All ocean waves are siblings; they are the children of the wind. When the air is calm the sea is generally smooth or almost so, but as a breeze picks up there are tiny wavelets born to run downwind along the surface, rather like the ripples that streak across your coffee-cup as you blow on it to cool it down.
The wavelets are nurtured by a waxing wind. They move in its general direction and they increase in height as time goes by; the stronger the wind the higher the resulting waves will ultimately be. At first, the waves are gentle curves on the surface of the water - smooth rounded undulations, reminiscent of a snake that slithers over grass. But when the wind increases above 10 m.p.h. or so, the waves become steeper, and the crest of each is pointed - like the ridgetiles on a roof. And if they grow steeper still, the waves become unstable, and "break" or topple over.
But besides wind speed, there is another factor which determines wave-height. We call it fetch - the distance available for waves to form. Given enough wind, the waves will grow and grow as they travel over miles of ocean; but if they have insufficient fetch, they have no room to grow.
Fetch has important influence on the aspect of the sea as viewed from shore in stormy weather. When the wind is blowing seawards from the land, the water appears anomalously calm despite a howling gale. There is insufficient fetch for the wind to have much impact on the water surface near the shore, and only some distance out to sea can waves of any size be seen developing. It is a fair bet in such circumstances that beyond the horizon the waves are much higher than any of those visible from land.
But a wave breaking on the Kerry coast in a strong westerly has had an almost infinite amount of fetch. It may have travelled thousands of miles, beginning life several days before, perhaps, as a minute undulation on the balmy waters of the Caribbean, but now unleashing an enormous power.
A breaking storm-wave, throwing thousands of tons of water at a coastline, can hurl rocks an awesome distance and destroy a concrete barrier in hours. And where the shores are not of rock, the waves can rapidly erode the coastline. In parts of Britain the waves have eaten back several miles of shore since Roman times, and across the Atlantic, stretches of Cape Cod in Massachusetts are being eroded at a rate of several feet per year.