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ger time over a long fetch. In general, longer-blowing, stronger
winds produce waves with greater wave heights, longer wave-
lengths, and longer periods, but a given wind produces waves
with a wide range of sizes and speeds. In addition, the wind
does not blow in just one direction, and shifting winds produce
a chaotic pattern of waves of many different heights and wave-
lengths. Thus, the surface of the ocean in the area of a storm or
strong wind has a complicated mixture of many sizes and speeds
of superimposed waves. The smaller waves soon die out from
friction within the water, and the larger ones grow as the wind
pushes against their crests. Ocean waves range in height from a
few centimeters up to more than 30 m (about 100 ft), but giant
waves more than 15 m (about 50 ft) are extremely rare.
The larger waves of the chaotic, superimposed mixture of
waves in a storm area last longer than the winds that formed
them, and they may travel for hundreds or thousands of kilo-
meters from their place of origin. The longer-wavelength waves
travel faster and last longer than the shorter-wavelength waves,
so the longer-wavelength waves tend to outrun the shorter-
wavelength waves as they die out from energy losses to water
friction. Thus, the irregular, superimposed waves created in the
area of a storm become transformed as they travel away from
FIGURE 24.15 The surface of the ocean is rarely, if ever, still. the area. They become regular groups of long-wavelength waves
Any disturbance can produce a wave, but most waves on the open with a low wave height that are called swell. The regular waves
ocean are formed by a local wind. of swell that you might observe near a shore may have been pro-
duced by a storm that occurred days before, thousands of kilo-
meters across the ocean.
waves are basically repeating series of these crests and troughs
The regular crests and troughs of swell carry energy across
that move across the surface like wrinkles (Figure 24.15).
the ocean, but they do not transport water across the open ocean.
The simplest form of an ocean wave can be described by
If you have ever been in a boat that is floating in swell, you know
measurements of three distinct characteristics: (1) the wave
that you move in a regular pattern of up and forward on each
height, which is the vertical distance between the top of a crest
crest, then backward and down on the following trough. The boat
and the bottom of the next trough, (2) the wavelength, which is
does not move along with the waves unless it is moved along by
the horizontal distance between two successive crests (or other
a wind or by some current. Likewise, a particle of water on the
successive parts of the wave), and (3) the wave period, which is
surface moves upward and forward with each wave crest, then
the time required for two successive crests (or other successive
backward and down on the following trough, tracing out a nearly
parts) of the wave to pass a given point (Figure 24.16).
circular path through this motion (Figure 24.17). The particle re-
The characteristics of an ocean wave formed by the wind
turns to its initial position, without any forward movement while
depend on three factors: (1) the wind speed, (2) the length of
tracing out the small circle. Note that the diameter of the circular
time that the wind blows, and (3) the fetch, which is the distance
path is equal to the wave height. Water particles farther below the
the wind blows across the open ocean. As you can imagine,
surface also trace out circular paths as a wave passes. The diam-
larger waves are produced by strong winds that blow for a lon-
eters of these circular paths below the surface are progressively
smaller with increasing depth. Below a depth equal to about one-
half the wavelength (wave base), there is no circular movement of
Wavelength the particles. Thus, you can tell how deeply the passage of a wave
disturbs the water below if you measure the wavelength.
Crest As swell moves from the deep ocean to the shore, the waves
Wave pass over shallower and shallower water depths. When a depth
height
is reached that is equal to about one-half the wavelength, the
circular motion of the water particles begins to reach the ocean
Trough
bottom. The water particles now move across the ocean bottom,
and the friction between the two results in the waves moving
slower as the wave height increases. These important modifi-
cations result in a change in the direction of travel and in an
FIGURE 24.16 The simplest form of ocean waves, showing
some basic characteristics. Most waves do not look like this repre- increasingly unstable situation as the wave height increases.
sentation because most are complicated mixtures of superimposed Most waves move toward the shore at some angle. As the
waves with a wide range of sizes and speeds. wave crest nearest the shore starts to slow, the part still over
24-15 CHAPTER 24 Earth’s Waters 611