, and expect a colorless column rather than the colored spots a sundog puts 22 degrees to the side.
wat. No beam is being sent up; a million falling crystals are sending the sun back down.
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Two effects can sit in the same sky and come from opposite halves of the physics.
How do we know?
The National Weather Service office in La Crosse sets out the mechanism and the conditions, alongside the refracting effects it contrasts with.
What is still unknown
A public explainer, which gives the mechanism without the crystal geometry behind it.
Learn something?
Where to next
Why is every look at the sky a look into the past?light
Why is a rainbow in fog white?light
Why do the colored rings follow your own shadow and nobody else’s?light