My friend Deric Hughes tipped me off to a new urban legend spreading around: rainbow clouds appearing in the sky shortly before earthquakes. Lots of folks are buzzing over this on Twitter, for example.
I’ll cut to the chase: these clouds are not physically related to earthquakes in any way. But how I know this will take a wee bit of explanation.
First, what’s a rainbow cloud? As you can see in the picture, it’s a cloud with the colors of the rainbow splashed across it. Sometimes these are called fire clouds, if the shape of the cloud resembles a fire (like in the picture above).
Second, what causes this effect? It’s pretty simple, actually. Ice crystals in the cloud act as little prisms, breaking up the sunlight into its component colors and spreading them out. It’s essentially the same thing that causes "real" rainbows, except with ice and not water droplets. The angle between the Sun, the cloud, and you is important as well, but the essential ingredients needed for this effect are icy clouds and sunlight. That’s it.