The same swell
How long does a swell take to travel from the North Atlantic to Galicia and to the Canaries?
Groundswell doesn't travel at one speed. Longer-period waves move faster, so a single storm arrives split: the long, ordered ones first, then the bulk.
It's a shared wave — the same energy leaving the North Atlantic, two paths, two arrival hours. Longer bands lead, shorter ones follow.
Each ray leaves the generation area at t=0 and travels at its period's group velocity. Where it crosses a coast line, that's when that period band arrives there. The rays are not the same wave — they are the same energy spread across bands.
The 11 s band now in the generation area reaches Pantín around 06:35 on Sat, after 657 km. Longer bands arrive first; the bulk follows behind.
The 11 s band now in the generation area reaches El Confital around 04:51 on Sun, after 2095 km. Longer bands arrive first; the bulk follows behind.
The 11 s band now in the generation area reaches Famara around 03:12 on Sun, after 1992 km. Longer bands arrive first; the bulk follows behind.
The 11 s band now in the generation area reaches El Cotillo around 03:57 on Sun, after 2039 km. Longer bands arrive first; the bulk follows behind.
Arrival gap between the two coasts: +11 h in Galicia versus +1 d 10 h in the Canaries, +23 h apart. Distance rules — a few hours for the Ártabra Atlantic, half a day or more for the southern end.
We compare arrival hour, not wave height. Our calculation says when the dominant band lands; the forecast says when the sea actually spikes (max swell in 72 h). No tuned constants.
Three constants and no fudge factor. If a number is wrong, the model is wrong — that is intentional.
- Deep-water group velocity: Cg = gT / 4π ≈ 1.5613 · T (m/s).
- Travel time: hours = (km · 1000) / Cg(T) / 3600.
- Great-circle distance (haversine, R = 6371 km) between declared coordinates.
It doesn't track a specific storm — the generation area is a fixed point. It ignores shelf refraction and island shadowing. It assumes deep water the whole way. That's why it always sits beside the forecast, never in its place.