When you hear about “swell,” it’s most likely in relation to surfing. Swells are more than just great waves: scientifically, swells are defined as “collections of waves produced by storm winds raging hundreds of miles out to sea, rather than the product of local winds along beaches.”
Swell is measured along a number of key aspects, such as direction, height, and swell period. These metrics show up on surf forecasts, but they’re also collected by researchers and marine organizations to learn about coastal impact, flood zones, to improve wave forecasts, and to optimize marine transport and logistics. This guide breaks down some basic aspects of swell, how this data is collected and uses for swell data outside of surfing.
When surfers talk about swell, they’re actually referring to a number of different measurements. The primary data points are related to wave sets: swell direction (SwD), swell height (SwH), and swell period (SwP).
- Swell direction: the direction from which the swells are coming. Swells differ from waves in that they are not produced by local wind and come in at a longer wavelength. Swell direction is expressed on a 16-point compass scale.
- Swell height: the estimated average height of the highest one-third of the swells. Swell height is measured from the very lowest point (trough) to the very highest point (peak) of each wave.
- Swell period: the time it takes for successive waves to pass the same point, expressed in seconds. If more than one swell is present, this is measured during the period of the swell containing the maximum energy.
Surfers also look at a set of measurements known as wind/wave metrics: wind-wave direction, wind-wave height, and wind-wave period. Wave height is impacted by wind speed and duration, as well as the direction of the wind and fetch – the distance over water that wind blows in a single direction. The wind-wave metrics are expressed in terms that relate to swell, combining a number of factors to give surfers the information they need.
- Wind-Wave Direction (WWD): wind-waves are produced by the local wind. If a swell is present, these waves arrive more frequently than the swells. Wind-wave direction tells you the direction from which the wind-waves are coming.
- Wind-Wave Height (WWH): similar to swell height, this is the average height of the highest one-third of the wind-waves.
- Wind-Wave Period (WWP): as with swell period, this is the peak period in seconds of the wind-waves.
Lastly, marine research organizations like NOAA use ocean buoys to collect two more detailed wave metrics: overall height and period (H0 and AVP) and steepness (STP). Overall height and period is the average height of the highest third of the waves. In cases where both swell and wind-waves are present, it will be calculated as the square root of the sum of the squares of the swell and wind-wave heights. Steepness is reported as either "VERY STEEP", "STEEP", "AVERAGE", or "SWELL”.
Surfers are primarily concerned with the size and period of the waves, the swell arrival times, and wind behavior, and the tide times. However, researchers and marine organizations can use wave data for other purposes.
While surf reports help surfers anticipate when to hit the waves, the data used to feed those reports comes from data sources that are also used by marine forecasters. “[Surf reports] are the result of series of computational analysis which gathers information from ocean buoys readings and weather satellites and transforms it into an easily readable chart or animated color-coded maps,” explains SurferToday.
Swell and wave data is used for a variety of purposes, both commercial and research related. Wind speed and wave height help marine renewable energy companies and others improve offshore engineering design. For marine research organizations, swell data provides input for modeling wind and wave climatology and track long-term trends in wind speed and wave height. Inevitably, this helps inform research into climate change and long-term changes in ocean currents.
Swell data impacts life on-shore as well. We can create better flood maps with a better understanding of trends in swell height, swell direction, and steepness. Better flood maps result in improved flood warning systems and a clearer understanding of the long-term changes happening on our coasts (such as erosion).
Most swell data comes from ocean buoys or satellites, and the ubiquity of these sources has improved dramatically over the last decade. Installations of low-cost drifter buoys that move along with ocean currents can provide better swell data from all over the world, expanding the reach and scope of our data collection like never before.
What’s also improved is the sharing of information. Private and public organizations are open to combining data to generate more accurate global wave forecasts. For instance, NOAA’s forecasting models, when used with private sources of data, can reduce forecasting error in some cases by more than 50%.
Swell data can provide valuable information when collected from disparate sources over time. While surfers use this data on a daily basis to hone their sport, researchers can use swell data for better environmental modeling and to understand the coastal impacts of ENSO over time.