It is a confused pattern that the waves make in the open sea – a mixture of countless different wave trains, intermingling, overtaking, passing, or sometimes engulfing one another; each group differing from the others in the place and manor of its origin, in its speed, its direction of movement; some doomed never to reach any shore, others destined to roll across half an ocean before they dissolve in thunder on a distant beach – Rachel Carson, The Sea Around Us
Years ago I took a ride in a helicopter that took off from a ship’s deck. Once we were airborne, I sat with my feet dangling out the door. The ship was out of sight and only my feet obstructed my view of the undulating waves below. The day was calm, so the waves weren’t breaking. I thought we were a short distance from the surface, maybe less than 50 m, until a seagull flew beneath us.
The bird was a tiny white speck, so we had to be several hundred metres up. It was surprising how wrong I was about our height, and at that moment I was very glad I was tethered to the helicopter. The fact that I couldn’t tell how high we were is a common occurrence when looking at the ocean surface.
Without a point of reference, like a shoreline or a seabird, there is no way to tell what size of motions you are looking at once you reach a certain distance above the water. Obviously, you could tell how far away you were if you were getting sprayed by the water – but by then you are very close. A skydiver over the open ocean wouldn’t be able to tell how high they were without an altimeter. A failed altimeter could create a potentially bad day for the skydiver (when would you open your parachute?).
As Rachel Carson describes above, the surface of the open ocean is complex. Ocean surface waves are mostly created by the wind and waves of all sizes occur simultaneously. Once created these waves can travel great distances and interact with waves created elsewhere adding even more complexity. The size and speed, thus energy, of a wave is related to wind strength and the distance over the sea the wind blows (fetch). For example, a large storm lasting several days creates a whole spectrum of waves. In the open ocean, more energetic waves travel faster than less-energetic ones resulting in a sorting of waves. Since, water is great at transmitting waves, these energetic waves can reach the opposite side of an ocean basin where they potentially create great fun for surfers.
Rachel Carson’s ‘The Sea Around Us’ is a wonderful descriptive read about the ocean. Since it was written in 1951, it provides an interesting window into how our understanding of the oceans has changed since then – for example: plate technonics wasn’t accepted then and there was no idea that hydrothermal vents existed.
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