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Here is a formula and some rough calculations: Speed in mph = (1.75) * (swell period in seconds)
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 (ft/s) = 2.56T
So a 17s swell will travel at about 30 mph, or 720 miles/day. An explanatory note:
In deep water it's a secondorder 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 20ft breaking wave, which in turn will be going about twice as fast as a 5ft 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.
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:

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