What Causes Square Waves In The Ocean?

A cross sea is caused by two sets of waves converging at different angles.
A cross sea is caused by two sets of waves converging at different angles.

Square waves, also known as cross sea or cross waves, are square-shaped grids that form on the surface of the ocean. The waves appear gentle on the surface, but they are strong enough to steer swimmers, surfers, boats, and even large ships into unintended courses.

What Causes Square Waves?

Square waves are caused by two different sets of waves converging at different angles. The distinguishing feature is that the angles are occasionally greater than 45°. Waves travel for hundreds of miles over the sea surface, and even on a calm day, storms raging far in the ocean can create rolling waves known as swells. Without the presence of the local winds to disrupt the swells, they travel over vast distances. When waves traveling in different directions collide they create the square pattern of the cross sea. Above the surface, the waves appear gentle, but what lurks beneath are currents strong enough to wreck ships. Cross waves are more dangerous than riptides as the square pattern makes it difficult to navigate.

Where Do Cross Waves Occur?

Square waves can occur in any part of the ocean as long as the conditions are right. They are common at the Isle of Rhe in France where they attract a significant number of tourists. Visitors are advised to stay out of the ocean during the waves. Cross waves are not permanent, they lose energy as they travel, and the local winds can also interfere, causing the square pattern to fade away.

Are Cross Seas Dangerous?

Generally, cross waves are impressive to look at, but in the real sense, they are extremely dangerous even in small swells. They pull surfers and swimmers far into the ocean while rocking boats and ships from all directions. They are relatively rare and can appear and disappear within a matter of minutes.

Are Cross Seas and Rip Currents Similar?

Cross waves differ from rip currents. Rip currents are narrow channels of powerful fast-moving water prevalent along the coast. The current drag swimmers and small boats further into the ocean. Panicked swimmers counter rip currents by trying to swim back to the shore but risk drowning due to fatigue. Every year thousands of people are rescued by coastguards and lifeguards along the West and East Coasts of the United States, while approximately 100 people lose their lives.


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