The Long Journey of a Wave
All scenes › Chapter 3 Swell crosses the ocean
Long waves go first
In deep water, long waves are faster, so the waves line up by length
View in 3DThe waves line up
In deep water, the longer the distance from crest to crest, the faster a wave travels. So even waves that leave a storm together end up lined up by length. The yellow marker is the 16-second waves and the blue marker is the 7-second waves. The long waves take the lead, while the short ones fall behind and slowly lose strength.
Why?A long wave takes longer to rise and fall once, but it moves much farther each time. A wave four times as long travels twice as fast. The 16-second waves are about 400 m crest to crest; the 7-second waves are about 77 m.
Learn moreSpeed that depends on length is called dispersion. A group of waves (and its energy) moves at half the crest speed in deep water. So covering 5,000 km takes about 4.6 days for 16-second waves and about 10.6 days for 7-second waves. When long, low swell reaches a beach first, it may be a message from a distant storm.
- Time since the storm
- 3 days
- 16-second waves
- about 45 km/h
- 7-second waves
- about 20 km/h
calculated from the textbook equations for water waves (linear wave theory)
View this scene in 3D →Read more
Wikipedia: Dispersion (water waves)
More in the series
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.