Swell height gets all the attention. Period, direction and wind are what actually decide whether Saturday is worth the drive.
You check the app on Thursday night. Three feet, Saturday morning, green bar, looks promising. You set an alarm, load the car in the dark, drive forty minutes, and arrive to a lumpy grey mess with whitewater stacked to the horizon. Nothing lied to you. The information was sitting right there in the columns you skipped.
Almost everyone learns to read a forecast the same way, by looking at the height and ignoring the rest. It works often enough to feel like a system and fails often enough to waste half your winter. This is the guide to the other numbers, written for anyone who has ever nodded along to a conversation about period and quietly had no idea.
What a surf forecast actually is
Worth clearing up first: a forecast is not a measurement. It is a model output, a computer's best guess at what the ocean will be doing several days from now.
Global weather models such as the American GFS and the European ECMWF produce a picture of atmospheric pressure and wind across the planet. Wave models like WAVEWATCH III take that wind field and calculate how much energy it dumps into the sea surface, then track that energy as it fans out across the ocean. Whatever app you use, that chain sits underneath it. The differences between apps come down to which model they lean on, how often they refresh, and how much local processing they apply before showing you a number.
Observations are separate. Wave buoys moored offshore, run in UK waters by Cefas WaveNet and the Channel Coastal Observatory, report what the sea is doing right now. When a buoy reading and a forecast disagree, believe the buoy. It is the only honest voice in the room.
Swell height, and why it flatters to deceive
The height on your forecast is almost always significant wave height, written as Hs. It has a specific meaning: the average height of the largest third of the waves passing a point. That definition matters because it builds in a lie of omission. If Hs is 2m, plenty of waves are under 2m and a handful are well over it. As a rough guide, the biggest wave in a few hours of swell will run somewhere near double the significant height.
Second problem. That figure describes swell in deep water offshore. It does not describe the wave breaking in front of you. As swell moves into shallow water it slows at the base, steepens, and stands up. How much it gains depends on the seabed and on the period. A long-period swell will jack up dramatically over a sandbank. A short, choppy sea will barely change.
So the height column is a starting point and nothing more. On its own, it cannot tell you whether Saturday is worth the fuel.
Swell period: the number that decides your session
Period is the gap in seconds between one wave crest and the next as they roll past a fixed point. It is the most useful figure on the entire forecast and the one people ignore hardest.
Here is why it carries so much weight. The energy in a wave train scales with the square of the height and, in terms of power delivered to your beach, roughly with the period as well. Double the period and you roughly double the power at the same height. That is the reason a 1.5m swell at 16 seconds will break heavier, faster and visibly bigger than a 1.5m swell at 8 seconds. Same number in the height column, completely different day.
Period also tells you where a swell was born. Waves sort themselves as they travel, with longer waves moving faster than shorter ones. A deep-water swell travels at roughly one and a half times its period in knots, so a 16-second swell is covering ground at about 24 knots, near 550 nautical miles a day. That swell has come from a low that spun up somewhere off Greenland or deep in the mid-Atlantic days earlier. An 8-second reading was made by wind blowing over your patch of sea yesterday afternoon.
Swell period, side by side
Same rough height, completely different wave. Longer gaps between crests mean more power arriving at the beach.
The practical takeaway: at anything above 13 or 14 seconds, mentally add to whatever the height column says. At 8 seconds, take a chunk off.
One catch worth knowing. Very long period swell, 18 seconds and beyond, can be so spread out that it arrives in long lulls, five or six waves every twenty minutes. That is a session that rewards patience and punishes anyone who paddles out on a tight schedule.
Swell direction: whether the energy reaches you at all
Direction is given in degrees or as a compass point, and it describes where the swell is coming from. A 290-degree swell is arriving out of the west-northwest.
Match that against the way your beach faces. Cornwall's north coast looks out roughly northwest through west, which is why Atlantic lows tracking across the top of the country light it up and why a south swell does almost nothing there. Head round to the Bristol Channel and the geometry changes completely, with a much narrower window of directions that can squeeze in past Lundy.
Two other effects come into play. Headlands block swell, throwing a shadow over anything tucked in behind them, which is exactly what makes a sheltered corner the right call on a big day. And refraction bends swell as it feels the bottom, so energy can wrap around a point and into a bay it was never aimed at, arriving smaller but far better organised.
This is where local knowledge beats any app. A generic forecast knows the swell direction; it does not always know that your beach only really switches on when the swell swings past 300 degrees.
Wind: the difference between a good day and a wasted one
You can have the best swell of the season on the chart and still find junk in the water. Wind decides that.
Offshore wind blows from the land out to sea. It holds the face up as the wave stands, grooms the surface smooth, and stretches out the moment before it breaks. This is what the dawn patrol crowd are chasing, and it is why so many of the best sessions happen before eight in the morning.
Onshore wind blows from the sea towards the land. It pushes waves over before they have organised themselves and shreds the surface into chop. A firm onshore can wreck a perfectly good swell in under an hour.
Cross-shore sits between the two and is mostly about strength. Light cross-shore is fine. Strong cross-shore chews the face up and makes the wave close out down the line.
Strength thresholds, roughly. Under 8 knots, direction barely registers and you can more or less ignore it. Between 8 and 15 knots, direction is doing real work either way. Above 15 knots onshore, the sensible move is to stay in bed or find somewhere sheltered.
Time of day matters too. In summer the sea breeze is a daily pattern rather than a weather event: the land heats up through the morning, air rises, cooler air pulls in off the water, and an onshore builds from late morning that had nothing to do with any weather system. That is why summer sessions front-load. In winter the wind is driven by pressure gradients and can hold offshore all day.
Tide: the local variable that ruins everything
Tide changes water depth, water depth changes how swell interacts with the seabed, and the seabed is what shapes a wave. Some banks only work on a pushing mid tide. Some reefs are unsurfable until the water drops off them. Plenty of Cornish beaches turn into a shore-dumping closeout at high water and reveal three separate peaks four hours later.
Add spring and neap cycles on top. Around a full or new moon, spring tides bring a bigger range and stronger flow, and some spots get flushed with current at the peak of the push. Neaps are gentler, with less water moving.
There is no shortcut here. You learn a spot's tide window by turning up at different states and paying attention, or by asking someone who already has.
Reading a forecast row properly
Take a line off a chart: 1.4m, 15s, 295 degrees, wind 7 knots from 110 degrees, low tide at 07:40.
Work through it. The 15-second period is genuine groundswell, so treat that 1.4m as conservative and expect it to stand up well overhead on the sets. The 295-degree direction is straight into a northwest-facing beach, no shadowing, energy arriving with nothing in the way. Wind from 110 degrees is easterly, which on a north coast beach is offshore, and 7 knots is light enough to groom without holding you out of the wave. Low tide at 07:40 means the push starts through the morning, so if your local prefers water on the bank, aim for 09:00 rather than first light.
That is a day worth setting an alarm for. Now change one figure. Same swell, wind at 18 knots from 300 degrees. That is straight onshore at strength, and Saturday is off.
The habit to build is scanning every column and asking what each one does to the others. It takes a few weeks to become automatic and it will save you dozens of pointless drives.
Mistakes that cost people sessions
Reading the height and stopping there. Covered above, and it accounts for more skunked mornings than anything else.
Trusting a five-day forecast like a promise. Beyond about 72 hours, model confidence drops off fast. Treat a Wednesday look at Sunday as a heads-up rather than a plan, and check again the night before.
Ignoring the secondary swell. Most charts list more than one swell train. A modest primary can be sitting on top of a second, shorter-period sea running from a different angle, and the interference between them is what turns a clean forecast into a confused, wobbly lineup.
Checking one source. Models disagree. When two of them show the same thing, confidence goes up. When they diverge sharply, that in itself is information: the setup is unstable and the swell might arrive early, late, or not at all.
Quick glossary
| Term | What it means |
|---|---|
| Hs / significant wave height | Average height of the biggest third of waves. Some will be considerably larger |
| Period | Seconds between wave crests. Higher means more energy and better shape |
| Groundswell | Swell generated by a distant storm, long period, organised |
| Windswell | Locally generated, short period, disorganised |
| Fetch | The distance of open water a wind blows across. Longer fetch, bigger swell |
| Refraction | Swell bending as it feels the seabed, allowing it to wrap into bays |
| Offshore | Wind blowing from land to sea. Grooms and holds up the wave face |
| Sets | Groups of larger waves arriving together, separated by lulls |
| Glass off | Evening drop in wind leaving the surface smooth |
Frequently asked questions
What is a good swell period for surfing?
Anything from 12 seconds upwards will generally produce a well-formed, powerful wave. Between 9 and 11 seconds is workable, particularly at a beach break with decent banks. Below 9 seconds you are looking at windswell, which is fine for learning and rarely much fun for anyone past that stage.
Is 2ft at 16 seconds better than 4ft at 8 seconds?
In almost every case, yes. The long-period swell carries far more energy, will stand up much taller as it hits shallow water, and will break with proper shape. The 4ft windswell will feel weak and disorganised despite the bigger number.
Why does the forecast say 3ft but the waves look twice that?
Two reasons stacked on top of each other. Significant wave height only describes the largest third of the waves, so the sets exceed it. And long-period swell gains substantial height as it shoals over the sandbanks, which the offshore figure does not account for.
What wind is best for surfing?
Light offshore, meaning wind blowing from the land out to sea at under about 10 knots. Glassy conditions with no wind at all are equally good. Strong onshore is the one to avoid.
How far ahead can you trust a surf forecast?
Around three days for reasonable confidence on swell size and timing. Wind is less reliable and can shift inside 24 hours. Anything beyond five days is a rough signal that swell is on the way rather than a plan you should commit to.
Do I still need to check the tide?
Yes. Tide can be the difference between a spot firing and the same spot being unsurfable, and it is the most location-specific variable of the lot. Learn your local's window.
None of this replaces standing on the cliff and looking at it. The forecast gets you to the right beach at the right hour; your own eyes decide whether you get changed. Give it a season of paying attention to period and direction and you will start calling days before your mates do.
