Out in the ocean, as the wind blows across a smooth water surface, air molecules push against the water. This friction between the air and water pushes up tiny ridges or ripples on the ocean surface. As the wind continues to blow, these ripples increase in size, eventually growing into waves that may reach many feet in height.

Three factors determine how large wind-generated waves can become. The first factor is wind speed, and the second factor is wind duration, or the length of time the wind blows. The final factor is the fetch, the distance over which the wind blows without a change in direction.
Seas refer to short-period waves that are still being created by winds or are very close to the area in which they were generated. Swells refer to waves that have moved out of the generating area, far from the influence of the winds that made them.

There are computer models that can take this information and predict wave heights at the shoreline. Many of the links on this page are showing output from these types of models.