WITH THE EXCEPTION OF EARTHQUAKES…

With the exception of earthquakes, only the wind makes waves. From the smallest ripples on a tiny pond to the great heaving swell of the deep ocean, all waves are generated by the wind.

Although the strong winds on San Francisco Bay can lead to some choppy water, it is nowhere near what you would find if these same winds occurred in open seas. Photo by Anthony Sandberg/OCSC

By Captain Ray
Published: October, 2009 

With the exception of earthquakes, only the wind makes waves. From the smallest ripples on a tiny pond to the great heaving swell of the deep ocean, all waves are generated by the wind. Friction from wind moving across the completely still surface of the water causes small ripples to form. Once the surface is no longer smooth, it becomes much easier for wind energy to be transmitted to the water, causing waves to form. Once waves begin to form, three factors determine whether waves will reach their full potential, known as fully developed seas. Let’s discuss each of them in turn.

The first of these wave-generating factors is wind speed.  The stronger the wind speed, the larger waves will become. If there were no other factors to limit wave development, a 20-knot breeze—very common here on San Francisco Bay—would create waves approximately nine feet high; a 30-knot wind—not uncommon here—would generate waves almost 20 feet high! But, since we don’t have waves of this magnitude on San Francisco Bay, there must be other forces at play.

The second factor to consider is the duration of the wind. It takes time for the wind to blow up waves. In addition, the wind direction has to remain constant to blow up big waves. In the lower ranges of wind speeds (10 to 20 knots), it may require only several hours for waves to become fully developed seas, but at higher wind speeds, it can take three or four days for total development. Here on San Francisco Bay, the wind tends to build during the afternoon, but dies off during the night. As the wind dies, so do the waves.

The third limiting factor in the development of waves is fetch. Fetch is the distance that the wind blows across open water. If there is not sufficient fetch, waves will not reach their full potential for a given wind speed. You can easily see the effect of increasing fetch on a small pond, or even in your backyard swimming pool. Look at the surface of the water where the wind first touches it and you will see that there are no waves. Now look farther out from the edge of the pool or the shoreline and notice how the height of the waves becomes greater as the distance from the shore increases. Any obstacle, such as an island or breakwater, forces the process of wave building to restart. Just watch what happens when your ferry passes downwind of Angel Island: In the wind shadow of the island, the waves are smaller or nonexistent. The amount of fetch necessary for waves to reach their full potential also varies with the speed of the wind: the stronger the wind, the greater fetch required. With a wind of 20 knots, about 150 miles of fetch would be needed for the waves to be fully developed. Regardless of the wind direction, San Francisco Bay does not provide the wind with that much uninterrupted open area.

The Earth has few places that provide all the conditions necessary—strong wind, consistent direction and fetch—to allow for the creation of fully developed seas. One such place is the Great Southern Ocean, between Africa’s Cape of Good Hope and Australia. Here, where strong winds blow constantly from the same direction, waves build to an average height of about 25 feet, with some reaching twice that height!