Swell from Distant Storms

Group speed and forecasting

An interactive lesson with 7 steps, about 6 minutes: you predict, drag and solve, with instant feedback on every answer. It runs in your browser and needs JavaScript.

What you’ll learn

• In deep water, wave energy travels at the group speed $c_g = gT/4\pi$, half the crest speed.
• Dispersion sorts storm waves by period, longest first.
• Arrival-time gaps reveal a storm’s distance.

Steps

1. Wave groups
2. Crests outrun their group
3. A 14-second swell
4. How far away is the storm?
5. Sorted by size
6. Period from wavelength
7. Across an ocean