How to read a surf forecast

A surf forecast looks like a wall of numbers. Most of them don’t matter. Five of them do.

Height

The “wave height” on Harper is significant wave height. It is basically the average of the biggest third of the waves rolling in. The biggest sets will run larger, the smaller waves smaller. If the chart reads 1.5 m, expect plenty of 2 m waves in the mix.

Period

Period is the gap in seconds between one wave crest and the next. The longer the gap, the more energy the swell is carrying, and the cleaner the lines show up.

• Less than 7 s: wind chop. Short-lived, breaks soft.
• 7 to 11 s: local wind swell and mid-period. Fun, but not lined up.
• 11 to 13 s: getting organised. Starting to look like proper surf.
• 13 to 15 s: classic groundswell. Clean lines marching in from the horizon.
• 15 to 18 s: long-range groundswell. Packs a real punch. That 1 m on the chart will feel bigger than it reads.
• 18 s and up: rare, massive storm swell. You cannot miss it.

Period matters more than height for how a wave actually breaks. That is why Harper colors the swell-height bars by period, so one glance tells you both how big and how clean.

Direction

Harper reports the direction the swell is coming from. A south swell means the energy is travelling up from the south. The arrow on the chart points the opposite way (where the swell is going), which is the marine convention.

Match the swell against your spot. A beach facing southwest needs a south to west swell window. A north swell will wrap around the coast and lose most of its juice before it reaches the sand.

Wind

Wind does not make waves at your spot (the swell is already on its way), but it decides whether those waves arrive clean or turn to mush. Harper reports it in knots (kn), with an arrow pointing where it is blowing to.

• Offshore (land to sea): grooms the wave face and holds it up longer. Every surfer’s best friend.
• Onshore (sea to land): chops the surface and pushes waves to close out.
• Cross-shore: somewhere in between.

Strength matters as much as direction. 10 kn is barely a breeze. 20 kn will chew up most sessions. 30 kn is blown out. Light is almost always better than strong, whichever way it blows.

Power

Wave power (kW/m, kilowatts per metre of wavefront) rolls height and period into a single number: how much juice the swell is really carrying. A 2 m 14 s swell packs way more than a 3 m 6 s swell, even if it looks smaller on the chart, because the long period puts far more energy behind every wave.

• Under 10 kW/m: small and soft.
• 10 to 50 kW/m: solid average day.
• 50 to 100 kW/m: energetic. Step up in skill and confidence.
• Over 100 kW/m: heavy water. Big-wave spots and pros only.

Power does not tell you how the waves break. Direction, wind, and the shape of the bottom do that. But it is the single best number for how serious a swell really is.

Wave systems

The ocean rarely sends one swell at a time. Out the back, you are usually feeling two or three trains of waves stacked on top of each other, each with its own height, period, and direction. Harper splits them into a primary swell, a secondary swell, and a tertiary swell. The headline combined height is what the ocean “feels” when you mash all of them together.

That combined number is fine for a glance. If you want to know how the lineup will actually look, read the individual swells.

• One clean primary, nothing else worth mentioning: this is the dream. A 1.5 m primary at 14 s with a tiny secondary is lined-up, easy-to-read surf. Trust the headline.
• Primary and secondary from the same direction, similar period: they stack constructively. Sets feel bigger than the combined number suggests, and lulls go quiet. Good if periods are long, sloppy if they aren’t.
• Two swells from different directions: peaks. Wedges. A-frames if you are lucky. Crossed-up closeouts if you aren’t. The bigger the angle between them, the more “wash-through” you get (one set rolling in while another is still backing off the sand).
• Long primary plus short, noisy secondary: this is the killer. A clean 1.5 m at 13 s feels nothing like the same 1.5 m with a 7 s wind swell layered on top. The short stuff puts bumps on the face of every long-period wave. The combined height looks identical. The session does not.

The dominant period is the period of whichever swell is carrying the most energy at that hour. If it is high (13 s+) you are mostly feeling groundswell, even if a wind sea is along for the ride. If it has dropped to 8 s, the wind sea has taken over and the groundswell is no longer the story.

Rule of thumb: if the secondary swell is more than half the height of the primary and the periods are far apart, look at each swell separately before you commit to the dawnie.

Wind sea

Wind sea is chop the local wind made on its way to you, not swell from a storm a thousand kilometres away. Its period is almost always short, 3 to 7 s. That short period is the giveaway: it has not had time or distance to organise into proper lines.

What it does to a session:

• Bumpy faces. The groundswell is still doing the work, but every wave has wind ripple on it. Harder to read, harder to set a rail.
• Paddle-around chop. That mid-paddle slap that turns a clean duck-dive into a faceful.
• Mixed lump on the wave. Sets look weirdly fat or weirdly peaky depending on whether the chop is in phase with the swell underneath.
• False size from the beach. Wind sea piles water against the sand and makes the shorebreak look angrier than the lineup actually is. Stand on the headland, not the carpark.

Heuristic: if the wind sea height is roughly equal to or bigger than the primary swell height, it is a victory at sea. The groundswell is still under there, but you will fight for every wave. If wind sea is a fraction of the primary (say a third or less), it is just texture. Often a clean offshore knocks it flat anyway.

Weather

Harper now shows hourly weather alongside the swell. Most of it is comfort stuff. A few bits actually move surf quality.

• Pressure (hPa). Read the trend, not the number. A falling pressure usually means a system is moving in: more wind, messier surface, faster-changing conditions. A slow rise the morning after a low has passed often means the wind drops out and the swell is still in the water. That is your dawnie window. This is a soft tell, not a forecast. Believe the wind chart first.
• Wind gust vs sustained. Sustained wind is the average, gusts are the spikes. A 12 kn average with 28 kn gusts will flip a lineup from clean to bumpy in a 20-minute window and you will not see it coming if you only read the average. Check the gust kph next to wind direction before you commit.
• Rain and shower probability. Does not change the waves. Does change the lineup. Some breaks empty out the second it starts spitting; others fill up with locals in 4/3s who were waiting for the tourists to leave. Useful for picking which break, not whether to surf.
• Apparent temperature and sea surface temperature. Wetsuit choice, mostly. Harper has a separate wetsuit guide that does the heavy lifting on this one. For the forecast: cold air plus warm water means fog on the inside; cold water plus warm air means a long walk back to the car questioning your life choices.

Putting it together

A good session usually means decent height, long period, solid power, swell lined up with your spot, and light offshore wind.

A bad session is short period, low power, wind onshore, or swell hitting the coast at a wrong angle, even if the height looks big.

Start with period and wind. The rest falls out from there.