Let's go surfing in the sky!

A large crashing ocean wave is a surfer's dream.  It's something they can ride their surf board on and potentially through, if they are really good!  But now imagine surfing in the sky?  Where would you get those amazing waves to ride, and what would they look like?  Well, wonder no more!



This picture was taken the morning of Friday, December 16th overlooking Birmingham, Alabama.  Large cresting waves were seen on the horizon and they really confused many of the locals.  The local television stations had many people calling in wondering what exactly was going on.

What they were actually seeing are "Kelvin-Helmholtz waves".  These waves form when a fast-moving layer of fluid slides over a slower and thicker layer of fluid, thereby dragging its surface.  For example, water waves form when the air (yes, air is considered a fluid) is moving faster than the water below.  When the difference between the wind and water speed is great enough, the waves "break" (their crests lurch forward) and they take on the classic shape of a Kelvin-Helmholtz wave as seen above!

So what happened here is that there was more than likely a slow moving cold layer of air near the surface (as evident by the thick cloud layer).  A warmer, faster moving layer of air aloft rode over top of it.  And since the cold air is denser and slower than the light, fast-moving warm air, the warmer air dragged and began scooping up the top of the cold layer, resulting in the cresting waves that appeared in the cloud!  Pretty cool, right!