This remarkable image was captured just off the western point of France's Isle of Rhé. It's a beautiful demonstration of a natural phenomenon known as a cross sea — one that can be explained by a little bit of physics and math.

A cross sea is a marine state with two wave systems traveling at oblique angels. It happens when water waves from one weather system continues despite a shift in the wind. Waves that are produced by the newer system run at an angle to the old. This creates a shifting, dangerous pattern. Indeed, until the older waves dissipate, cross seas are a hazard for boaters and swimmers.

It's an example of the Kadomtsev–Petviashvili (or KP equation) equation at work. It looks a little something like this:


Named after Boris Borisovich Kadomtsev and Vladimir Iosifovich Petviashvili, it's a partial differential equation that describes nonlinear wave motion. The equation can be applied to physics as a way to model water waves of long wavelengths that have particular qualities — like those produced by interacting weather systems.

[h/t @SciencePorn]


Image: Michel Griffon/Wikimedia Commons.