A cross sea or square waves in the ocean seen from Phares des Baleines (Lighthouse of the Whales) on Île de Ré. (Photo: Wikimedia/Michel Griffon)
A cross sea or square waves in the ocean seen from Phares des Baleines (Lighthouse of the Whales) on Île de Ré. (Photo: Wikimedia/Michel Griffon)

Square waves in the ocean are a phenomenon which you may observe on the Isle de Ré in France and in some parts of New Zealand, square waves are beautiful, amazing, and potentially deadly. So, what causes them and just how dangerous are they? Let’s explore this phenomenon a little so that you can get a better idea why it’s not a good idea to play a little ‘sea- chess’ with your friends when you spot this occurring.

Square waves in the ocean are also called a ‘cross sea’… and for a good reason

A cross sea is exactly as it sounds. Violent riptides are ready to pluck even the strongest swimmers from surface-safety to the depths below and not only this, but ships have even been tossed about when encountering this phenomenon. This is due to the nature of what you are seeing. While it manifests as an orderly grid that may span the water for as far as you can see or simply decorate the shallow waters of the beach, what you are observing is not the product of something below, but rather two weather systems colliding.

Read More: Banish the travel sickness blues with motion sickness glasses

More of a danger to ships than to people

While the rip tides CAN pose a danger to swimmers, most of the information which you see in this regard is a bit exaggerated in order to get your attention. Most swimmers observing sqaure waves in the ocean are going to be in relatively shallow water and have a good chance of getting out of it once they’ve noticed it. As it also occurs close to beaches, in areas where it is a tourist attraction, swimmers will have been warned in advance and these ripples can even be quite safely observed in depths of water only inches deep. Plus they don’t tend to last very long, sometimes dissipating in minutes.

When ships observe a cross sea, however, it is a different story. In fact scientists around the globe are involved in studies looking at cross seas and the occurrence of rogue waves.

However brief, when two weather systems are colliding like this you can get waves as high as 10 feet and the design of ships, fine for a single weather system, are in very real danger of rolling when this occurs. If you happen to be in a small boat and not very far out then it is likely that you are going to be okay. Square waves in the ocean are indeed as dangerous as you’ve heard, but mostly in the deeps.

Just avoid shouting something along the lines of ‘Mother-in-law to E4!’ when you are seeing this phenomenon with your family and you should be just fine!