How are waves formed?

Every wave starts somewhere - from wind chop to the bombing sets at outer reefs. Understanding how these waves form helps us read forecasts better, predict conditions, and maybe even score better sessions. Let's start at the beginning.

In general, the waves that we surfers care about come in three forms: tides, which we've covered before, and two types of wind-driven waves. Capillary waves are the small ripples you see when a light breeze hits smooth water - they're controlled by surface tension, like water straining against itself. Surface gravity waves are what we actually surf - they're the larger waves where gravity pulls the water back down after wind pushes it up. Every swell starts as tiny capillary waves before growing into proper surface gravity waves.

Both air and water are considered fluids, meaning they don't have a rigid shape and respond pretty easily to pressure. Picture what happens when a gust of wind blows over glassy water - it gets a lot of texture. Water is more viscous than air, which means it resists changing shape more. This viscosity plays a key role in how wind energy transforms into the waves we ride.

It starts with capillary waves - tiny ripples that bounce up and down due to surface tension rather than gravity. When wind drags across the water, it creates friction between air and water, essentially wrinkling the surface. Once these ripples form, the pressure difference in the air adds momentum, making them bigger. This same mechanism that creates surface texture on a calm day can develop into significant wave energy under the right conditions.

Here’s a picture of both the tiny capillary waves and their transition into gravity waves.

These small waves give the wind something to push against. As long as wind keeps blowing on them in their direction of travel, they'll keep gaining energy. The transformation from ripples to surfable waves depends on three key factors: wind strength, how long it blows, and the distance over which it blows (what we call fetch). Strong winds blowing for extended periods across large stretches of ocean create the conditions for swell formation.

Wind isn't just creating waves in one spot. It was originally believed that waves in Hawaii and California came from relatively nearby - that the equatorial trade wind region would cause waves from further south to decay. Then in 1963, oceanography legend Walter Munk highlighted just how far waves could travel in his documentary "Waves Across the Pacific." He tracked swells over vast distances, proving that waves can maintain their energy across thousands of miles of open ocean. This explains how Southern Ocean storms can send solid surf to Hawaii, or why California can get perfect waves from weather systems near Japan. The ocean, it turns out, is remarkably efficient at moving energy across entire basins.

On the way out from their stormy birthplace, they sort themselves by period - a process we call dispersion. Longer period swells travel faster than shorter ones, which is why a distant storm's arrival often follows a predictable pattern. The longest period waves arrive first, gradually transitioning to shorter periods over subsequent days. Storms closer to shore generate waves that arrive with their periods more closely grouped together, since they haven't traveled far enough to spread out.

A diagram of wind pumping energy into waves. This has a little more than what we’re talking about, with shear instabilities and the velocity profiles, but is still a good diagram for understanding the pressure setup. Credit: Seung Joon Yang - Own work.

Through wave-wave interaction (math-speak for really complicated energy transfer), the most stable wave patterns emerge as distinct groups with similar periods. As more energy pumps in, these waves grow into surface gravity waves, where gravity takes over from surface tension as the main force at work.

Surface conditions reveal the ongoing interaction between wind and water. A glassy morning shows us swells in their purest form, undisturbed by local winds. As winds pick up through the day, they transfer energy into the water, creating surface texture and eventually wind waves that interact with the underlying swells.

In essence, the waves we surf form through the same process as afternoon wind chop - they're just born further away and have had time to organize into more powerful, refined swells. Understanding these wave dynamics helps explain why checking wind and wave period forecasts matters so much for scoring good surf in the days ahead.

Further Reading:

Waves Across the Pacific

Capillary Waves

Surface Gravity Waves