Humans have long been fascinated with ocean waves without knowing what caused them. In the 1830s, Sir Francis Beaufort, a British admiral and hydrographer, established a standard scale to objectively classify sea states and thereby the size and potential destructiveness of ocean waves. Over time, the "Beaufort Scale" has been modified and consists of sea states ranging from 0 to 12, which correspond to wind speed, wave height and sea conditions.
While the Beaufort Scale is universally accepted as the means for describing sea states and wave heights, the parameters that generate waves are wind speed, wind duration and "fetch," or the distance over which the wind blows. Pierson and Moskowitz proposed the simplest model of this phenomenon and mathematically described wave height as a function of the dependent variables. Generally speaking, in an open body of water, the harder the wind blows over great distances, the greater the wave height.
Ocean waves can also be generated by seismic activity such as earthquakes, volcanoes and underwater landslides. These are generally referred to as tsunamis and are independent of wind activity. Complicating factors in predicting wind driven wave heights include ocean and intertidal currents, as well as constructive and destructive effects of the waves themselves. In the former case, currents running opposite to the wind will build waves to greater heights than predicted; this occurs regularly in the Gulf Stream. In the latter case, waves travel in groups or "trains" and transmit energy to other waves that are out of period, thus generating waves that are larger than predicted, the so-called "seventh wave" effect.