A day at the beach can change quickly when ocean conditions shift. Most swimmers are familiar with ordinary waves rolling toward shore, but occasionally the surface of the water can develop a strange checkerboard or crisscross pattern. These are commonly called “square waves,” although the more accurate term is a cross sea.
The pattern can look fascinating from the beach, almost as if someone has drawn a giant grid across the ocean. But it is not simply an unusual visual effect. A cross sea means wave systems are moving in
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