TonnrReading a week of forecasts and finding the right window
You surf already and want to pick your day. Swell and wind sea, energy, fetch, a week read at a glance, and why two spots a few kilometres apart do not work on the same day.
Swell and wind sea
The week's table shows two things in the waves: the swell, which comes from afar, and the wind sea, raised by the day's wind. The total height is the sum of the two, and it is what many sites announce.
The two are not worth the same. Tonnr shows them apart: the swell alone says what there will be to surf, and the wind sea says whether it will be clean. Two metres announced, one of them wind sea, is one metre of swell in a mess.
Look at the swell's line first, then its period. Then ask what the wind adds: if it is light or offshore, nothing; if it has blown from the sea for hours, a wind sea that chops everything up.
| Height | Period | |
|---|---|---|
| Swell | 1.2 m | 13 s |
| Wind sea | 0.3 m | 5 s |
| Total | 1.2 m | – |
The swell stays at 1.2 m and 13 s. Raise the local wind: the wind sea adds up, and the total grows without the session getting any better.
Remember: The total height lies; the swell alone tells the session.
Energy
Height and period are read together, and one figure holds them: the energy, which the week's table writes in kilojoules per metre of crest. It grows with the square of one and the square of the other.
1 m at 14 s carries as much energy as 2 m at 7 s. The first is a clean swell that will stand up at the shore; the second, a short sea that crumbles. At equal energy, the long period always wins.
On a spot you know, note the energy of the days you liked. It is often a better marker than the height, because it does not change meaning from one swell to the next.
1 m at 14 s and 2 m at 7 s both make 384 kJ/m. Slide both: the period weighs as much as the height.
Remember: Two figures in one: at equal energy, take the longer period.
Fetch
A swell is born of a wind that blows long, hard, in the same direction, over a wide stretch of ocean: the fetch. The longer the fetch and the stronger the wind, the bigger the swell, and the longer its period.
Once on its way, the swell travels alone, and the long periods go faster: a 15 s swell moves nearly 1,000 km a day. That is why a storm on the other side of the Atlantic announces itself four or five days ahead, and a low passing near the coast raises a short sea without warning.
On the map, a red patch out at sea is a storm; the color that spreads towards the coast over the following days is its swell arriving, sorted by distance.
The beach is 2,000 km away. Strengthen the wind and lengthen the fetch: the swell grows, its period lengthens, and it arrives sooner.
Remember: A long period is a distant storm: you can see it coming days ahead.
Reading a week
A week is read at a glance: first the swell and its period, day by day, to see when it comes in; then the wind, hour by hour, to see when it stays clean; then the tide, to pick the hour.
The right window is when the three cross: enough swell, a period that holds, and a wind under 10 kn or offshore. It often lasts three hours, not a day: the morning before the breeze picks up, or the evening when it drops.
That is exactly what an alert of Tonnr watches for you: you say what your spot likes, it reads the week and tells you the evening before.
Run through the three days: the hours that work are marked under the bars. Look for when the swell is there and the wind is still asleep.
Remember: Look for the hour, not the day.
Two neighbouring spots
Two spots five kilometres apart do not work on the same day, and that is normal. Each has its window, its exposure, its bottom and its tide.
A beach that faces west takes everything that comes from the Atlantic and saturates fast. A cove that faces south, behind a headland, only receives the swells that turn, by refraction, and stays surfable when the beach is closed out. The same wind is cross-shore here and offshore there.
That is the game: when the exposed one closes out, the sheltered one lights up. Each spot's page says what it likes; compare them before you load the car.
Turn the swell: each spot has its ring. A north-west swell lights up the beach; a south-west swell wakes the cove.
Remember: When your spot closes out, its sheltered neighbour may be opening.
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.
Next guide: Advanced · Models, buoys and secondary swells: reading a forecast in depth