Origin of surfable waves

Probably useful for surfers looking to extend their understanding of ocean waves beyond the star rating in your favorite forecasting app.

Stay for some explanation about waves, swell, period, wavelength, dispersion, surf forecasting. Part 2 for surfing statistics in Japan, attached dashboard & nerdy things about using python, Dash, docker, pandas, AWS for serving dashboards.

Photo by erika m on Unsplash. Could be from Japan, who knows.

After starting to write about my dashboarding of surfing conditions in Japan, I noticed that I got too deep into the formation of surfable waves and surf forecasting so splitting the post in half. Anyway recently some people asked me how to read surf forecasts so maybe this post can help to understand what is needed for surfable waves. If you already know that stuff, I still recommend checking the videos I linked here. In the part 2 more about the dashboard project & surfing conditions in Japan.

Formation of swell

To be able to surf, waves needs to have a certain size and steep enough face so that a surfer can glide on it. Furthermore, the size is not everything. If the wave period is small and the sea is a mixture of different wave patterns, the conditions still suck.

Yea dude surf’s up. Waves near or inside the storm. We need some travel time for these waves to clean up. Prob will be pumping somewhere a day or two later.

The birth place of surfable waves are the strong winds like in the photo above. Initially friction between the wind and the sea creates ripples, which will be pushed by the wind to even bigger ripples, eventually growing into big messy waves. For those interested in this part of the process, check the video here.

The waves are created by the friction between the water and wind at the storm area. The waves might be high but it’s all a mess. After the storm ends and the waves travel at the open sea, the clean long period waves separate, “disperse”, away making the surfable long period waves, the “swell”.

At some point the storm ends, moves away or waves propagate outside of the storm area. While the actual dynamics of ocean waves are complicated, roughly at open deep sea the wave speed is proportional to the square root of the wave length and at shallow waters to the square root of the depth. That means that at the open sea, the large wavelength (long period) waves travel faster than the short period ones. After a while the long period waves are separated from the small messy wavefronts, creating the “swell”. To sound smart, call it “dispersion” of waves (different wavelengths of waves traveling at different speeds). To know more, check this video (tells you also more about sets for example).

“Yea dude the waves are totally dispersed”. Now the surf is actually up, the wave height is decent and the period is long. Waves have travelled far enough from the storm so that the dispersion of waves have done its magic and the messy wind ripples have turned into a surfable swell.

So after the waves have travelled and cleaned up to a proper swell they at some point approach the shore, hopefully at your favorite surf spot. There’s a bunch of factors determining how the wave itself breaks and depending on surfers skills or preferences different types of waves are in demand. Roughly the wave breaks when the wave height is 70% of the bottom depth. More about the shoreline dynamics here.

In surf forecasts as well as wave records data the swell period, height and direction are normally reported. Why talk about the wave period instead of the wavelength? Because, once the waves approach shallow waters, the speed of the wave and the wave length decreases, but the period stays constant. A decent surfable swell starts from 9s period. Well depends on opinion, might be still quite crappy on Indo standards but great somewhere else. Swell height from maybe 0.7m starts to be “surfable”, but again matter of opinion, spot and size of your surfboard.

Wind direction

For good surf conditions, one need decent swell, but that is not enough. Wind from the sea, onshore wind, creates small waves which mix up with the already cleaned swell. This makes the sea bumby and “mushy”. In addition the wind presses the waves to break earlier making the waves less steep.

The offshore wind has less effect on the waves, possibly making them even more steeper compared to no wind case. Since we surf close to the beach, the wind from the land doesn’t have time to make any significant waves to mix up with the beautiful base swell.

As a summary, we want offshore wind direction or all most no wind for good conditions.

Onshore wind makes mushy waves.

So to summarize, for good surfing conditions to formulate, one needs a storm or at least strong winds far away to generate the ripples, which will then pack into bigger waves. These waves need some time, typically at least a day, to travel at the open sea. During this time the long period waves travel faster and separate from the low period mess. After this separation of periods, the long period waves are cleanly separated to “a swell”, which should hit the beach in a suitable angle. Lastly, when the swell hits the beach, the wind should be blowing from the beach to the sea (“offshore wind”).

Diffraction and refraction

As mentioned before, in shallow waters the wave speed is proportional to the square root of the depth. Thus the shallower it comes, the slower the wave moves. This leads into waves wrapping a round obstacles like the peninsula or island below, phenomena called diffraction. Refraction is when waves push through for example a space between two islands.

This wave bending phenomena creates surf into places which are not in clear line of sigh to the original storm centers as well as affect the surf spot dynamics often like in the photo below.

Diffraction of waves, especially the long period waves have the ability to “wrap around” peninsulas and islands, making surfable waves even behind the peninsulas or islands.

Example of a surf forecast

We need decent swell height and period in a suitable angle, and no wind or offshore wind. Let’s take a snapshot of todays forecasts around Chiba, in Japan provided by windy.

Today the conditions are sure better at Torami, north of Isumi where both primary and secondary swell can hit the shore and where the wind blows from the land to the sea. The primary swell is at 9s period, which is decent, but the wave height is not too big, so most likely small but clean surf. Any place with a beach facing east but having something block the wind from the south would work today.

The surf forecasting is a bit of an art form and normally requires knowledge of the local spots + sometimes the forecasts are completely off, but hopefully you got something out of this post. Next part is more focused on introducing a dashboard to visualize surfable waves in Japan.

Screenshot of the app exploring statistics of surf conditions in Japan.