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Seawater Bow

No rain made this unusual and spectacular bow. It was formed entirely by salt water spray cast up from a very stormy sea by high winds. Seawater bows are subtly different from those formed by rain.

Imaged by Sandy Robertson (parked in a safe spot!) on Alderney, Channel Islands. The high breakwater in the distance is where he captured his equally spectacular "Waterfall Sea" mirages.

Image ©Sandy Robertson, shown with permission
Sea spray and raindrop bows compared. This photograph taken in the Pacific Ocean by J Dijkema shows both.

The lower sea spray bow is some 0.8 degrees smaller in radius than the upper rain bow.





A 21 degree radius bow (half that of a rainbow) made by glass spheres.

Artist Charles Monkhouse glued glass beads onto boards and illuminated them with a distant powerful spotlight.



Another day, another sea spray bow captured by Sandy Robertson over the Alderney breakwater.

 
To form a bow we need something transparent and near spherical - plus some sunlight.

The sun's rays refract twice as they enter and leave the sphere. The refractions are wavelength dependent and thus make the bow's colours.

The refractions also depend on the sphere (or drop) material. A water drop gives a bow of radius 42 degrees. A glass sphere refracts more strongly and the bow shrinks to to half that size. A diamond sphere refracts so strongly that it cannot form a bow.

The salt in seawater increases its refraction slightly and sea spray bows are slightly smaller by about 0.8 degrees radius than rainbows. The famous picture at right shows the difference. The lower sea spray bow is noticeably smaller than the rainbow above it.