An observer sitting on a beach can make at least an intelligent guess whether the surf spilling out onto the sand before him has been produced by a gale close offshore or by a distant storm. Young waves, only recently shaped by the wind, have a steep, peaked shape even well out at sea. From far out on the horizon you can see them forming whitecaps as they come in; bits of foam are spilling down their fronts and boiling and bubbling over the advancing face, and the final breaking of the wave is a prolonged and deliberate process. But if a wave, on coming into the surf zone, rears high as though gathering all its strength for the final act of its life, if the crest forms all along its advancing front and then begins to curl forward, if the whole mass of water plunges suddenly with a booming roar into its trough—then you may take it that these waves are visitors from some very distant part of the ocean, that they have traveled long and far before their final dissolution at your feet.
What is true of the Atlantic wave we have followed is true, in general, of wind waves the world over. The incidents in the life of a wave are many. How long it will live, how far it will travel, to what manner of end it will come are all determined, in large measure, by the conditions it meets in its progression across the face of the sea. For the one essential quality of a wave is that it moves; anything that retards or stops its motion dooms it to dissolution and death.
Forces within the sea itself may affect a wave most profoundly. Some of the most terrible furies of the ocean are unleashed when tidal currents cross the path of the waves or move in direct opposition to them. This is the cause of the famous ‘roosts’ of Scotland, like the one off Sumburgh Head, at the southernmost tip of the Shetland Islands. During northeasterly winds the roost is quiescent, but when the wind-born waves roll in from any other quarter they encounter the tidal currents, either streaming shoreward in flood or seaward on the ebb. It is like the meeting of two wild beasts. The battle of the waves and tides is fought over an area of sea that may be three miles wide when the tides are running at full strength, first off Sumburgh Head, then gradually shifting seaward, subsiding only with the temporary slackening of the tide. ‘In this confused, tumbling, and bursting sea, vessels often become entirely unmanageable and sometimes founder,’ says the British Islands Pilot, ‘while others have been tossed about for days together.’ Such dangerous waters have been personified in many parts of the world by names that are handed down through generations of seafaring men. As in the time of our grandfathers and of their grandfathers, the Bore of Duncansby and the Merry Men of Mey rage at opposite ends of the Pentland Firth, which separates the Orkney Islands from the northern tip of Scotland. The sailing directions for the Firth in the North Sea Pilot for 1875 contained a warning to mariners, which is repeated verbatim in the modern Pilot:
Before entering the Pentland Firth all vessels should be prepared to batten down, and the hatches of small vessels ought to be secured even in the finest weather, as it is difficult to see what may be going on in the distance, and the transition from smooth water to a broken sea is so sudden that no time is given for making arrangements.
Both roosts are caused by the meeting of swells from the open ocean and opposing tidal currents, so that at the east end of the Firth the Bore of Duncansby is to be feared with easterly swells and a flood tide, and at the west end the Merry Men of Mey stage their revelries with the ebb tides and a westerly swell. Then, according to the Pilot, ‘a sea is raised which cannot be imagined by those who have never experienced it.’
Such a rip may offer protection to the near-by coast by the very fury and uncompromisingness of the struggle between waves and tide. Thomas Stevenson long ago observed that as long as the Sumburgh roost was breaking and cresting heavily off the Head there was little surf on shore; once the strength of the tide was spent and it could no longer run down the seas a heavy surf rolled in against the coast and rose to great heights on the cliffs. And in the western Atlantic, the confused and swiftly running tidal currents at the mouth of the Bay of Fundy offer such strong opposition to waves approaching from any quarter from southwest to southeast that such surf as develops within the Bay is almost entirely local in its origin.
Out in the open sea, a train of waves encountering a hostile wind may be rapidly destroyed, for the power that created a wave may also destroy it. So a fresh trade wind in the Atlantic has often flattened out the swells as they rolled down from Iceland toward Africa. Or a friendly wind, suddenly springing up to blow in the direction the waves are moving, may cause their height to increase at the rate of a foot or two per minute. Once a group of moving ridges has been created, the wind has only to fall into the troughs between them to push up their crests rapidly.
Rocky ledges, shoals of sand or clay or rock, and coastal islands in the mouths of bays all play their part in the fate of the waves that advance toward shore. The long swells that roll from the open ocean toward the shores of northern New England seldom reach it in full strength. Their energy is spent in passing over that great submerged highland known as Georges Bank, the crests of whose highest hills approach the surface over the Cultivator Shoals. The hindrance of these submarine hills, and of the tidal currents that swirl around and across them, robs the long ocean swells of their power. Or islands scattered within a bay or about its mouth may so absorb the strength of the waves that the head of the bay is free from surf. Even scattered reefs off a coast may offer it great protection, by causing the highest waves to break there, so that they never reach the shore.
Ice, snow, rain—all are enemies of the waves and under proper conditions may knock down a sea or cushion the force of surf on a beach. Within loose pack ice a vessel may count on smooth seas even if a gale is raging and surf is breaking heavily about the edges of the pack. Ice crystals forming in the sea will smooth the waves by increasing the friction between water particles; even the delicate, crystalline form of a snowflake has such an effect on a smaller scale. A hail storm will knock down a rough sea, and even a sudden downpour of rain may often turn the surface of the ocean to oiled-silk smoothness, rippling to the passage of the swells.
The divers of ancient times who carried oil in their mouths to release beneath the surface when rough water made their work difficult were applying what every seaman today knows—that oil appears to have a calming effect on the free waves of the open ocean. Instructions for the use of oil in emergencies at sea are carried by most official sailing directions of maritime nations. Oil has little effect on surf, however, once the dissolution of the wave form has begun.
In the Southern Ocean where the waves are not destroyed by breaking on any beach, the great swells produced by the westerly winds roll around and around the world. Here the longest waves, and those with the greatest sidewise expanse of crest, are formed. Here, it might be supposed, the highest waves would also be found. Yet there is no evidence that the waves of the Southern Ocean surpass the giants of any other ocean. A long series of reports culled from the publications of engineers and ships’ officers show that waves higher than 25 feet from trough to crest are rare in all oceans. Storm waves may grow twice as high, and if a full gale blows long enough in one direction to have a fetch of 600 to 800 miles, the resulting waves may be even higher. The greatest possible height of storm waves at sea is a much debated question, with most textbooks citing a conservative 60 feet, and mariners stubbornly describing much higher waves. Throughout the century that has followed the report of Dumont d’Urville that he encountered a wave 100 feet high off the Cape of Good Hope, science generally has viewed such figures with skepticism. Yet there is one record of a giant wave which, because of the method of measurement, seems to be accepted as reliable.