Wave heights of 15 to 20 feet are not unusual, but there are limits to growth. During very intense winter storms, waves of 40 and 50 feet have been reported by ships. The ocean liner Queen Elizabeth II reported a wave measuring 100 feet in 1996. The wave becomes higher with a shorter wave length and a slower speed. As the wave form changes, it becomes steeper with a narrower crest which allows the wave to become unstable.
Waves normally break when the forward speed of the crest exceeds the speed of the wave itself.
The collapse of the wave creates a frothy surf of foam and air-filled bubbles. The type of breaker depends on the steepness of the beach and the steepness of the wave before it reaches shallow water. Spilling breakers occur on a gently sloping beach. They gradually break over a considerable distance from the shore. When a crest does break, the surf slides down the face of the wave.