"When a double rainbow occurs, the colors are reversed in the second rainbow (red nearest the horizon, rather than violet). Why?"

The reversing of colors in a rainbow comes down to how light interacts with raindrops and if there is enough room in a water droplet for light to bounce enough times. 

One weather word that you should know in this discussion is “refraction.” This is the process of light being bent in a prism – or a raindrop – and split into various colors.   

When refraction occurs one time, you get a typical single bright rainbow. This can occur in nearly any size of raindrop, although mist droplets or fog droplets will release a grey or white rainbow (known as a mistbow, fogbow or moonbow). 

Larger raindrops result in a dazzling display that occurs because different wavelengths of light travel at varying speeds when passing through water. Longer wavelengths appear red, while shorter ones show as blue or violet, creating the iconic arc of colors.

What about double rainbows, which occur when light bounces twice inside a raindrop? With additional bounces, this light comes out of the raindrop in the opposite array compared to the first bow and at a slightly higher angle in the sky.

In theory, under the right conditions, light can bounce around in a raindrop multiple times and create triple or even quadruple rainbows. 

Additional bounces require larger raindrops and enough room in the sky for multiple arcs.

Each time, some of the light will escape the raindrop, so the colors will be less vivid with each time refraction takes place. This is why triple or quadruple rainbows are much less common. 

Each new reversal of color will occur above the first rainbow. 

Beware of social media images that may generate AI depictions that show the arcs crossing or that all of the rainbows are of the same brilliance. They just simply aren’t realistic.