Wave height is defined as the difference between the highest point, or crest, and the lowest point, or trough, of a wave. Wave height is usually measured with buoys, which are loosely attached to the ocean floor and float along the surface of the water. Each buoy contains an accelerometer, which measures the vertical displacement of the buoy as the buoy rises and falls with the wave.
Wave height may also be measured using pressure sensors, which maintain a fixed position underwater. The pressure sensors measure the height of the water column above them. As wave crests pass over the sensors, the measured height of the water column increases, and wave troughs reflect lower heights. Wave height is then calculated by subtracting the height of the trough from the height of the crest.
Waves are formed by the wind blowing across the surface of water. The height of a wave is determined by the speed and fetch of the wind. Wind fetch is the distance for which wind blows over water at a similar speed and direction. A combination of high-speed winds and long fetch lengths creates the highest waves.
Wave height also affects wave steepness. Wave steepness is expressed as the ratio between wave height and wave length. When the height of a wave is close to the length of a wave, such as three foot waves that occur three seconds apart, a wave is considered severely steep.