The Physics of Waves
Part of the sport of surfing is the search for big, interesting waves that are fun to ride. These waves can be huge, like Mavericks off the coast of San Francisco, which can reach up to 50 feet (15 meters). Another famous surfing wave, the Banzai Pipeline, breaks over a reef off the coast of Oahu, Hawaii. It's one of many plunging waves that creates a pipe-like space, or barrel, that surfers can fit inside. Some of these waves are so big that swimmers can't swim out to them safely. Reaching these waves involves tow-in skiing, or traveling to the wave by being towed behind a personal watercraft.
No matter how big or interestingly-shaped popular surfing waves are, they all form because of two basic factors:
- The interaction between wind and water
- The interaction between water and land
A third influence is the tide. Lots of different factors contribute to the Earth's tides, but the pull of the moon's gravity on the Earth is the biggest. There are also other factors that can contribute to tsunamis and other rare types of waves, but wind, water and land do most of the work when it comes to the waves used for surfing.
To understand how they form, it's helpful to know a few basic facts about ocean waves. Waves are essentially energy moving through matter. If you looked at the cross-section of an idealized ocean wave, it would look like a transverse wave. The surface of the wave moves up and down, which is perpendicular to the left-to-right direction that the wave itself moves.
But ocean waves are a little more complicated than ordinary transverse waves. They're really orbital progressive waves. The water molecules that make up the wave move in circles, or orbits, as the wave progresses. You can visualize this movement by thinking of the particles near the wave's surface. If the wave is passing in front of you from left to right, the particles move in a circle in a clockwise direction. They move up the wave, across its crest and down into its trough.
The ocean's orbital waves get their start when wind blows on the open ocean. A gentle wind doesn't have much of an affect - it makes ripples in the water that spread the same way ripples do in a pond or a fish tank. But the stronger the wind becomes, the more it pushes against the water. It transfers energy to the water as it makes peaks and white caps in the water's surface. This region of white caps is chaotic, and the water can move choppily in random directions. The churning peaks give the wind more surface area to grab on to, which lets the wind force the water into even higher caps.
The height and shape of the white caps comes from three primary factors:
- How long the wind blew over the water
- How hard it blew
- The surface area of the ocean that the wind affected, or the fetch
A very hard wind blowing for a long time over a large expanse of ocean will lead to large, frothy white caps. These eventually become large waves, which is why surf conditions are often good after a storm at sea. Satellite data used to track surface winds from outer space has helped forecasters predict where the surf will be high based on oceanic weather patterns.