Square Waves in Ocean Settings Are Rare But Dangerous

Before we get into the physics of cross seas, let's review how ocean waves form. As the National Oceanic and Atmospheric Administration notes, "Waves are created by energy passing through water, causing it to move in a circular motion. However, water does not actually travel in waves."

The wind is mostly responsible for the creation of ocean waves, which shouldn't be confused with waves created by earthquakes, hurricanes, landslides and volcanic eruptions. As the wind blows, it transfers energy to the surface of the water creating a constant disturbance that results in a wave crest.

The moon and the sun can also create waves, called tidal waves. Note that a tidal wave is not the same as a tsunami.

Waves generally run parallel to the shore, but a cross sea is different. It occurs when opposing swells collide. When the wind from two weather systems — one old, the other more recent — meet, the waves of the newer weather system run at an oblique angle to the old weather system, whose waves continue on despite the shift in the wind.

The result looks like a quilt-like pattern of squares on the ocean surface, with the waves moving in a grid-like arrangement.

Cross seas generally occur along coastal areas, and they are rare. Yet, there is one place along the west coast of France where cross seas occur with astonishing regularity. The geology of the Isle of Rhé makes it perfectly situated for their formation.

The site is a tourist attraction that brings thousands to the island's lighthouse each year to view the phenomenon at a safe distance. It's a bit odd to think that waves intersecting at different angles would draw so many people, but really, when was the last time you saw nature create squares in water?

With a squared sea, the water can be difficult to navigate for boaters, as well as swimmers. Part of what makes square waves dangerous is that they generate powerful rip currents, and powerful waves, which can reach nearly 10 feet (3 meters) high, more than enough to swamp a large boat. A study found that "a large percentage of ship accidents occurred in crossing sea states."

You can usually spot two opposing swells in shallow waters, such as those off the Isle of Rhé, and off Tel Aviv, Israel. Scientists say cross seas are an example of the Kadomstev-Petviashvili equation at work. The formula describes nonlinear wave motion and is often used to explain how different weather systems interact with one another.

In short, if you see a square wave, you want to get out of the water immediately as they pose real danger. However, it might not be so easy to see the pattern the two swells make when you're swimming, so your best bet is not too go too far out.