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Swell Speed
"Is there a way of calculating the speed of a
swell or does it travel at a set speed?"


Here is a formula and some rough calculations:
Speed in mph = (1.75) * (swell period in seconds)

Period 10 12 14 17 20
Miles per hour (mph) 17.5 21.0 24.5 29.7 35.0
Miles per day 420 504 588 714 840

Sources:  Formula provided by Tim Maddux (formerly of Santa Barbara, CA); table provided by Gioni (Santa Cruz, CA). Both of alt.surfing. *See below for more info.

"Waves in deep water travel at a speed relative to their wavelength, longer waves travel faster. When they hit shallow water the speed is governed by the depth, the shallower the slower until they run out of water and stop."
-- Jose Borrero of alt.surfing (Los Angeles, CA)

How far is it?

More Info for the Geeks
The energy of a swell moves at the group velocity, which is 1/2 the speed of the individual wave crests.

Cg = 0.5Cp = 0.5(gT/2pi)

For T in seconds,
Cg (m/s) = 0.78T
Cg (kph) = 2.81T
Cg (km/day) = 67T

Cg (ft/s) = 2.56T
Cg (mph) = 1.75T
Cg (mi/day) = 42T

So a 17-s swell will travel at about 30 mph, or 720 miles/day.

An explanatory note:
"Well, what I've always posted here is linear wave theory, which solves the equations of motion for a fluid using the approximation that water particles on the surface must remain at the average surface, along with some other simplifications. The simplification I mentioned eliminates the dependence of the wave speed on height.

In deep water it's a second-order effect (< 10% of the whole) that only becomes important in shallow water when nonlinearities take over.

We surfers see the effect in shallow water, particularly as the waves are just about to break. In shallow water, wave speed is C=sqrt(gD) where D is the water depth. Since the waves break in water about equal to their height, Laird and Peter Mel have to tow into their 80 ft waves, which will go about twice as fast as a 20-ft breaking wave, which in turn will be going about twice as fast as a 5-ft breaking wave.

Now, I have to confess that that equation is also from linear theory, and I don't have the correction for higher order terms handy."

Hope that helps.
- Tim Maddux, as posted to the usenet newsgroup, alt.surfing.


What Time Is It?
What is UTC ? How do I tell at what time a satellite picture was taken? UTC stands for Universal Time Coordinated, what used to be called Greenwich Mean Time (GMT) and Zulu Time (Z). This is the time at the Prime Meridian (0° Longitude) given in hours and minutes on a 24 hour clock.

On most satellite pictures and radar images the time will be given. If it's not in local time then it will usually be given as UTC, GMT, or Z time. To convert this to your local time it is necessary to subtract the appropriate number of hours for the Western Hemisphere or add the correct number of hours for the Eastern Hemisphere. And don't forget the extra hour adjustment for Daylight Savings Time or Winter Time over Standard Time for your zone.

Source: Huricane Research Division FAQ, Atlantic Oceanographic and Meteorological Laboratory, Ocean and Atmospheric Research, National Oceanic and Atmospheric Administration


Two Good Sources on Waves:
  • Jack Morelock, Wilson Ramirez & Dennis Hubbard. Marine Geology, go the the section on Physical Oceanography Sections, click on "Waves."
  • Willard Bascom. Waves and Beaches: The Dynamics of the Ocean Surface, (rev. ed. 1980).

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    Last updated on: 07/01/09