Hikers at Deception Pass State Park, northwest of Seattle, Washington, were greeted by a mesmerizing sight last weekend. Kim Knight Wilkinson was there to photograph this incredible phenomena, known as a fogbow.

Fogbows, sometimes referred to as cloudbows, white rainbows, or mistbows,¹ are a similar atmospheric phenomena to rainbows,² ³ ⁴ however, as implied by its name, they form in patches of fog rather than in falling raindrops. Like rainbows, they are in the shape of a circular arc¹ ⁴ and are massive in size; both broad and towering.² ⁴ However, unlike rainbows, which are more colorful and luminescent, fogbows are usually faint and white in color,¹ ⁴ ⁵ though they may have hints of blue on the inside and/or red on the outside edges.² ⁴ While both rainbows and fogbows are formed by the interaction between sunlight and water droplets in the air, the size of the water droplets are considerably smaller in fogbows than they are in rainbows.³ ⁴ ⁵

In the case of a rainbow, raindrops are large enough to act as a prism that reflects a small fraction of the sunlight within the droplet, then refracting (bending) that white light along a geometric pathway, producing the splendid colors we normally see opposite of the sun along a circular arc.² ⁵

Fogbows, however, consist of tiny water droplets that are only between 10 and 40 μm (0.0006 and 0.002 inches) in diameter,⁶ according to Sadeghi et al. (2012). As such, these droplets are too small to reflect and refract sunlight like an ordinary rainbow.⁵ Instead, the droplets diffract the light extensively,³ ⁴ ⁵ as the diffraction pattern, in terms of the angle θ for small water droplets is of great magnitude, the colors produced by the scattering overlap each other producing a haze of bright white or faint gray (Sadeghi et al., 2012) known as a fogbow.² ³ ⁵ ⁶

Your best chances of seeing a fogbow are during the morning or evening hours when the sun is less than 30° or 40° above the horizon to your back, and the light is shining through a thin layer of clouds or fog in front of you.² ⁴ The sun can be at a higher angle, if and only if you are standing on a large hill, mountain, or are in an aircraft and look down at the sight below from above.² ⁴

REFERENCES

[1] “Cloudbow.” AMS Glossary. Accessed March 24, 2020. http://glossary.ametsoc.org/wiki/Cloudbow.

[2] Cowley, Les. “Fogbow.” Atmospheric Optics. Accessed March 24, 2020. https://www.atoptics.co.uk/droplets/fogbow.htm.

[3] Breslin, Sean. “Spectacular Fogbow Captured in Southern Wisconsin.” The Weather Channel, February 3, 2016. Accessed March 24, 2020. https://weather.com/photos/places/news/southern-wisconsin-fogbow-images.

[4] Kanuckel, Amber. “What Are Fogbows?” Farmers’ Almanac, December 18, 2019. Accessed March 24, 2020. https://www.farmersalmanac.com/what-are-fogbows-22862.

[5] Erdman, Jon. “Fogbow, a Colorless Rainbow, Appears in Midwest.” The Weather Channel, August 22, 2016. Accessed March 24, 2020. https://weather.com/science/weather-explainers/news/fogbows-explained.

[6] Sadeghi, Iman, Adolfo Munoz, Philip Laven, Wojciech Jarosz, Francisco Seron, Diego Gutierrez, and Henrik Wann Jensen. “Physically-Based Simulation of Rainbows.” ACM Transactions on Graphics 31, no. 1 (January 2012): 1–12. https://doi.org/10.1145/2077341.2077344. (PDF)

Categories: Weather