Hawaii High Surf, Ocean Rip Currents, Ocean Safety Sign

• Hawaii's Wave Sources
• Safety
• Terminology
• Warning Signs (OSLSD, Hawaii)

High surf is the term used to describe higher than normal sea-surface wave activity occuring between a shoreline and the outermost line of breakers.

Ocean surface waves are primarily generated by three natural causes: wind, seismic disturbances, and the gravitational pull of the moon and the sun. Oceanographers call all three "gravity waves" since once they have been generated, gravity is the force that drives them in an attempt to restore the ocean surface to a flat plane. Although tsunamis are certainly spectacular if you are in the right place at the wrong time, they are relatively rare. And the tides (although they are always with us) are relatively slow to shift and difficult to observe as waves. On a day-to-day basis, wind-generated waves are the most visible to us. Ripples, chop, rough seas or plunging breakers are what we think of when we hear the word "waves", and their source is the movement of air across water.

Wind is the result of solar energy acting on the earth's atmosphere. The great patterns of circulation of the global winds give rise to the various dynamics of high and low pressure, of calm and storm. Huge north and south Pacific low-pressure systems generate enormous waves. More localized thermal differentials excite the ocean's surface with racing patterns of energy. Smooth coastal waters oscillate gently with the decaying echoes of storms half a world away.

The primary mechanism of wave genesis is the friction between the atmosphere and the surface of the ocean. A puff of less than two knots will raise miniscule wrinkles (called capillary waves) on the surface almost immediately. As the puff dies down, these waves quickly disappear due to the resistance of the water's surface tension. However, when a breeze of two knots or more develops, and is sustained for a time, "gravity waves" begin to form as the wind drags across the water. Ripples at first, these waves continue to grow as the wind continues to blow. In fact, it becomes increasingly easy for the wind to transfer its energy to the water, since it can now push directly against the backs of the ripples. The more jagged and uneven the surface, the more there is for the wind to push against.

The size of wind waves is a function of three factors: the strength of the wind (force), the length of time it blows (duration), and the amount of open water over which it blows (the fetch). If the wind is strong enough and blows long enough, waves of considerable size can develop. However, for a given wind strength, there is a limit to the amount of energy that can be transferred from the atmosphere to the ocean, and when that limit has been reached, the seas are said to be fully developed.

Waves generated by storms seldom need fetches of more than 600-700 nautical miles to reach full height. According to oceanographers, 900 nautical miles is probably room enough to develop the largest storm waves that have ever been reliably estimated.

The table below estimates wave heights for the world's oceans based on a frequency study for wave heights developed by Bigelow and Edmundson in 1947.

Wave height	0'-3'	3'-4'	4'-7'	7'-12'	12'-20'
Frequency of occurrence	20%	25%	20%	15%	10%

Once a pattern of waves radiates free of the winds that created it, the confused chaos of the apparently random sea organizes itself into even lines of "swell." The original wind waves decay, and their energy is consolidated into waves of greater length and increasing speed. As a rule, the ten-second period (distance between wave crest to wave crest) is the dividing line between sea and swell, although there is naturally some overlap. Sea is shorter in wave length, steeper, more jagged, and more confused than swell.

Swell moves across the open ocean in trains of waves of similar period that radiate downwind from a wind source. Responding to the downward force of gravity, the lines of swell spread their forms, lose some height, and distribute some of their energy sideways, lengthening the wave front as they expand away from the source.