In this episode of Surf Simply's animated exploration of surf science 'Surfing Explained', we introduce some of the metrics used on surf & wave forecasting websites in the first of a series on wave mechanics and meteorology.

We're going to cover subjects like Wave Period, Wind and Tide patterns, and how waves change as they approach shore.

Script:
Between the time when wind blows over a distant ocean to the moment you paddle into a wave, there have been an almost incalculable number of influences and interactions that have shaped your wave into the rising wall of water you are planning to ride, and were going to be looking at some of those impactful, and interesting areas of surf forecasting.

For starters, if we take a typical wave forecasting chart, there will be three main values listed, describing an upcoming swell; the swell height in feet or meters, the swell period measured in seconds, and the swell direction given in degrees or cardinal & inter-cardinal directions like north or north-east.

The swell height in feet or meters is the top to bottom, trough to crest, height of the waves when they’re out in the open ocean - not a shoreline measurement since local bathymetry such as the steepness of an individual beach will morph the final height of a wave when it breaks at the shore - but more on that in the future.

This measurement is also the average height of the biggest 1/3rd of waves in a swell, so we can assume the actual wave heights will range to be both shorter and taller than the number listed.

Open ocean waves range from a mere ripple on the ocean surface, to over 50ft in height during the most extreme weather, and often we focus our attention on swell height as it’s defining measurement, but in reality both swell period, and direction significantly impact what you might expect to see at your local beach.

Generally in science we talk about wavelength, which would be measured in feet or meters for surfing waves. However measuring the ocean comes with many challenges, and so we use the gap in time between two waves, normally measured in seconds, as a substitute. This is known as the Swell Period.

The gap between the waves gives us a much better indication of how much power is contained within the wave itself, as much like an iceberg the majority is often hidden below the ocean surface.

Wave energy travels in an orbital path similar to a rolling ball, and measuring the gap between the waves essentially allows us to measure the diameter of that ball and by doing so determine the depth of the energy.

Like the swell height, this number is also an average, although this time of the full range not just the top 1/3rd, so we can expect some variation to what is listed on the report. Swell periods generally range from 5 seconds for a low energy wave to 25 seconds for those ‘Perfect Storm’ purple blobs we sometimes see in news reports, and this correlates with wavelengths between 40 and 900 metres in the deep ocean.

As swell period grows, the area of our circle, and the amount of energy it contains, expands exponentially. These more powerful waves also travel much faster through the ocean, so that the largest waves from a particular storm normally arrive at the beach first, which is why conditions and wave size can often change in just a few hours.

Finally the swell direction. This is a compass direction and depending on your forecast this will be given in either degrees, cardinal and intercardinal directions or maybe just an arrow. With arrows, North is always up, and swell direction is always given as where the swell is coming from, rather than where it is going. So a swell that is traveling from south west to north east is listed as SW or 225 degrees.

Waves are effected by the angle and geography of the coastline, and can slow or bend from drag when in contact with shallower waters. Therefore the more directly a swell lines up with a beach, the more likely it is to arrive with all it’s energy intact and the bigger and more powerful the waves are likely to be.

Bigger and more powerful doesn’t always mean better. Predicting the characteristics of a wave that has travelled a thousand miles with greatly changing conditions is a complicated science.

So when or what kind of wave will finally arrive at your particular beach with a reasonable accuracy is a big ask of the forecasting companies, so having a personal understanding of your coastline and conditions can greatly improve your chance of getting out of bed on the right day.

So join us next week as we dive deeper into some of the science we mentioned in this video.

Thanks for watching, and we’ll see you then.

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