This may come as a shock, but everything you thought you knew about rainbows is wrong. Not only do they not all contain the seven colors that make up the famous mnemonic Richard of York Gave Battle in Vain (or other, ruder variations), but their strength is also not dependent entirely on the size of the water droplets in the air.
According to research presented last week at the American Geophysical Union Fall Meeting, 12 different types of rainbow exist, each of which can be recognized according to the number and combination of colors it contains, among other things.
Lead researcher Jean Ricard, of the National Meteorological Research Center in France, introduced a new classification model for rainbows at the convention, while at the same time explaining that many of the differences between rainbows are caused by the position of the Sun in the sky. This goes against all previous explanations for rainbow variation, which have attributed these differences to the size of water particles in the air.
To refute this existing theory, Ricard and his team presented a series of images of wildly differing types of rainbow, some of which appeared to be no more than a single streak of red light. They explain that this effect is caused when the Sun is very low in the sky – typically at sunrise or sunset. Under such conditions, light from the Sun has to pass through more of the Earth’s atmosphere than when the Sun is higher in the sky. As a consequence, only the longer, red wavelengths are able to pass through.
The higher the Sun rises, however, the more visible are the colors towards the violet end of the spectrum, with greens and blues becoming increasingly prevalent when the Sun’s angle is above 70 degrees.
When the Sun is at a low angle, only red wavelengths of light can pass through the Earth's atmosphere, producing rainbows with a single red band of color. Rainbow Sunset by Steve Jurvetson via Flickr. CC BY 2.0
As such, the team’s system places rainbows into categories based on how many colors can be seen. They also class them according to whether or not additional bows – called supernumerary bows – exist alongside the main arc. When these are present, they tend to be interspersed by dark regions called Alexander bands. Accordingly, the classification scale also accounts for the strength of these Alexander bands.
Yet while the angle of the Sun may have a major impact on the type of rainbow produced, the size and position of water droplets in the air is also an important factor in determining the constitution of a rainbow. For instance, Ricard points out that rainbows are not static, but are in fact constantly in flux as water droplets fall, producing continuous changes in the refraction of sunlight that produces the rainbow.