Square Waves Are a Thing — and If You See Them, Get Out of the Water Immediately


Stacey Leasca
Tue, March 1, 2022

Cross sea waves in La Rochelle, France

Adrian Hij/Getty Images

Heading to the coast for a seaside trip is what vacation dreams are made of. Soaking in the sun while feeling the warm sand between your toes as the shore laps against your feet makes for a magical scene. However, there are still a few things to be wary of when spending time at the beach — namely, some very specific water safety tips to keep in mind.

While you may know a thing or two about traditional rip currents and changing tides, you may not be aware of the dangers of square waves. Yes, this is a real thing — and a truly stunning phenomenon at that — but it's also one of the most dangerous sights to see in the water.

Known as a "cross sea," a square wave occurs when two swells meet to form a square, often resembling a checkerboard pattern. As the European Space Agency explained in 2010, "The conditions are quite common in the ocean and occur when a windsea and a swell, or two swell systems, coexist." It pointed to a 2004 study that showed "a large percentage of ship accidents occurred in crossing sea states."

HowStuffWorks further explained, these square waves are rather rare, but when they do occur, they generally can be found along coastal areas. A prime place to view them from a safe distance is along the western coast of France on the Île de Ré. (If you want to really get into it, HowStuffWorks also pointed to a scientific breakdown of the Kadomtsev-Petviashvili equation, which is why these waves form in the first place.)

But these cross seas can form swells up to 10 feet high, as well as create unique wind patterns, making it difficult for boaters to navigate and swimmers to make their way through. So, again, while rare, if you do stumble across this, avoid heading out via a boat or swimming in the potentially rough seas. Instead, choose to spend your time relaxing on the sand, or just splash in the shallows for a refreshing dip and wait for better conditions to take your ocean plunge in peace and safety.