Never stop complaining that CGI looks fake, people. It gets results. While Disney was funding research into making CGI rainbows look more realistic, the researchers found a mechanism to create rare twinned rainbows!

Twinned rainbows are not double rainbows. Although both include two rainbows for the price of one, double rainbows stay separate from each other. Twinned rainbows start from the same base before lifting one arch above another. They're much more rare. When researchers, doing work sponsored by Disney, wanted to model one, they found that no one entirely understood the twinned rainbow phenomenon. They set to work with computer models, trying to figure exactly what was happening.

This work started by counteracting what is commonly assumed about raindrops. I learned in school that raindrops only get their characteristic teardrop shape when they're sliding down window panes. In the air, they're spherical drops. Because of their shape, when light passes through them it is scattered in a certain way. Lorenz-Mie theory is a series of equations that explain exactly how the light scatters. It explains how light is bent as it goes through a sphere. It explains how light is polarized - its waves all moving in the same direction - as it goes through a sphere. It explains rainbows. Sadly, it doesn't explain twinned rainbows.

Because drops, especially large ones, aren't spherical at all as they fall. Instead, they form shapes that the researchers called burgeroids. The name explains the shape, but not why the shape occurs. As drops get bigger, they face air resistance as they fall. This flattens them out on the bottom and creates a wide, flat curve on the top. The curves on the sides are relatively sharp and dramatic. These effects increase with size. At about a half millimeter, a drop will look like a sphere. By three millimeters it looks pretty much like it could flop down on your plate and you'd eat it. The small drops form rainbows that can be explained by Lorenz-Mie Scattering. The larger burgeroid drops can't.