Shaking raindrops wash out rainbows.

Shaking raindrops wash out rainbows

Not every raindrop contributes to a rainbow's appearance. for instance, large drops, which get flattened by air pressure into a shape resembling a hamburger bun, are too distorted to add significantly to the colors seen in a rainbow's upper portion. The recent discovery that small, nearly spherical drops can oscillate has now led scientists to add a further restriction on the sizes of raindrops that help create a rainbow.

"Our discovery of the natural oscillations [of raindrops having diameters between 1 and 1.5 millimeters] would seem to exclude most raindrops from contributing to the rainbow," says Kenneth V. Beard of the University of Illinois at Urbana-Champaign. That means raindrops less than 1 mm in diameter -- practically drizzle -- must be the major contributors. "They don't suffer from the distorting effects of oscillations, and they are nearly spherical," Beard told SCIENCE NEWS.

The brilliant, multicolored arc of a rainbow represents the combined effect of reflections within innumerable raindrops. Different colors of light emerge from each drop at different angles, spraying reflected light over a large part of the sky opposite the sun. As raindrops fall, they flash different colors toward a stationary observer, who sees the scattered light as a band of colors spread across the sky.

Raindrop oscillations complicate this picture. As a drop's shape changes, the angles at which it reflects particular colors change, and because the oscillations of all the drops aren't coordinated, the observer sees a mix of colors coming from any given position in the sky. Any rainbow colors produced by oscillating drops are in effect washed out.

"When you add all this up, you get some scattered light -- a whiteness -- but no colors," Beard says. "Droplets that create rainbows can't be oscillating and must be quite small."

Calculations based on theory suggest that droplets between 0.5 and 1 mm in diameter create the most brilliant colors. It's not surprising, then, that Hawaii -- with its abundance of light showers -- is one of the best places to see rainbows.