Height and Period Must Travel Together

A North Sea afternoon is often two seas at once.

A leftover swell from the north-west. A wind sea building from the west. Sometimes a second, shorter pulse on top. The instruments and the models can describe each of those as its own train: a height, a period, a direction. Together they are the total sea. Apart they are different animals.

The forecast used to be willing to pick a height from one and a period from another.

Total sea height is the easy number. It is what people mean by "it's a metre." Period and direction are often cleaner on the swell component. Stitch them and you get a tidy line: a metre at 8 seconds from 300 degrees. That line can describe a sea that was never in the water. The metre included the wind chop. The 8 seconds belonged to a swell that was only part of the energy. The direction might belong to neither by the time you are on the sand.

Why the stitch is tempting

Surfers want one wave.

Apps want one row. Height, period, direction, go. Mixed seas make that row feel incomplete, so the code completes it. The total height looks more honest than a smaller swell height. The swell period looks more honest than a mean period dragged down by chop. Each field is defensible. The combination is not.

On this coast the stitch is especially costly. A short-period wind sea at a metre is a different session from a longer swell at a metre. Sandbanks filter them differently. A harbour wall cares about direction more than height. If the period you showed belonged to a train that was only 0.4 m, the bank may never see the wave you described.

The North Sea delivers mixed seas often enough that this is not an edge case. It is a normal Tuesday.

What we changed

Each component is now a complete tuple or it is nothing. Total sea, primary swell, secondary swell, wind sea: height, period, direction, and the period statistic, together. If a field is missing, that component is unused. The selected wave is a copy of one complete tuple. It is never a collage.

The app still prefers a complete primary swell when it has one. If it does not, it falls through to a complete total sea, then wind sea. It will not fill a hole in the swell with a number from the total. A missing sea is a missing sea.

Period also has to say what it is. Mean period and peak period are not interchangeable. Treating them as one number overstates how clean the swell is. The snapshot now carries the statistic with the value.

What to trust

When the sea is mixed, believe the component, not the collage.

If the swell is small and the total is bigger, you are looking at wind sea on top of a weaker pulse. Plan for that. If only one complete train exists, that is the forecast. A beautiful period attached to the wrong height is how people drive to a beach that is only chop.

The honest product is slightly less tidy. It is also a sea that could have been in the water.