As these peaks move away from the wind, they spend some of their energy through motion. This causes the peaks to smooth out into rounded swells. The swells collide with one another, and some of them combine through constructive interference. The larger, rounded swells begin to travel in approximately the same direction as the prevailing wind that originally created the whitecaps.
The swells become breaking waves when they reach shallower water. This can happen at the shoreline, at a point extending into the ocean or when the waves pass over an obstruction like a sandbar or a reef. Here's what happens when the wave gets to water that's about half the depth of its wavelength:
- The swells slow down as the water beneath them gets shallower. As a result, the waves get closer together, much the way a line of cars will get closer together if the car in front starts to slow down.
- The leading edge of the swell becomes increasingly vertical as it slows while the trailing edge continues to look like a rounded slope.
- The waves get taller as the solid surface under them and the waves' energy pushes the water upwards.
Eventually, the wave crests, or breaks -- the fast-moving back of the wave spills over the slowing front of the wave. The exact shape of the ocean floor has a dramatic difference on how the waves break. If the shore slopes gently upward, the wave will gently spill over as it crests. A steep slope can cause waves that break suddenly and dramatically.
The cresting waves can travel for thousands of miles before reaching shore and becoming conducive to surfing. They have an enormous amount of power and momentum. For these and other reasons, they can become dangerous to even experienced surfers. Next, we'll look at surfing safety and the dangers inherent in the ocean.