Reading the forecast

Every number in Rosa, in plain terms.

Nothing here is secret. If a figure looks wrong to you, your eyes win - and we would rather you knew exactly what we are claiming than trusted it blindly.

The word: Flat, Poor, Fair, Good, Epic

The one-word verdict on each hour, from a score out of 100. Three things feed it: the size of the surf, the period of the swell, and the wind. Good size and long period with light or offshore wind scores high; small, short, or blown-out scores low.

Wind can only ever polish waves that already exist. A glassy offshore morning with no swell stays Poor - it cannot be talked up into Good by the wind alone.

Surf - the range, and the number under it

This is the height of the breaking wave at the spot, not the wave height out at sea. Rosa takes the open-ocean swell and walks it into the beach over the real shape of the seabed, so it grows, bends, and finally breaks the way it actually would there.

One honest limit on that. The public seabed maps we use (GEBCO) stop well short of the surf zone - at the average spot the shallowest depth they give us is about 7 metres, and waves do most of their growing in the last metre or two. So Rosa walks the swell in as far as the map goes and then finishes the last stretch with a published formula that predicts breaker height from the swell's height, period and angle alone. Until 2 August 2026 it did not finish that stretch at all, and every spot read too small as a result.

When more than one swell is running, Rosa asks how far apart they are pointing. Swells arriving from roughly the same direction are one body of water and their energy adds up. Swells crossing from clearly different directions do not: they break at different angles on different parts of the beach, and the wave you actually ride belongs to one of them, so the bigger one sets the number. Until 3 August 2026 Rosa added every swell together regardless of direction, which made busy multi-swell days read too big.

We show a range because no forecast is precise to the centimetre. Words like waist-high or overhead are the same number said out loud.

The smaller number underneath is a different measurement: the height of that swell out in deep water, before it reaches the coast, carrying its unit so you can tell the two apart. Either one can be the bigger, and that is not a contradiction. A long-period groundswell arrives at the beach taller than it was at sea - it stands up as the water shallows, which is why a 1.5 m reading offshore can be well overhead at the reef. A short-period wind sea does the opposite and loses most of itself on the way in. Period and the angle it meets the coast at decide which happens.

Swell - where the energy is coming from

A swell is one train of waves travelling from one storm. There are usually several at once, so Rosa lists them separately rather than mashing them into one figure.

The direction is where the swell is coming from. Each spot faces a certain way and only picks up swell from a band of directions - swell arriving outside that band still shows in the list, but it does not count toward the score, because the headland or the coast blocks it.

Period - and why it says 8s when the maths uses 6s

Period is the gap in seconds between waves, and it matters as much as height: long-period swell carries far more energy and stands up taller when it hits shallow water. Short-period windswell is weaker than its height suggests.

The models publish two of them. The peak period is the dominant rhythm of the swell - the number surf reports quote, as in "3ft at 8 seconds". The mean period is the average across the whole sea state, and it always reads lower because short, choppy waves drag it down.

Rosa displays the peak period, because that is the convention you already know and the honest description of the swell. As of 30 July 2026 the surf height is judged on the peak period too: we used to rate how rideable a swell was from the mean period, and it under-called real Mediterranean surf by up to double against what we saw in the water. Two smaller pieces still run on the mean period - the arithmetic that bends waves as they shoal in, and the Period bar on the rating card - so the seconds on screen can still sit a little above some of the seconds doing the work. We would rather show you the number surfers use and say this plainly than quietly show the lower one.

Wind

Shown in knots, with the direction it is blowing from, and a word for what that means at this particular spot:

• Offshore - blowing from the land out to sea. Holds the wave face up and grooms it. The good one.
• Cross-shore - along the beach. Tolerable when light, messy when not.
• Onshore - blowing in from the sea. Crumbles the waves and turns the surface to chop.

The same wind direction is offshore at one beach and onshore at the next, so the word is worked out per spot, not per region.

kJ - how much punch the swell carries

Wave energy, in kilojoules per metre of wave crest. Height alone does not tell you whether a swell is serious: energy grows with the square of the height and the square of the period, so a long-period swell can carry several times the punch of a wind chop of the same height.

Use it to compare days. Two mornings both listed at chest-high are not the same session if one reads 40 kJ and the other 300 kJ. Only swell the spot actually faces is counted.

hPa - air pressure

Sea-level air pressure, the same units a barometer uses. It is a weather signal, not a surf one: falling pressure means a system moving in - wind and, a day or two later, swell. Steady high pressure usually means settled, lighter mornings.

Water and air temperature, and the wetsuit

The wetsuit suggestion comes from the water temperature alone, using the industry-standard bands. It is a starting point - if you run cold, or the wind is up, go thicker.

Tide

The height of the water against its average level, hour by hour, with the high and low marked, for the full forecast - even two weeks out. This is the astronomical tide, computed from each spot's own harmonic constants (the same maths behind a printed tide table). It does not include what the weather pushes on top - storm surge can sit above or below it by a few tens of centimetres.

How much it matters depends entirely on where you are. On the Mediterranean the tide moves maybe fifteen centimetres and is barely worth a glance. In Costa Rica it swings a metre and a half or more, and it changes everything.

Light

First light, sunrise, sunset and last light for that spot's own date and location. First and last light are the useful ones - you can usually surf a good half hour before the sun is up.

Confidence, and how fresh this is

Rosa runs several times a day and always shows when it last ran. Confidence reflects two things: how far ahead the forecast is looking, and how much the different weather models agree with each other right now. Tomorrow is worth trusting; day twelve is a hint about a trend, nothing more.

The day strip and the three bars

Each day shows its best few hours as coloured bars, so you can find the good day without opening every one. The Size, Period and Wind bars break the chosen day's rating into its three ingredients - a quick way to see why a day scored what it did, and which of the three is letting it down.

Day ratings only count daylight hours. A perfect 3am is not a session.

Colours

The coloured rail down the left of each hour is the rating, in visual form - the same scale as the words, so you can scan a day without reading a single number.

Units

Metres or feet, and Celsius or Fahrenheit, are yours to set in Profile. Wind is always in knots and period always in seconds, because that is what surf forecasts everywhere use.

Swell alerts

If you turn them on, Rosa notifies you when a real swell is lining up for a spot you follow - a sustained run of well-above-normal energy that peaks at Good or better. One swell gets one notification, not one per spot per hour.

What Rosa does not know

It does not know the sandbank shifted last week, that the river mouth is closed out, that the car park fills at seven, or that the peak works on a dropping tide only. Local knowledge beats every model, including ours.

Forecasts are estimates. Check the conditions with your own eyes before you paddle out, and never let a green number talk you into water you would not otherwise enter.