November 02, 2020

A Rainbow Without Colors: The Rare Fog Bow is Seen in Seattle

 This morning I was able to view something that I always wanted to see.  

The white, colorless Fog Bow.

And I will show it to you now.  I knew there was going to be some interesting fog action this morning, so I biked down to Magnuson Park on my way to the UW.

The sun had just come up....and this is what I saw--an arch of white light, the fog bow.


And here is another shot.  Pretty much all white...and much wider than a normal rainbow.


The fog bow is in the same location as a normal rainbow (opposite of the sun), but why no colors like a normal rainbow?

First, fog droplets--which are really cloud droplets-- are MUCH smaller than the raindrops that produce normal rainbows.  Raindrops are typically 2000 microns (millionths of a meter) in size, cloud droplets are about 20 microns in size.  To put it another way, raindrops are about 100 times as large as cloud/fog droplets.

So why does size matter?

For a normal rainbow, the sun's light enters the raindrop and reflects off the back.  And the light is broken up into various wavelengths of light (different colors) as the light goes between the air and the droplet.  The light is bent or refracted at the interface between air and water, with amount of bending dependent on the wavelength of light.  This process is called dispersion.




But for a cloud droplet, something else happens at well.  The droplet is so small that it is similar to the wavelength of visible light (roughly .5 microns) and the wave-like nature of light is revealed through a process called diffraction.  Such diffraction cause the light to be scattered in a range of directions, mixing the colors together.  Mixing all colors together gives you white.

In any case, I was excited to see the ethereal white fog bow.  Plus, the scene at the park this morning, with fog interspersed with sunny skies, was magical.

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10 comments:

  1. This comment has been removed by a blog administrator.

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  2. Really excellent & surprising post, Cliff about fog bows - which I'd never heard of. Kudos.

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  3. Beautiful phenomenon. Thanks for sharing and the explanation, Cliff.

    Is this what a typical park looks like in Seattle? I see more asphalt than anything else. If so I'm glad my small-town parks generally have trees, grass, and swing sets.

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    1. Magnuson Park is the old San Point Naval air station, so when it became a park it was treeless. Cliff simply rode to the parking lot near the water and boat launch, so yeah not surprising to see asphalt. The park itself is 350 acres, most of it is vegetation of some type.

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  4. Thanks for the awesome explanation Cliff. I’ve seen some pictures recently and no explanation was ever given. I love the physics and optical reasons for this.

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  5. Thanks for the explanation, Cliff. I was fortunate to see a fogbow years ago in Bellfountain, Oregon. But, have you ever seen a moonbow? I have also seen a rainbow cast by the light of a very bright moon, also in Bellfountain. Twas a very strange sight.

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  6. I see these fog-bows on a daily basis on the coast and while boating on the ocean in Ucluelet in summer - often shallow, dense fog with sun coming through the top.

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  7. Cliff, thank you for the great explanation of Pacific cyclones in your podcast. You cited the Columbus Day storm of 1962 as the most powerful you were aware of. You also mentioned the destructive nature of our tall trees cleaving houses when they topple over, as opposed to deciduous trees 'simply' dropping limbs. In 1982 my parents moved to Eugene and their backyard was the home of a "Columbus Day Topper" which locals understood to mean a mature Douglas Fir that had snapped in the middle rather than toppling as a result of the wind storm. Huge, healthy, half-height trees. They were a unique feature of the plantscape in that area until recent years, with most of them now having succumbed to old age (and land developers). As for Seattle, check out www.thelast6000.org - it appears that there may be a couple "Columbus Day Topper" candidates among these trees.

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  8. Beautiful, Cliff. Your last photo, toward the sun, shows a corona, also a diffraction phenomenon (unrelated to any viruses).

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