Surf's up! Those are the words you've been waiting to hear. You finally made it to the beach. You have your surfboard in hand. You're ready to go. You've just been waiting for the lifeguard on duty to give the go ahead. She utters those magic words and you're off, diving into the surf in search of a wave to ride back to shore.
If you've ever been to the ocean and spent time on the beach, you know that there are few joys as precious as feeling waves crash all around you, cooling your body on a hot summer day. All you have to do is sit back and relax at the water's edge, as wave after endless wave takes away the heat and stress of the day.
But where do all those endless waves come from? Is there a giant sitting in the ocean on the other side of the world, kicking his feet to make round after round of waves? Or perhaps there's an underwater wave machine that runs night and day to create those never-ending whitecaps? Or could science provide the answer to the question of waves?
Science can and indeed does provide the answer to what causes waves. Oceanographers who study waves will tell you they're the result of something else that starts with a “w": wind! If you've ever blown on a hot bowl of soup and noticed the ripples caused on the surface by your breath, then you already have an idea of how wind creates waves in the ocean.
Although certain waves called tsunamis can be created by underwater earthquakes, most waves result from the work of wind on the ocean's surface. Through the forces of friction and pressure, wind transfers energy to the surface of the sea, creating first ripples and eventually the swells we call waves.
In any group of waves, there will be several peaks called crests that are separated by low points called troughs. Scientists measure several aspects of waves. Their height is called amplitude, while the distance between successive crests is called wavelength. The time that elapses between successive wave crests is called the period.
The more wind there is, the more and bigger waves there will be. The maximum size of waves will depend upon three factors: wind speed, wind duration, and the fetch, which is a measure of the size of the area over which the wind is blowing. Once waves have reached their maximum size for a particular wind speed and fetch, oceanographers say that the sea surface is “fully developed."
In the open sea, waves tend to travel at a constant speed, unaffected by the depth of the water. Once they begin to approach a shoreline and encounter shallow water, however, then waves begin to change and resemble the waves we're familiar with as they crash onto the shore.
When we think about waves, we usually only think about the part that's visible above the level of the water. In reality, however, waves extend down through the water column to the ocean floor. As the water level decreases closer to shore, the bottom part of a wave begins to drag along the ocean floor.
As the bottom part of the wave begins to drag along the ocean floor, the upper part of the wave begins to move faster than the rest of the wave. The closer to shore the wave gets, the more the bottom drags and the faster the upper portion travels.
Eventually, the upper part of the wave travels fast enough that it begins to tilt forward. That tilt eventually turns into a curl, creating what we call a breaker. It's at this point that we see the traditional rolling shape of a crashing wave.