Swell period, the time in seconds between one wave crest and the next passing a fixed point, matters as much as swell height because it sets how much energy each wave carries and how deep that energy reaches. With swell period explained in one line: in deep water, wave power rises with the square of height but in direct proportion to period, so a 1.5 m swell at 14 seconds carries roughly 75% more energy per meter of crest than the same 1.5 m at 8 seconds. Long-period swell also travels faster, feels the seabed earlier and grows more as it reaches shallow water. On Portugal's coast that difference is built into the geography. Monthly averaged peak periods offshore run 9–13 s on the northwestern coast and 6–10 s along the south (ScienceDirect, 2022), which is a large part of why a forecast of "1.5 m" means something different in Viana do Castelo than in Tavira. Height tells you how big. Period tells you how hard.
Swell period explained: what the seconds actually measure
A buoy or a model reports swell as three numbers: height, period and direction. Height is the one everyone reads first. Period is the one that explains why two days with the same height feel nothing alike.
Short period means local wind
Swell at 6–8 s was usually made by wind not far away. The crests are close together, the faces are lumpy and the energy sits near the surface. In Portugal that often means the nortada, the summer north wind, building its own short, messy sea.
Long period means a distant storm
Swell at 12 s and above has usually traveled a long way from a deep Atlantic low. The weaker, shorter waves have dropped out along the way. What arrives is organized: clean lines, longer gaps between sets, and a lot of energy moving well below the surface.
A rough rule from standard wave physics: deep-water wavelength is about 1.56 × period². An 8 s swell has crests around 100 m apart. A 14 s swell, around 306 m.
How much more power does a longer period carry?
The standard deep-water approximation for wave power is about 0.49 × height² × period, in kilowatts per meter of wave crest (height in meters, period in seconds). Doubling the height quadruples the power. Doubling the period doubles it.
How does period compare with height in practice?
That second part is the bit most people underrate.
Here is our own arithmetic using that formula. These are illustrations, not measurements.
| Swell height | Period | Approx. power (kW/m) |
|---|---|---|
| 1.0 m | 8 s | 3.9 |
| 1.0 m | 14 s | 6.9 |
| 1.5 m | 8 s | 8.8 |
| 1.5 m | 14 s | 15.4 |
| 2.0 m | 8 s | 15.7 |
Read the last three rows together. A 1.5 m swell at 14 s carries about the same energy as a 2.0 m swell at 8 s. On a forecast the second looks bigger. In the water the first often feels heavier, because more of that energy is concentrated into fewer, longer waves.
Why does long-period swell grow more near shore?
Long-period swell also starts to feel the seabed at a depth of roughly half its wavelength, so a 14 s swell is already being shaped by the bottom about 150 m down. By the time it reaches a sandbar or reef, it has had more room to steepen and grow.
Why do Portugal's north and south coasts feel so different?
Because the north gets more height and more period at the same time, and the two multiply. The south coast, tucked away from the main northwest swell track, gets less of both.
What do the measured figures show?
| Measure | Figure | Source |
|---|---|---|
| NW coast, monthly avg. wave height (winter / summer) | 2.8 m / 1.2 m | ScienceDirect, 2022 |
| South coast, monthly avg. wave height (winter / summer) | 1.5 m / 0.6 m | ScienceDirect, 2022 |
| Monthly avg. peak period, NW / south | 9–13 s / 6–10 s | ScienceDirect, 2022 |
| Mean annual offshore height and peak period (56-year hindcast) | 2.3 m, 10 s | ResearchGate |
| Average winter wave power, Aguçadoura / Sines | ~40 kW/m / 29 kW/m | 33-year hindcast, 1979–2012 |
| Spatial average wave power, winter / summer | 38.8 kW/m / 6.4 kW/m | 33-year hindcast, 1979–2012 |
Which sea states carry the most energy?
The same 33-year study found the most energetic sea states sit at 2–4 m height with energy periods of 10–11.5 s. That is a period band, not just a size band. Portugal's energy agency, the DGEG, says the northern region has higher maximum wave energy, while swell is more constant through the year in the center.
What does long-period swell do at Nazaré?
Nazaré is where period stops being theory. A wave-tracking site drawing on MONICAN and Open-Meteo data listed conditions on October 13, 2026 of 7 ft at 13.8 s from 319° (Big Waves Nazaré). That is not a large height. It is a long period, from the northwest, at the start of what the same source describes as the October to March big-wave season, when deep Atlantic lows send the largest, longest-period swells.
The extreme end has been measured, not guessed. During storm Herminia, the Instituto Hidrográfico (the Portuguese Navy's hydrographic office) recorded a maximum wave height of 17 m at the Nazaré coastal buoy early on January 28, 2025. The oceanic Nazaré buoy recorded 15.6 m maximum and 9.4 m significant height; the oceanic Leixões buoy, 15 m maximum and 9.5 m significant (Região Sul).
We will leave the canyon for another post. The short version for this one: the same swell carries a lot more weight when the period is long.
How should you read swell height vs period on a forecast?
Read them as a pair, never one alone. A few habits that help when reading swell data for the Portuguese coast:
- Check period before you get excited about height. 1.2 m at 7 s and 1.2 m at 13 s are different days.
- Expect long-period swell to break bigger than the offshore height suggests at spots that face it, because it steepens more as it shoals.
- Expect longer lulls. Long-period sets arrive further apart. A quiet ten minutes is not a flat day.
- Look at direction too. A long-period swell from the wrong angle can miss a beach that faces away from it entirely.
- Treat short-period swell with onshore wind as one problem, not two. It is usually the same weather system.
What does period mean for swimmers, fishing and diving?
The same number, read the other way round.
For swimmers
A long-period swell can look calm from the sand during a lull and then deliver a set that knocks you over in the shorebreak. Short-period chop is uncomfortable but more predictable. Our swim board ranks beaches 0–10 for calmness, a separate scale from the 0–5 surf score, with a day picker for the calmest hours. Rips, flags and lifeguards are not in that data, so read the water and the beach as well as the number.
For fishing and diving
Long-period energy reaches deeper, which tends to mean more surge around rocks and reef, and usually stirs the bottom more. Anyone snorkeling or spearfishing near structure will feel a 14 s swell well below the surface even when the top looks tidy. Pair period with the maré, the tide, and the hour before deciding where to go in.
Where period sits in our 0–5 surf score
The Peel Watch score is multiplicative: 10 × swellQuality × windFactor × tideFactor × skillFit × daylight, halved and printed to one decimal from 0.0 to 5.0. Swell character feeds the swellQuality term. Size is checked separately against each level's breaking-height band and hard cap, from L1 Novice (0.3–0.9 m, cap 1.3 m) to L5 Expert (1.3 m+, no cap).
Why can a long-period day score a 4 for one surfer and a 0 for another?
That is why a long-period day can be a 4 for an intermediate and a 0 for a beginner. The extra energy pushes breaking height past the L1 cap, and one dealbreaker sinks the hour.
How accurate is the score?
Limits, stated plainly: our accuracy panel puts average swell-height error at about ±0.1 m, growing to ±0.2 m at five days out, and wind error at ±1–3 kn. We do not publish a separate period error figure, so treat a 0.2 difference between two spots as noise.
If you want to see this on real water, open any spot page on the dashboard and compare tomorrow's height, period and score side by side. It is the quickest way to learn what 13 seconds feels like before you are standing in it.