Surfer’s guide to Swell direction

Introduction to Swell direction

Swell direction as shown on a Surf Forecast shows in degrees of a compass the direction the waves are travelling from, when they reach the coastline. The bathymetry of the coastline will bend the waves and change the direction and nature of the waves according to the laws of Physics. Tidal depth of the ocean before the wave breaks, in conjunction with wave period, wave height, local winds and bathymetry where the wave is breaking, along with the swell direction, are all factors that dictate how the wave will break.

While swell direction does influence wave behaviour, the science behind how waves break is much more complex. Swell direction is certainly important in shaping how a wave will break, but it is just one of many factors at play. Read on if you want to find out why sometimes the waves break great and sometimes not.

How Swell Direction is a factor in wave size

In the open ocean, swell direction is largely irrelevant when evaluating the energy and nature of waves. However, it becomes critically important when it comes to surfing at your local beach. Even if the swell is powerful and nearby, if the waves are travelling away from your coast, you simply will not get any surf at all.

Surfline and other swell models have a weakness to present data from primary swells and secondary swells that deliver no surf, typically because they are blocked by a headland or the waves are going to a different coastline. These phantom swells should be totally discounted when using the surf forecast to determine how big the surf will be at your local spot.

A swell approaching at an obtuse angle will generate smaller waves compared to one arriving directly onshore, because wave energy diminishes as the swell bends toward the coast. For example, Uluwatu faces west, so the more westerly the swell direction, the larger the waves will be. A swell coming from 200 degrees is roughly one-third smaller than one arriving at 220 degrees. When reading a swell forecast, it is important to adjust the expected surf size accordingly, considering the swell angle—typically by increasing or decreasing the forecast by a certain amount based on how directly the swell hits the shore.

Offshore islands and nearby landmasses can also influence swell size, depending on their position relative to the wave source. For example, a 200-degree south swell hitting Balian directly will produce strong waves, while a 220-degree southwest swell may be partially blocked by the headland of East Java. The effect of swell direction varies between locations; for instance, swell arriving from the south affects Balian differently than Uluwatu. This variation explains why some days Balian is bigger than Uluwatu (during south swells) and other days smaller (during west swells).

Swell direction also explains why Uluwatu is always bigger than Padang Padang and Bingin. The swell arrives at Uluwatu first due to its source direction, but then it must wrap around Uluwatu’s headland, causing some loss of size and energy. This wrapping is called refraction — the bending of waves as they slow over shallower water — and while it costs some size, it can also clean a wave up and improve its shape. This is why, although waves at Uluwatu might sometimes be a bit disorganised, breaks around the corner like Baby Padang, Impossibles and Bingin often produce cleaner waves. You can read more about how swell bends and focuses over the sea floor in our Science of Waves guide.

Now that you have a better understanding of swell direction, you are ready to read a swell forecast, but to be fully prepared, it is better to also read about wave energy, winds, wave height and wave period.

If you are interested in the technical and scientific aspects of surfing, then our Beginner, Intermediate, and Advanced Video Surfing Tutorials are for you.