One evening in 2012, in the dead of summer, I was heading south back to Austin from Dallas. Along the I-35 corridor, intermittent storms popped up and down just to the east over flat ranch land. It was around 7:30, and the sun was getting lower on the horizon.
As a passenger, I could let my eyes take in that sky, and just north of Waco I saw something that made me think three little letters: U, F, and O.
At first, I thought my eyes were playing tricks on me. Maybe the car window was refracting light in some weird way, so I rolled down the window. But it didn’t go away, and I’d never seen anything like it before. It was as if a rainbow got itself trapped inside a wisp of a cloud. Take a look:
Two years later, I’ve finally learned what this phenomenon was: an iridescent cloud. And while it’s pretty simple to explain, it’s pretty rare to see.
As with any rainbow in the sky, the conditions have to be right. In both cases, you’re most likely to see them after a storm when the sun is low on the horizon. In both cases, you’re looking at the the interaction of light through particles of H2O.*
But a few more things need to happen for an iridescent cloud to appear.
As the cumulus storm clouds push air upwards, that air both expands and cools. That’s all pretty normal; air is thinner and cooler at every ascending altitude. The unusual part happens when cooled water droplets or even ice crystals pull together above that cumulus cloud to form a cloud cap, also known as a “pileus.”
Those droplets and crystals are quite small, creating a pileus that’s thin and almost translucent (again, the water particles have expanded). Add some dramatic backlighting from a low-hanging sun that’s obscured by the storm clouds below, and voila: you’ve got a rare glimpse of mother of pearl in the sky.
Isn’t that remarkable?!
Summer’s almost here, and while I always hope for the rainbow in sky after an early evening storm, this time I’ll train my eyes to those storm clouds, hoping for another remarkable site.
If you want to see more, look at this Googled set of images of iridescent clouds.
*BTW… rainbows refract light while iridescent clouds diffract light. Look for a future BSP post that experiments with this difference!