WHY DO WAVES COME IN SETS?

This is a very complex question which has been a personal quest of mine to figure out for nearly 30 years — and this is what I’ve learned:

The wind in the midst of storms usually does blow in intervals, which we call gusts. The primary swell producing component of wind is consistency. So the lower-sustained, most consistent end of the wind speed spectrum is what really generates the swell. But still, the more the wind gusts above that range, the more that energy is transferred into the water.

Once the swell leaves the storm-generating area, the swell energy bonds together and travels in groups of waves we call sets. The waves within the sets cycle together with other waves which helps to conserve their energy over vast distances of ocean.

So how come some sets arrive every 15 minutes with only one or two waves, and other sets arrive every 5 minutes with 10 waves in each set?

On the other hand, strong high pressure following a storm creates stronger, more consistent sustained wind speeds which will not only result in a bigger swell, but also more waves per set and more consistent sets.

One part of the answer goes back to the wind. Low pressure systems without a solid high pressure system filling behind in the low will result in very gusty, flukey winds. The winds may range between 20-50 knots but aren’t sustained enough to transfer enough energy into the water. As a result there may be some big waves, but the number of waves per set will be less, and the consistency of the sets will be less frequent.

On the other hand, strong high pressure following a storm creates stronger, more consistent sustained wind speeds which will not only result in a bigger swell, but also more waves per set and more consistent sets.

I’ve had the unfortunate experience of being caught in the middle of way too many storms on a 50-foot sailboat we used to race to Hawaii and deep Mexico. After many of these storms, I’ve been able to monitor exactly what the surf climate was along the shore compared to what we had to
sail through days prior.

The wind is usually gusty in these storms — which are also the major swell producers of surf along our coast. The wind speeds may hover between 25-30 knots, but then suddenly gust to 40-45 knots for a few minutes in the midst of squalls or trough lines. On the sailboat we were always looking for these squalls so we could reduce sail and not threaten the mast and rigging. Sometime there would be
associated clouds and other times we could only see the wind on the water.

Nighttime sailing was even more fun because the squalls would slam you in the dark. Other storms had strong high pressure filling in behind the low, and we were blown away with how consistently strong the wind speeds were in those types of storms. These storms also always had incredible consistency to the seas and the resulting surf along shore.

A second part of the answer is the length and “width” of the fetch, which will combine with the consistency of the sustained wind speeds to determine the number of waves per set as well as the consistency. Everybody know fetch length is an important component, but the width of the fetch plays a huge role in the number of waves per set and the set consistency.

A third part of the answer is how centered the swell focus is on your spot. If a swell coming up from the Southern Hemisphere and the focus is centered on Mainland Mexico, this is where you’ll find the maximum energy, which is the maximum number of sets and the most waves per set. As you move up toward California, you get out on the edge of the swell. As a result you find less consistent sets AND less waves per each set.

A final part of the answer are the local effects of the bathymetry and the ocean floor outside of your spot. If the ocean floor is favorable much more energy will wrap in (refraction) which will result in more sets and more waves per set. Other areas with less favorable bathymetry will result in less consistent sets and fewer waves per sets.

So, not an easy thing to figure out due to so many variables. But if we do see a storm with high wind speeds, a long and wide fetch, with a strong high pressure filling in behind, and the fetch is pointed toward us at the right spot, we can bet on lots and lots of waves.

—Surfline founder Sean Collins