Swell forecasts: how to read wave height, period, direction and wind

Significant wave height: an average, not a ceiling

‘West-north-westerly swell, 1.5 m, 12 s.’ What does that line really say? First, what the text says. To describe the state of the sea, Météo-France, the French weather service, uses ‘neither the maximum wave height (too large) nor the mean height (too small)’, but the average of the highest third of the waves, the significant wave heightThe average height, from trough to crest, of the highest third of the waves; written H1/3 or Hs. [1]. The American buoys of the National Data Buoy Center (NDBC) calculate it over 20 minutes [2]; according to the World Meteorological Organization (WMO), it roughly matches the height an observer would estimate by eye [3].

What it means for you: some of the waves will be bigger than the forecast figure. Under the statistical law the WMO uses, about one wave in seven exceeds the significant wave height, one in a hundred exceeds it by half, and over a few hours the highest reaches roughly double [3]. The Met Office puts it plainly: when 1 m is forecast offshore, anyone at sea for several hours stands a good chance of meeting at least one wave of 1.5 to 2 m [4].

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Measurements bear the theory out. In early November 2023, the Oléron buoy in the Bay of Biscay recorded a significant wave height of almost 10 m and a single wave of 18 m, 1.8 times as high, reports Germany’s national weather service (DWD) [5].

The period: the time between two crests, and what it gives away

The period is the time between two successive crests passing, in seconds [2, 4]. The sea mixes waves of every period, so a forecast keeps just one: the peak periodThe period of the waves carrying the most energy, at the top of the wave spectrum., that of the most energetic waves, or an average period. NDBC buoys give both, labelled DPD and APD [2]; Met Office beach forecasts give the peak period [4]. Before comparing two sources, check which one each displays.

Météo-France classes swell by its period: short under 8 s, medium from 8 to 12 s, long above 12 s [8]. That was the word it used in August 2025 to announce ‘a long north-westerly swell’ from former hurricane Erin, with waves of 4 to 5 m off the tip of Brittany [6]. The Met Office draws the same line in its own way: a ‘swell’ wave, of 10 seconds or more, is usually more powerful and breaks with more force than a ‘choppy’ wave of 6 seconds or less [4].

The period also sets the size and speed of the waves. In deep water, the WMO gives two simple relationships [3]:

The sums speak for themselves: a 7 s wave is about 75 m from crest to crest and travels at 40 km/h; a 14 s swell is about 300 m long and runs at nearly 80 km/h. The energy, for its part, travels at half the speed of the crests, at the speed of the wave groups [3].

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Why, at the same height, the period makes the difference

Let us be precise, since that is the whole point: height matters no less than period. The energy of a wave grows with the square of its height [3]: double the height and the energy quadruples. But forecasts often set two swells of the same height side by side, and the period then separates them, for three physical reasons set out in the WMO guide [3].

• It carries more energy. The power of a swell, per metre of wave front, is about 0.49 × H² × T kilowatts, where H is the significant wave height in metres and T the so-called energy period, in seconds. At 1.5 m, a 7 s swell brings about 8 kW per metre of coast, a 14 s swell about 15 kW: twice as much.
• It feels the seabed sooner. A wave feels the bottom when the depth falls below half its length, and only its period stays the same all the way to the coast. The 7 s swell touches the bottom at about 40 m deep, the 14 s swell from 150 m: it has much further to go to refract.
• It grows more as it arrives. As the depth decreases, the energy is concentrated and the wave rises before breaking. In 3 m of water, a swell of 1 m offshore stands about 1.1 m high if its period is 7 s, but 1.4 m at 14 s, according to linear theory, leaving aside refraction and friction on the seabed.

Météo-France sums it up for the general public: the longer a swell’s period, the more energy it carries, and the more powerful it is when it breaks on the coast [6]. The Met Office builds it into its beach forecasts: a longer period increases the surf height, as do waves directed towards the beach and a steep beach, where waves break more quickly and energetically [4].

Swell, wind sea and several trains: one sea, several figures

The sea you are looking at mixes two families of waves. The wind seaThe waves raised by the wind blowing locally, at the time they are observed. is born of the local wind; its height depends on the wind’s speed, how long it blows and the distance over which it blows, the fetchThe stretch of sea over which the wind blows from one direction. [7]. Swell is made of waves formed elsewhere that have travelled here; it always looks more regular, less chaotic [8]. When the wind drops, the wind sea gradually turns into swell [7]: an ‘old’ wind sea, as the DWD neatly puts it [5].

Forecasts separate the two when it helps. Météo-France always describes the total sea by its significant wave height, and adds the swell when it is significant: ‘very rough by north-westerly swell 5 m’ is the example its marine guide gives [1]. In the United States, the coastal forecasts of the National Weather Service (NWS) list each wave train with its direction, height and period, on the West and East Coasts, along the Gulf, in Hawaii, Guam and Puerto Rico [9].

The smallest of the three, the 18 s train, has probably come furthest, and it is the one that will grow most, in proportion, as it reaches shallow water. Another trap: heights do not add up. For the NWS, the square of the combined sea height is the sum of the squares of the swell and the wind sea [10]: 1.5 m of swell and 1 m of wind sea make about 1.8 m, not 2.5 m. Finally, the peak period jumps from one train to another: when two sources have similar energy, explains the Coastal Data Information Program (CDIP) at the Scripps Institution of Oceanography, it can switch from 18 s to 5 s from one hour to the next [11].

When two trains of similar height cross at an angle of 45 to 135°, Météo-France speaks of a cross seaTwo wave trains of similar significant height whose directions are 45 to 135° apart. [1], and the DWD notes that such a sea produces tall individual waves more often than the usual statistics predict [5]. Trains of similar period from the same distant storm also explain sets: when their crests coincide, the waves grow; when they cancel out, the sea calms down for a while [11].

Direction: where the swell comes from, and where the arrow points

A swell direction is given, in principle, like a wind direction: where it comes from. The NDBC gives the direction the waves at the dominant period are coming from, as an angle measured clockwise from true north [2]: 90° for an easterly swell, 270° for a westerly one. The CDIP uses the same convention [11], and Météo-France bulletins likewise speak of a ‘north-westerly swell’ [1].

What changes from one country to another is sometimes the drawing. On Met Office beach forecasts, the arrow shows the average direction the waves are going, 1 or 2 miles out to sea [4]: an arrow pointing east there means a westerly swell. An arrow has no opinions; it obeys its key. Read the key before anything else.

Direction is then read against the way the beach faces. If the swell heads straight for an unsheltered beach, nearly all its energy reaches it; if it runs parallel to the coast or away from it, the waves reaching the shore are much smaller than offshore [4]. As it nears the seabed, the crest turns to lie parallel to the depth contours, which is refraction [3]: two neighbouring beaches facing different ways do not get the same swell.

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Offshore and onshore winds: what the wind does to the waves

The wind counts twice: it raises the wind sea, and it shapes the waves arriving from offshore. The Met Office distinguishes three cases [4]. An offshore windA wind blowing from the land out to sea; ‘vent de terre’ in French. blows from the land out to sea; an onshore wind, from the sea towards the land; a cross-shore wind, parallel to the shoreline. When onshore winds are strong, the waves are likely to be rough; when cross-shore winds are strong, they become increasingly choppy [4].

On breaking itself, laboratory experiments summarised by researchers at the Scripps Institution of Oceanography show that an onshore wind widens the surf zone and makes waves break at a lower height-to-depth ratio, while an offshore wind has the opposite effect [12]. Put simply, an onshore wind brings breaking forward, an offshore wind holds it back.

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The wind also turns with the time of day. Météo-France calls a sea breeze the wind blowing from the sea towards the land during the day, especially in the afternoon, and a land breeze the one blowing out to sea, mostly in the second half of the night [8]. A forecast read at breakfast does not necessarily describe the afternoon’s wind: look at the time each line refers to.

The tide, the figure to cross-check with all the others

A swell forecast says nothing about the tide, which nonetheless changes the depth of water over sandbanks and rocks. And it is depth that decides where a wave feels the bottom, grows and breaks [3]: the same spot does not behave the same way at low water and at high water. Tide times and heights are covered separately in our guide to tides.

The cross-check matters for the whole coastline too. Météo-France points out that coastal flooding comes from a combination of the tide, a surge caused by low pressure and wind, and big swells breaking, and that the higher the French tidal coefficient, the higher the sea at high tide [13]. During storm Xynthia in February 2010, the surge reached 1.53 m at La Rochelle, at high tide, with a coefficient of 102 [13]. That is what France’s ‘waves and coastal flooding’ weather warning is for.

Rip currents, for their part, appear in none of these lines. How to spot them, and what to do if you are caught in one, is covered in our beginner’s guide to surfing.

Where to read an official forecast, country by country

Every country publishes its marine forecasts free of charge, with its own words and units. The words for the state of the sea, though, are shared on both sides of the Channel: Météo-France and the Met Office both describe the sea using the Douglas scale, from ‘calm’ to ‘phenomenal’ [1, 14].

In France, Météo-France’s coastal forecast covers the coastal strip up to 20 nautical miles from the shore [1]. In the United Kingdom, the Met Office inshore waters forecast extends up to 12 nautical miles, in 19 areas, and reminds users that the internet is not part of the official Maritime Safety Information system: at sea, radio broadcasts remain the reference [15]. In Germany, the DWD publishes maps of significant wave height, wind sea and swell combined, in steps of 0.5 m [16], and the BSH, the federal maritime and hydrographic agency, shows the raw measurements from its stations in the North Sea and the Baltic in real time [17].

Commercial forecasting websites and apps exist too; we do not recommend any of them. They show the same families of figures: before relying on one, look in its help pages for which height and which period it gives, and for what location. If I had to keep just one habit, it would be this: never read a height without the period that goes with it.

Worth watching

Surf Simply’s animated surf-science series explains the three numbers on a swell forecast, from period to significant wave height (the average of the highest third) and direction.

The video is hosted by YouTube (Google): nothing loads until you play it, and Google may then set cookies.

Arnaud Couderc

Fact-checking, regulations and safety

Arnaud Couderc reviews our contributors’ articles and writes for Outfarer about regulations, safety, fishing, the sea and outdoor pursuits. He traces every rule back to its official text and every figure to its source, so that you set off with sound benchmarks.

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