Passage V

More than a Silver Lining

[1]

Iridescent clouds—in which patches, bands, or waves of color appears—are arresting phenomena. Aptly nicknamed “rainbow clouds,” flaunting displays that range from pastel suffusions to vivid, fiery streaks. But the conditions that iridescence requires are far more allusive than those that produce rainbows, making iridescent clouds comparatively rare.

[2]

[A] Think of every cloud as a vast collection of wet or icy particles. For iridescence to occur, many of these particles must be roughly uniform in size and shape. [B] They must also be minute: smaller than most cloud particles. Finally, a portion of the cloud (typically along the edges) must be nearly transparent. Together, these traits cause sunlight passing through the cloud to diffract consistently.

[3]

Its consistency that allows people to see individual colors. [C] Visible sunlight is actually multiple wavelengths of light, each with its own color. Sunlight usually appears as “white” light because all the wavelengths are traveling at the same angle, and the eye can’t discern them individually. But when sunlight collides with a cloud particle, the component wavelengths are deflected at different angles, an event known as diffraction. [D] When the diffraction is consistent (when all the red light waves deflect at one angle, the blues at another, and so on), the eye can detect individual colors.

[4]

Diffraction happens in all clouds, but most cloud particles are too large and irregular to diffract sunlight consistently. One particle might deflect red waves at the same angle an adjacent particle deflects green. Or, in really close-knit clouds, each light wave is likely to collide with a series of particles, deflecting at a different angle every time. In both cases, the different light waves cross paths, and the cloud appears white.

[5]

In iridescent clouds, for example, deflection angles don’t conflict. As sunlight passes through the cloud, patches, bands, or waves of color become visible.