Going deeper

Smaller circles deeper down

This time there are dots below the red one, at different depths. The deeper you go, the faster the circles shrink. At 25 m deep, a quarter of the crest-to-crest distance, the circle is about 21% of the surface one; at half, 50 m, it is about 4%. Below that, the water hardly moves even as waves pass overhead.

Why?A wave is a rolling motion that starts at the surface, so its push weakens the deeper you go. That is why the water a few tens of meters down stays calm even during a storm.

Learn moreCircle size shrinks by the same fraction for every step down. Each time you go down one eighth of the crest-to-crest distance, the circle becomes about half as big (46%). So a wave only “feels” down to about half its crest-to-crest distance. The crest-to-crest distance is called the wavelength.

Circle size at 50 m deep

4%

12.5 m deep

46%

25 m deep

21%

50 m deep

4%

calculated from the textbook equations for water waves (linear wave theory)