Swell Forecasts: How to Read Wave Height, Period, Direction, and Wind

Significant wave height: an average, not a ceiling

“West-northwest swell, 5 ft (1.5 m) at 12 seconds.” What does that line really say? First, what the text says. To describe the state of the sea, France’s Météo-France 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 National Data Buoy Center (NDBC) buoys 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 waves will be bigger than the forecast number. 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 biggest reaches roughly double [3]. The UK Met Office puts it plainly: when 3.3 ft (1 m) is forecast offshore, anyone at sea for several hours has a good chance of meeting at least one wave of 5 to 6.5 ft (1.5 to 2 m) [4].

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Measurements back up the theory. In early November 2023, the Oléron buoy in France’s Bay of Biscay recorded a significant wave height of almost 33 ft (10 m) and a single wave of 59 ft (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 ocean 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, labeled 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’s the word it used in August 2025 to announce “a long northwest swell” from former Hurricane Erin, with waves of 13 to 16 ft (4 to 5 m) off the tip of Brittany [6]. The Met Office draws the same line 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, here in feet and knots [3]:

Run the numbers and it’s striking: a 7 s wave is about 250 ft (75 m) from crest to crest and travels at about 24 mph; a 14 s swell is about 1,000 ft (300 m) long and runs at nearly 49 mph. The energy, meanwhile, 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’s be precise, since that’s 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 put two swells of the same height side by side, and the period then separates them, for three physical reasons laid out in the WMO guide [3].

• It carries more energy. The power of a swell, per meter of wave front, is about 0.49 × H² × T kilowatts, where H is the significant wave height in meters and T the so-called energy period, in seconds. At 5 ft (1.5 m), a 7 s swell brings about 2.4 kW per foot of coastline (8 kW per meter), a 14 s swell about 4.7 kW per foot (15 kW per meter): twice as much.
• It feels the bottom sooner. A wave feels the bottom when the depth drops below half its length, and only its period stays the same all the way to the coast. The 7 s swell touches bottom in about 125 ft (40 m) of water, the 14 s swell from 500 ft (150 m): it has much farther to go to refract.
• It grows more as it arrives. As the water gets shallower, the energy is squeezed together and the wave rises before breaking. In 10 ft (3 m) of water, a swell of 3.3 ft (1 m) offshore stands about 3.6 ft (1.1 m) tall if its period is 7 s, but 4.6 ft (1.4 m) at 14 s, according to linear theory, leaving aside refraction and friction on the bottom.

Météo-France sums it up for the 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 aimed at the beach and a steep beach, where waves break more quickly and energetically [4].

Swell, wind waves, and several trains: one ocean, several numbers

The ocean mixes two families of waves. Wind wavesWaves raised by the wind blowing locally, at the time they’re observed; also called wind sea. are born of the local wind; their height depends on the wind’s speed, how long it blows, and the distance over which it blows, the fetchThe stretch of water over which the wind blows from one direction. [7]. Swell is made of waves formed elsewhere that have traveled to you; it always looks more regular, less chaotic [8]. When the wind dies down, wind waves gradually turn into swell [7]: an “old” wind sea, as the DWD neatly puts it [5].

Forecasts split the two out when it helps. Météo-France always describes the total sea by its significant wave height and adds the swell when it’s significant: “very rough with a 5 m northwest swell” is the example its marine guide gives [1]. In the U.S., National Weather Service (NWS) coastal waters forecasts 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]. A range such as “Seas 2 to 4 ft” signals uncertainty or changing conditions, not that every wave falls between 2 and 4 ft [18].

The 18 s train, the smallest, probably came the farthest and will grow the most, proportionally, in shallow water. Another trap: heights don’t add up. For the NWS, the square of the combined seas is the sum of the squares of the swell and the wind waves [10]: a 5 ft (1.5 m) swell and 3 ft (1 m) of wind waves make about 6 ft (1.8 m), not 8 ft (2.5 m). Finally, the peak period jumps between trains: when two sources have similar energy, says the Coastal Data Information Program (CDIP) at Scripps Institution of Oceanography, it can switch from 18 s to 5 s from hour to hour [11].

When two trains of similar height cross at an angle of 45 to 135°, Météo-France calls it 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 line up, the waves grow; when they cancel out, the ocean 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 east swell, 270° for a west swell. The CDIP uses the same convention [11], and Météo-France bulletins likewise speak of a “northwest swell” [1].

What changes from one country to the next is sometimes the drawing. On UK Met Office beach forecasts, the arrow shows the average direction the waves are heading, 1 or 2 miles out [4]: an arrow pointing east there means a west swell. An arrow has no opinions; it follows its legend. Read the legend first.

Then read the direction 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 shore are much smaller than offshore [4]. As it nears the bottom, the crest turns to line up with the depth contours, which is refraction [3]: two nearby beaches facing different ways don’t 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 waves, and it shapes the waves arriving from offshore. The Met Office describes three cases [4]. An offshore windA wind blowing from the land out to sea. blows from the land out to sea; an onshore wind, from the sea toward 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 get increasingly choppy [4].

On breaking itself, lab experiments summarized by researchers at 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 makes waves break sooner, an offshore wind holds them up.

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The wind also shifts with the time of day. Météo-France calls a sea breeze the wind blowing from the sea toward 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 you read over breakfast doesn’t necessarily describe the afternoon wind: check the time each line covers.

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

A swell forecast says nothing about the tide, which still changes the depth of water over sandbars and rocks. And it’s depth that decides where a wave feels the bottom, grows, and breaks [3]: the same break doesn’t behave the same way at low tide and at high tide. 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 storm surge caused by low pressure and wind, and big breaking swells, and that the higher France’s tidal coefficient (a 20 to 120 scale), the higher the sea at high tide [13]. During storm Xynthia in February 2010, the surge reached 5 ft (1.53 m) at La Rochelle, at high tide, with a coefficient of 102 [13]. That’s what France’s “waves and coastal flooding” weather alert is for.

Rip currents, for their part, show up in none of these lines. How to spot them, and what to do if you’re 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 for free, in its own words and units. In Europe, the words for the state of the sea are shared on both sides of the English 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 out to 20 nautical miles [1]. In the UK, the Met Office inshore waters forecast extends out to 12 nautical miles, in 19 areas, and reminds users that the internet isn’t 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 0.5 m (1.6 ft) steps [16], and the BSH, the federal maritime and hydrographic agency, shows raw real-time measurements from its stations in the North Sea and the Baltic [17].

Commercial forecast websites and apps exist too; we don’t recommend any of them. They show the same families of numbers: 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 breaks down the three numbers on a swell forecast: period, significant wave height (the average of the biggest third of the waves), 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 ocean, and outdoor activities. He traces every rule back to its official text and every number to its source, so you head out with solid benchmarks.

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