TonnrModels, buoys and secondary swells: reading a forecast in depth
What the figures are made of: the models and their resolution, the gap between a buoy and a model, swells that overlap, the shadow of an island, and an alert drawn from your sessions.
The models and their resolution
A forecast is the output of a model: a program that computes the ocean on a grid. Tonnr reads the waves of a model at 8 km, and the wind at 2 to 13 km depending on the region. A cell sees nothing smaller than itself: a three-kilometre cove does not exist for a model at 25 km.
The model computes out at sea, in deep water. The figure of a coastal cell is the swell arriving, not the wave breaking: the bottom, the headland and the bay that transform it are below its resolution.
Each run starts again from the latest measurements and corrects the previous one. The forecast therefore changes every day, and the far days change the most. A spot's page says at what time its forecast was read.
Change the resolution: at 25 km the cove disappears into a cell of land; at 2 km it has its own.
Remember: A figure holds for its cell, out at sea, at the hour of its run.
Buoy against model
A buoy measures; a model computes. The buoy says the sea of half an hour ago, with its sets and its mess; the model says a smoothed sea, now and for seven days.
The two always differ a little, often by 0.3 m in height. The gap grows with the horizon: at two days, the model is reliable; at four, it gives the trend; at seven, it guesses. When buoy and model disagree at the very same hour, the buoy is right.
On the map, Tonnr shows the two side by side at the same point: the buoy's curve up to now, the model's going on. Read today's gap, it tells you how far to trust the week.
Move through the week: the cone widens. The solid line is the buoy; the dashed one, the model.
Remember: The buoy says what is; the model says what is coming, less and less surely.
Secondary swells
There is rarely one swell alone. A secondary swell, from another storm or another direction, lies over the main one. The week's table separates them: the main one, then the others.
Two swells cross and make sets: when their crests coincide, the waves add up; in between, the sea calms down. A short secondary swell from the local wind chops up the main one; a long one from another direction gives peaks that move.
Read the main swell for the size, and the secondary one for the shape. A spot whose window takes only one of the two does the sorting by itself.
| Main swell | 1.5 m · 14 s | WNW |
|---|---|---|
| Secondary swell | 0.6 m · 8 s | SSW |
Raise the secondary swell: the two trains cross, and the bigger becomes the main one.
Remember: The main swell makes the size, the secondary one makes the shape.
Shadow and refraction
A swell goes straight. An island, a cape, a peninsula cast a shadow on it: behind them, it arrives weakened or not at all. That is why a whole coast can be flat with two metres out at sea.
Refraction softens the shadow: as it nears the shore, the swell slows where the bottom rises, and bends. It gets around a headland, wraps into a bay, and arrives square to a beach it was aiming at from the side. It is what makes point breaks peel.
Two spots under the same swell can be in the shadow for one and exposed for the other, a few degrees apart. The exact direction therefore counts more than the height when there is an obstacle between the open sea and you.
Turn the swell around the island: the spot enters its shadow and leaves it. At the shore, refraction brings back a little of the swell the shadow hides.
Remember: Behind an island, a few degrees of direction make all the difference.
An alert drawn from your sessions
All this leads to one thing: knowing what your spot likes, and letting it tell you. Tonnr keeps a log of your sessions: where, when, a rating, and what the sea was doing, which it finds even for a session written afterwards.
From your best-rated sessions, it draws the conditions you like at a spot: the height, the period, the wind, in the same words as an alert. The more you rate, the tighter the range.
An alert made of those figures watches the week for you and tells you the evening before. That is the end of the course: you no longer read the forecast, it reads you.
- 1.4 m · 13 s · 6 kn
- 1.1 m · 12 s · 9 kn
- 0.6 m · 8 s · 14 kn
- 1.8 m · 14 s · 5 kn
- 2.4 m · 11 s · 22 kn
- 0.9 m · 10 s · 11 kn
Tell me when the waves are 1.1–1.8 m, with a period of 12 s or more, the wind is 9 kn or less.
Raise the lowest rating: only the best sessions remain, and the alert tightens on what you really like.
Remember: Rate your sessions: your alert writes itself.
You can read the sentence now.
Find your spot, read what the sea is doing there right now, and once you know what it likes, Tonnr tells you.