At sunrise and sunset, only ROY shows up for rainbow duty—G and BIV don’t appear for the earliest and latest color shows in the sky. Rainbows, it seems, are far more complex than the simple mnemonic for the colors of the rainbow that we all learned as children. 
 
 Inspired by the rainbow of rainbows, atmospheric scientist Jean Ricard has created a new rainbow classification scheme, which he presents Friday at a meeting of the American Geophysical Union. 
 
 While creating such a system may seem like a frivolous endeavor, it could help people to look beyond what they expect to find and “challenge our understanding of rainbows,” said Ricard, of France's National Centre for Meteorological Research, at a press conference on Thursday. 
 
 All the Rainbows 
 
 Draw a rainbow, and you’ll probably include lines of red, orange, yellow, green, blue, and violet. Maybe you’ll throw in indigo, though scientists don’t consider it to be a rainbow flavor. 
 
 But rainbows can easily get more complicated: A second bow—creating what is usually called a “double rainbow”—can appear more faintly above the first, with the colors in reverse order. There can be a dark band between the arcs and sometimes even bright fringes, called supernumerary bows, on the top and bottom of the bands. 
 
 Then again, some rainbows have none of those features, and entire colors can be absent. 
 
 Since the 1950s, rainbow classification has been based on the size of the raindrops that create them. The bigger the drops, the more vivid the colors. 
 
 Another attempt organized them by the height of the sun above the horizon. At about 70 degrees, a rainbow is dominated by blues and greens. Closer to the horizon, there are mostly reds and yellows. 
 
 “At sunset or sunrise, the color of the sun and the intensity of the incoming light change dramatically,” Ricard said. When the sun is low in the horizon, rays of light must pass through more of the Earth’s atmosphere. “The red manages to go through,” he explained. “Other wavelengths are completely gone.” 
 
 Catch the Rainbows 
 
 To capture this rainbow diversity, Ricard and his colleagues gathered hundreds of pictures of rainbows, sorting them into 12 categories based on the visibility of the six colors, the strength of the dark band, and whether any supernumerary bands can be seen. One type lacks a band of green, for instance, another is missing blue and violet, and a third type has only red and blue. 
 
 The system is so simple that most anyone could look at a picture of a rainbow, put it in a class, and understand what's going on, he said. A misty red rainbow, for instance, could only be created near sunrise or sunset with tiny raindrops. 
 
 “The public continues to be fascinated by rainbow sightings, and I’m often asked to interpret folks’ photographs of rainbows,” said Raymond Lee, a meteorologist at the U.S. Naval Academy in Annapolis, Maryland. “Thus, any agreed-upon scientific principles that help explain how colors vary from one rainbow to the next is a real plus.” 
 
 
 
 Studying rainbows could even help in the search for extraterrestrial life. If rainbows or glories—rainbows often spotted around the shadows of airplanes—were sighted on exoplanets, Ricard noted, it could be a sign that they are host to water—and even life. ||||| SAN FRANCISCO — Why are there so many songs about rainbows? 
 
 Perhaps because there are so many different types, each with its own distinctive features, new research suggests. 
 
 There are 12 types of rainbows, distinguished by various characteristics, the study suggests. Fat droplets of water or tiny sprays of mist will affect them, along with the angle of the sun. Rainbows can even appear as twins, triplets or quadruplets, Jean Ricard, a researcher at the National Meteorological Research Center, in France, said here yesterday (Dec. 17) at the annual meeting of the American Geophysical Union. 
 
 And even a single rainbow is always changing, he said. 
 
 "They don't look alike because when we look at a rainbow, one second later, the drops which form the primary bow and the secondary bow are not the same, because they are falling," Ricard said here in a news briefing. "If you look carefully after a few minutes you will start to see some changes in each rainbow." [Infographic: Earth's Atmosphere – Top to Bottom] 
 
 Classification scheme 
 
 Scientists have understood the basics of rainbow formation since at least Descartes' time: Sunlight interacts with water droplets in the sky, and the light is both reflected and refracted as it enters and leaves the raindrop. Because different wavelengths of light — which correspond to different colors — slow down by different speeds when they hit a raindrop, the different colors get bent at different angles, separating into the rainbow's distinctive hues. (The bizarre phenomenon known as a fire rainbow is neither a fire nor a rainbow, because it occurs when light refracts through ice crystals, not raindrops.) 
 
 In the past scientists tried to classify rainbows based on the colors in the rainbows, or the size of the droplets they refracted through. But those classifications often missed certain types of rainbows. 
 
 A double rainbow appeared in the summer sky above Toulouse following an afternoon storm. Credit: CNRM 
 
 To capture all the myriad "flavors" of rainbows, Ricard and his colleagues tried to figure out the minimum set of characteristics that would describe all rainbows. It turns out that rainbows can have up to four characteristics . There is the primary bow, with red on top and blue-violent on the bottom. Above that, secondary reflections inside a rain droplet always form a secondary, fainter bow above the primary bow, with the colors reversed. Between the two is a dark region, called Alexander's band, where little light from the raindrops light reflects. And sometimes, there are additional bows, called supernumerary bows, which may occur when the light rays spread and cancel each other out via diffraction and interference in the atmosphere. 
 
 Types of rainbows 
 
 Based on those characteristics, they determined there are 12 different types of rainbows, with imaginative names like RB_1, RB2, etc. The rainbows vary by whether all colors are visible, whether they have a strong Alexander's band, and whether there are supernumerary bows. Some of the more striking rainbows include only red arcs, and then there are yellow-and-orange rainbows. 
 
 When Ricard and his colleagues analyzed the physics, they found that the height of the sun in the sky was the biggest single factor affecting the rainbow's appearance. For instance, when the sun is very low in the sky, such as at sunset, light is much less intense and must travel much farther to reach the eye. Only the red wavelengths are able to make it through the atmosphere at this time, he said 
 
 "At sunset or sunrise, the color of the sun and the intensity of the incoming light changes dramatically," Ricard said. 
 
 The size of the droplet also affected a rainbow's appearance, though to a lesser degree. Wider drops make for less vivid rainbows with more widely spaced hues, he said. 
 
 While rainbow research may seem more suited to daydreamers and poets, it may have practical applications, Ricard said. For instance, if scientists can spy rainbows on exoplanets, that may be a sign of atmospheric water. And where there's water, there's often life. 
 
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– How long have you spent thinking about rainbows—like really thinking about them? National Geographic reports atmospheric scientists Jean Ricard has spent enough time thinking about rainbows to develop a new rainbow classification system, which he presented Friday at an American Geophysical Union meeting. According to the presentation, Ricard created the system—which puts rainbows into 12 categories—because the classic ROY G. BIV rainbow described by textbooks hardly ever exists in nature. Rainbows are actually more complex than that. Ricard and his team found that rainbows can have up to four characteristics: the main band, a fainter double rainbow, a dark space between the two, and any additional bands, Live Science reports. National Geographic reports things get complicated because of all the variables—hence the 12 categories. For example, some rainbows exclude entire colors. Not to mention rainbows are constantly changing. "They don't look alike because when we look at a rainbow, one second later, the drops which form the primary bow and the secondary bow are not the same, because they are falling," Live Science quotes Ricard as saying. For decades scientists had been classifying rainbows based on raindrop size, according to the presentation. But after studying hundreds of pictures of rainbows, Ricard found the height of the sun actually has a bigger effect. National Geographic reports classifying rainbows might seem like a "frivolous endeavor," but Ricard says it can "challenge our understanding of rainbows." (Did the Grateful Dead conjure a rainbow?)