That’s called “constructive” and “destructive interference”…when crests line up it’s constructive (sets). But when crests line up with troughs, it’s destructive (lulls).
I teach my students that wave speed = wavelength X period. So wavelength is the unknown variable in most cases, since that information is seldom given by bouy reports (at least around here).
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I teach my students that wave speed = wavelength X period. So wavelength is the unknown variable in most cases, since that information is seldom given by bouy reports (at least around here).
it’s been my understanding that the wave period x 2.2 is the speed at which it travels, or close to that.
Where does that number (2.2) come from? Is it an average or something? If speed is measured in meters per second and period is always in seconds, then 2.2 must be meters? All waves are not 2.2 meters apart.
Also…breaking waves slow down as they approach the shoreline because of friction with the bottom, so as the speed changes, so does wavelength change - the wavelength of breaking waves slowed by the bottom is shorter than the open ocean swells they once were. I’m assuming, then, that period must also change, unless the change in speed perfectly compensates for the change in wavelength.
Nj arn’t you a phyisics teacher?
The speed (in nautical miles per hour) of an individual deep water wave is about 3 times it’s period (in seconds). That is, an individual wave with a 13 second period travels at 39 kts/hr. Contrary to what you might intuitively think, there is a linear relationship between wave period and wave speed. But because most deep water waves move in groups, the group speed is half that of an individual wave (within the group), or in this example about 19.5 kts/hr. As the wave moves into shallow water, the group speed and the individual wave speed become the same, so the individual wave starts traveling at the group speed, or 19.5 kts per hour. This wave speed formula is approximate, and actually wave speeds are a fraction different, but this is close enough for all but the most detailed surf forecaster.
That makes sense… knots/hour is the key. I was applying the wave characteristics of sound, light, etc. to ocean waves, so the units didn’t make sense.
Cool…
Seems that some of you are confusing “lull” with “null”.
A null is also known as cancellation. This occurs when a peak and trough coincide, thus cancelling each other out. In audio, this is known as phase cancellation, since the two are out of phase by 180 degrees. Two waves that are in phase will “sum”, essentially doubling the height.
Also, according to Bascom, in Waves and Beaches, the speed of a swell in deep water (greater than 1/2 the wave length) is 3.5 times the period.
NJsurfer - thanks for getting me thinking about waves. I was intuitivly thinking of ocean waves, but as I read your reply I started reading about waves on some physics links. It all quickly went over my head. So, I really don’t have much to back my info up except the Pacific Storm Surf Page. That guy is one of the few who gives forecasts w/o the hype and corny adjectives.
http://polar.ncep.noaa.gov/…n/nww3_pa.anim.2.gif
Thanks for sparking some interest!
==epac