Swells 101: How Are Waves Formed?
Posted by DAVID KELLY
It might seem like the simplest of questions, but the reality is that many surfers don’t actually know where their waves come from. And little wonder. In an age of computer-guided swell models and customized spot forecasts, it’s easy to live from surf reports to surf reports rather than understanding the science ourselves.
But 20 years ago the idea of an online swell model didn’t even exist, and 20 years before that the only surf reports in existence were word of mouth from the guy who got to the beach earlier than you. Go back a couple of hundred years and waves were the stuff of legend, their origins inspiring confusion and wonder, or at the very least leading to myths such as the ever-popular “full-moon swell.” But swells have always been much simpler than that—although they are created by powerful, destructive forces, they certainly aren’t the handiwork of gods and demigods. Instead, they are easily explained by science, which is exactly why we are able to have such consistently accurate forecasts.
Waves are caused by the action of wind over the surface of the water. As all of us have seen on breezy days, wind causes ripples on the water, and the stronger that wind is, the bigger the ripples get. If a strong wind blows in one direction (also known as a “fetch”) for a long enough time, the ripples will become bigger and bigger—and the bigger they become, the more surface area they have for the wind to act upon, which makes them even bigger still. Thus, waves are the product of storms blowing over the ocean. The bigger the storm and the longer it blows in one direction, the bigger the waves will become.
But this is only the first step in the creation of what we know as a swell. The waves that are produced by the wind move through the ocean, just as other types of waves move through literally every substance on earth, from liquids to gases, and even solids. And they move in relatively straight lines, as long as they don’t run into anything that might impede their progress or cause them to turn or refract. The farther they travel, the cleaner and stronger the waves get—although they also suffer from decay as they move through the water, and eventually get smaller and smaller. Thus, the ideal location of a storm that is producing swell (at least from a surfer’s perspective) is far enough away to create clean, powerful waves, but not so far that the waves have decayed too much by the time they arrive.
The major storm-producing regions are in the far northern and southern latitudes. Storms are the product of a combination of factors, including changes in surface and atmospheric pressure, temperatures, and moisture levels. The colder waters of the northern and southern reaches of the various oceans tend to create the strongest storms, so, for instance, the Pacific tends to see storms originating up near Russia or the Gulf of Alaska, or down near the tip of Chile, while the Atlantic and Southern Oceans see storms generated off the coast of Canada or down south of Cape Town. Coastlines such as those in the Pacific Northwest, Chile, South Africa, and Portugal tend to bear the brunt of these storms and the swells they produce, which is why they are some of the most wave-rich regions on the planet and home to many of the biggest waves in existence.
Hawaii is a special case, as it enjoys a rather central location in the Pacific. Although it is quite a way north of the equator, it is not so far north as to completely miss out on the swells generated in the South Pacific. And just about everyone who has ever surfed knows that the North Shore receives some of the biggest swells in the world each winter, courtesy of storms in the North Pacific. In fact, winter is the single biggest producer of swell in general. While swells can be created by tropical and subtropical storms during the summer, fall, and spring, the biggest storms consistently occur during winter, when the difference between the temperature of the ocean surface and the temperature of the atmosphere is at its greatest.
Thus, coastlines that face north enjoy swell during the northern hemisphere winter (approximately November through March), while south-facing coastlines get their biggest swells during the southern hemisphere winter (May through September). This seasonal pattern has implications for just about everyone, from locals where the storms generate (who suffer damage and destruction) to surfers thousands of miles away (who wait patiently for winter to bring them swells). So the next time you put on Bruce Brown’s classic surf flick The Endless Summer, you can smugly inform your family and friends that Hollywood had it completely wrong. Surfers aren’t chasing the endless summer—it’s winter that we are obsessed with!
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