13.4: Types of Waves
- Page ID
- 31681
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Oceanographers and mariners use a variety of ways to classify waves—none official that I’m aware of—but we’re stuck with what the experts define, so here we go.
The shortest waves—those with wavelengths less than 1.73 centimeters—are called capillary waves because they are small, like the capillaries in your body. Capillary waves are most easily recognized as the slight ruffling of the ocean’s surface as a wind blows across it. The term “capillary” actually refers to the surface tension of water in a tube that causes the surface of a column of water to form a slight bulge or depression. Sometimes capillary waves are referred to as ripples—a better name, perhaps, but confusing in that the term has also been applied to waves slightly larger than capillary waves.
Capillary waves are the most abundant wave in the world ocean. It takes only the slightest amount of wind to cause them. Their shape is distinctive, with rounded crests and V-shaped troughs. You can easily make your own by blowing on a bowl of water. It’s said they resemble cats’ paws on water. As an owner of four cats, I really don’t see the resemblance, but I guess a thirsty cat’s habit of gently pawing a bowl of water to gauge the distance between its face and the water surface may have something to do with it.
Be that as it may, these very short waves behave differently from how larger ocean waves behave because they are under the control of surface tension. When the wind stops, capillary waves disappear immediately as surface tension restores the ocean’s surface. Another interesting thing about capillary waves is that as the wind blows harder, their wavelengths become shorter, just the opposite of larger waves. Despite their small size, capillary waves play an important role in the formation of larger waves because the roughness they create over the ocean’s surface permits a faster exchange of wind energy into the ocean.
Sailors and ocean folks refer to wind-generated waves, or wind waves, by a variety of names. Each roughly corresponds to the wave’s size. Ripples—a bit larger than capillary waves—and wind chop—short and steep irregular waves—represent the waves you see when the wind begins to blow or has been blowing for a short time. Fully developed seas refer to the maximum wave size for a given wind speed over a period of several hours or a few days. Swell refers to the waves that travel out beyond the winds that generate them. Swell is what you see and experience as the rise and fall of the sea surface before they inter the seafloor. Swell moves the fastest of any wind-generated wave and produces the largest and fastest surf, ideal for big wave surfers, defined as surfers who ride waves greater than 20 feet (6.2 m) in height.
Oceanographers and physicists use a more formal classification system for waves based on their forward motion. To understand this system, we need to take a ride on the freeway.