Surf Forecast Reader
What swell height, period and wind actually mean — including why the same height at a longer period is a completely different day.
Type in the numbers from any forecast — buoy readings, an app, a magic-seaweed table — and this will tell you what they mean. It fetches nothing and knows nothing about your beach; it is a reading tool, not a forecast.
What each number is telling you
Period is the number to read first
Everyone looks at swell height. Experienced surfers look at period, because it tells you where the swell came from and therefore what it will do when it arrives. Energy scales with the square of height times the period — so the same 1.5 metres at 18 seconds carries more than twice the energy of 1.5 metres at 8, and breaks like a completely different ocean.
Long-period swell also shoals harder. A long wave reaches deeper into the water column, so it starts feeling the bottom further out and stands up more dramatically — which is why an 18-second reading of 1.5 metres can break at three metres on the right bank, while an 8-second 1.5 metres breaks at about 1.5 metres and closes out.
And it wraps. Long-period swell refracts strongly around headlands and into bays, which is why some spots only ever work on a proper groundswell and stay flat through a week of wind swell at the same nominal height.
What this cannot tell you
- Swell direction against your beach. The most important variable of all and entirely local — a spot that lights up on a west swell can be flat on a north of twice the size.
- Tide. Some reefs only work on a push, some beaches only on the drop, and some spots close out entirely at high water. This is knowledge you get from going, not from a table.
- Bathymetry. What the sand or reef is doing this month decides whether the wave has a shape at all. Sandbars move; the forecast does not know.
- Crowd, current, and whether it is any good for you. A perfect forecast at a heavy reef break is a bad day for most surfers, and the number above cannot tell the difference.
How to use
- Read swell height and period together, never separately.
- Check swell direction against the spot's exposure.
- Check wind direction relative to the shore.
- Check the tide the spot works on.
Frequently asked questions
Why does swell period matter so much?
Because energy rises steeply with period as well as height. A 3-foot swell at 8 seconds is weak local windswell; the same height at 16 seconds is ground swell carrying far more energy, and it will break with much more power and further out. Period is the single most informative number on a forecast.
Where does long-period swell come from?
Distant storms. Waves sort themselves as they travel, with longer waves outrunning shorter ones, so swell arriving from thousands of miles away has a long period and arrives cleanly. Local wind produces short-period chop that has had no time or distance to organise.
What wind direction is best?
Offshore — blowing from the land out to sea — because it holds the wave face up and grooms it clean. Onshore wind breaks the face down and produces disorganised surf. Light offshore is generally the ideal, since strong offshore makes it hard to get into waves.
Why does the forecast height not match what I see?
Because forecast heights are usually open-ocean significant wave height, not the face height at your beach. How much of that swell reaches a given spot depends on its exposure, the swell direction, and the shape of the seabed. Local knowledge is what bridges the gap.
Does swell direction matter more than size?
Very often, yes. A spot sheltered from a particular direction gets almost nothing from a large swell arriving at the wrong angle, while a modest swell from the right direction can produce excellent conditions. Knowing a spot's window matters more than reading the height.
How does tide fit in?
Differently at every spot. Some breaks only work on a pushing tide, others go flat at high water or become dangerous at low. This is entirely local and cannot be read off a forecast — it is the part you learn by watching a spot across the tide.
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