One of the joys of going to a beach is listening to the breaking of waves. Waves are typically associated with the ocean, but can also form on lakes, rivers, canals or any body of water with a free surface.
Waves are caused by wind blowing over the water surface, dragging it in a certain direction. As the wind only affects the surface, the water below rises to fill the space, causing a circular movement. This appears as a wave on the surface. The faster the wind blows, the more the surface is shifted and the bigger the waves become. Other factors that determine the wave size are: water depth, distance of water that the wind blows over (fetch), the width of the area of the fetch and the duration the wind blows over the area. Because of these factors, some lakes may be as wavy as the sea while others are completely tepid.
The waves formed by the wind merge to form bigger waves in the ocean. The resulting wave is known as a swell. When the swell reaches the shore, the depth of the water reduces, causing the wave to rise in height and become steeper. If the wave is high enough, the base becomes unstable and the wave collapses, which is what causes waves to break.
Although it sounds like a simple process, the consequences can be deadly. Wind waves can reach heights above 30m given that the conditions are right (usually during extremely serious storms). Such a wave can flip a cruise ship with ease like a rubber toy.