Rainbows are caused when sunlight is refracted and broken into its constituent colors by raindrops, creating these dramatic semicircular bands of light. But what conditions are needed to create double, triple, and even the elusive quadruple rainbows?
The actual physics of multiple rainbows is pretty straightforward — they're just the result of the water particles reflecting light at two or more different angles. Indeed, most primary rainbows are accompanied by a second, dimmer rainbow, though usually these are so pale that they go unnoticed, and triple and quadruple rainbows are even more difficult to see.
For conditions to align just right and produce a really noticeable double rainbow — the stuff of viral video legend, basically — you need a water particle that is specially suited to multiple reflections. That's the finding of a team led by researchers at UC San Diego, who used computer simulations to figure out the precise conditions needed to recreate various different rainbows.
The type of raindrops needed for noticeable double rainbows are known, rather amusingly, as "burgeroids." These are big raindrops that have been flattened out by the surrounding air, creating a larger surface on which light can be reflected and split apart to form the pair of rainbows. You can see a simulated example of what burgeroids can do in the image up top - maybe not the brightest double rainbow ever, but far more visible than the usual pair.
Read their original paper here. Image by Jacobs School of Engineering/UCSD.