The Long Journey of a Wave
All scenes › Chapter 3 Swell crosses the ocean
Swell
Smooth, evenly spaced waves crossing a calm sea
View in 3DSmooth, even swell
Waves that have lined up keep going even where there is no wind. Waves like this, with round crests and even spacing, are called swell. 12-second swell travels about 810 km a day. Even swell from a storm 5,000 km away arrives in about 6 days.
Why?Short, steep waves break easily and lose their strength. Long, low swell does not break, so it carries the energy it got from the wind a long way, almost without loss.
Learn moreThe crests of 12-second swell move at about 67 km/h, but the energy moves at half that, about 34 km/h. The crests are about 225 m apart. In 1964, researchers set up six stations across the Pacific from New Zealand to Alaska and recorded swell from southern storms travelling all the way to Alaska.
- Time for 12-second swell to travel 5,000 km
- about 6.2 days
- Crest to crest
- about 225 m
- Speed of the energy
- about 34 km/h
calculated from the textbook equations for water waves (linear wave theory)
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Wikipedia: Swell (ocean) · Snodgrass et al. (1966), Propagation of ocean swell across the Pacific
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The numbers come from the textbook equations for water waves (linear wave theory, the dispersion relation and shoaling). In the 3D view, wave heights are drawn larger; the seabed slope and the shape of breaking waves are rough sketches.