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My teenage kids went to the beach with a few friends that afternoon. It was supposed to be an ordinary day: sunshine, warm weather, plenty of people around, and the kind of waves that make teenagers want to stay in the water for hours.
When they came home later, however, they didn’t talk about the weather or how much fun they had.
They told me about a stranger who had suddenly started yelling at them.
“Get out of the water!”
At first, they thought he was simply being rude.
Then he shouted something that caught their attention.
“A square wave is forming! Get out!”
They didn’t know what he meant.
They said the man sounded so serious that they immediately stopped what they were doing and got out of the water. They didn’t even think to ask him what a square wave was until they were safely back on the beach.
When they told me the story, I had the same reaction many people would have.
What exactly is a “square wave”?
It sounds almost harmless. You might imagine a strange wave that somehow looks like a square.
But in the ocean, the phenomenon people commonly call a “square wave” can be associated with a potentially dangerous sea condition known as a cross sea.
And understanding what is happening beneath the surface makes the warning much easier to understand.
A cross sea occurs when waves traveling in different directions meet and cross one another. Instead of all the waves moving more or less in the same direction toward the shore, two wave systems can intersect at an angle.
From above, the surface can sometimes develop a distinctive grid-like or checkerboard appearance.
This is where the term “square waves” comes from.
The water may appear to form squares or rectangular patterns, especially when the wave systems meet at approximately right angles. Photographs of this phenomenon can look almost unreal, as though someone has drawn a geometric pattern across the surface of the ocean.
But the appearance isn’t necessarily the most important part.
The real concern is the combination of different wave directions and changing currents.
When waves move in different directions, the water’s surface can become complicated and unpredictable. Swimmers who are comfortable in ordinary waves may suddenly find themselves dealing with water moving in a direction they weren’t expecting.
That is why the stranger in the story may have been so concerned.
He wasn’t necessarily warning them because the ocean had literally turned into a collection of perfect squares.
He was warning them about the conditions that can create that unusual appearance.
Imagine standing on a beach and watching two sets of waves approach from different directions.
One group of waves moves toward the shore.
Another wave system approaches from the side.
Where the two systems meet, their energy interacts. Depending on the conditions, the surface can develop alternating lines and patterns that resemble a checkerboard.
From a distance, it can look fascinating.
For a swimmer in the water, however, unusual wave and current patterns deserve respect.
One of the biggest problems is that the surface appearance doesn’t necessarily tell you exactly how powerful the water movement is underneath.
A person standing on the beach may see a beautiful geometric pattern and want to take photographs. Someone swimming in the water may experience something completely different.
They may find themselves being pushed sideways, pulled away from where they entered the water, or forced to work harder than expected to remain in position.
This is especially concerning for inexperienced swimmers.
Many people think about waves primarily in terms of their height.
They ask, “How big are the waves?”
But wave direction, currents, wind, tides, and the shape of the seabed can all affect conditions.
A relatively modest-looking ocean can still produce challenging currents.
That’s one reason experienced swimmers and lifeguards pay attention to more than just the size of individual waves.