News Brief: Why rainbows can lose some hues

A new analysis finds one contributor may be how near the sun is to the horizon

red rainbow

This rainbow is missing a number of its normal colored bands.

Graham Hickey/Flickr (CC BY-SA 2.0)

SAN FRANCISCO, Calif. — Some rainbows are missing a few colors. When the sun is low in the sky, those colorful arcs may contain only a fraction of the traditional red-to-violet hues. Or that’s what researchers reported December 17. They shared their new findings here at the American Geophysical Union’s fall meeting.

Rainbows emerge as sunlight bends while passing through water droplets in the air. The water acts like a prism. It bends different parts of the light spectrum — or colors — by different amounts. This breaks the rainbow’s light into bands of red, orange, yellow, green, blue, indigo and violet. What results is a vibrant and colorful arc.

But sometimes a rainbow will be short a few hues.

Previous work had suggested that these rainbows — some are even single-colored — are due mostly to water droplets being an unusual size. But, Jean Ricard now reports, the sun’s height above the horizon can play a bigger role than previously thought. Ricard is an atmospheric scientist at the National Centre for Meteorological Research in Toulouse, France.

He and his colleagues reviewed hundreds of photos of rainbows. At sunset, light has to travel a longer distance through the atmosphere than at other times of the day. So water droplets in the air can scatter away the cooler colors during this long, sunset journey, he notes. The redder light remains. This thickens the rainbow’s red band. It can even squeeze out the violet, blue and orange bands. When that happens, the rainbow may end up totally red.

Power Words   

(for more about Power Words, click here)

atmosphere  (adj. atmospheric) The envelope of gases surrounding Earth or another planet.

geophysics   (adj. geophysical) The study of matter and energy on Earth and how they interact.

meteorology  (adj. meteorological) The study of weather as it pertains to future projects or an understanding of long-term trends (climate). People who work in this field are called meteorologists.

prism     A triangular wedge of glass or another clear substance that can bend the components of white light into a rainbow-like succession of colored bands.

rainbow     An arc of color displayed across the sky during or just after a rain. It’s caused when water droplets in the atmosphere bend (or diffract) white sunlight into a number of its component hues: usually red, orange, yellow, green, blue, indigo and violet.

spectrum  (plural: spectra) A range of related things that appear in some order. (in light and energy) The range of electromagnetic radiation types; they span from gamma rays to X rays, ultraviolet light, visible light, infrared energy, microwaves and radio waves.

More Stories from Science News Explores on Physics