How Big Is This Wave?

[Brent]

But the water in front of the wave is not flat... if you look closely at the wave, and the as-yet unbroken wave behind it you'll be able to see the underwater profile of the beach is very steep...or otherwise the waves wouldn't stand up so sudden. The example presented is like a shorebreak.

The height of a wave is measured properly from sea level to its peak, correct?....not from the artifical trough created by the retreating water caused by the steep beach profile ahead of it. i.e It's a standing wave situation and a bit of a trick question.

[WaveJunkie]

What the hell ??? I searched Google --

http://oceanworld.tamu.edu/resources/oc ... r16_06.htm

(Just because I found it, linked it, read it . . . doesn't mean I understand it. *LMAO* )

16.6 Measurement of Waves

Because waves influence so many processes and operations at sea, many techniques have been invented for measuring waves. Here are a few of the more commonly used. Stewart (1980) gives a more complete description of wave measurement techniques, including methods for measuring the directional distribution of waves.

Sea State Estimated by Observers at Sea
This is perhaps the most common observation included in early tabulations of wave-heights. These are the significant wave-heights summarized in the U.S. Navy's Marine Climatological Atlas and other such reports printed before the age of satellites.

Accelerometer Mounted on Meteorological or Other Buoy
This is a less common measurement, although it is often used for measuring waves during short experiments at sea. For example, accelerometers on weather ships measured wave-height used by Pierson & Moskowitz and the waves shown in Figure 16.2. The most accurate measurements are made using an accelerometer stabilized by a gyro so the axis of the accelerometer is always vertical.

Double integration of vertical acceleration gives displacement. The double integration, however, amplifies low-frequency noise, leading to the low frequency signals seen in Figures 16.4 and 16.5. In addition, the buoy's heave is not sensitive to wavelengths less than the buoy's diameter, and buoys measure only waves having wavelengths greater than the diameter of the buoy. Overall, careful measurements are accurate to ± 10% or better.

Wave Gages
Gauges may be mounted on platforms or on the seafloor in shallow water. Many different types of sensors are used to measure the height of the wave or subsurface pressure which is related to wave-height. Sound, infrared beams, and radio waves can be used to determine the distance from the sensor to the sea surface provided the sensor can be mounted on a stable platform that does not interfere with the waves. Pressure gauges described in §6.8 can be used to measure the depth from the sea surface to the gauge. Arrays of bottom-mounted pressure gauges are useful for determining wave directions. Thus arrays are widely used just offshore of the surf zone to determine offshore wave directions.

Pressure gauge must be located within a quarter of a wavelength of the surface because wave-induced pressure fluctuations decrease exponentially with depth. Thus, both gauges and pressure sensors are restricted to shallow water or to large platforms on the continental shelf. Again, accuracy is ± 10% or better.

Satellite Altimeters
The satellite altimeters used to measure surface geostrophic currents also measure wave-height. Altimeters were flown on Seasat in 1978, Geosat from 1985 to 1988, ERS-1 & 2 from 1991, Topex/Poseidon from 1992, and Jason from 2001. Altimeter data have been used to produce monthly mean maps of wave-heights and the variability of wave energy density in time and space. The next step, just begun, is to use altimeter observation with wave forecasting programs, to increase the accuracy of wave forecasts.

The altimeter technique works as follows. Radio pulse from a satellite altimeter reflect first from the wave crests, later from the wave troughs. The reflection stretches the altimeter pulse in time, and the stretching is measured and used to calculate wave-height (Figure 16.12). Accuracy is ± 10%.
16.11 Shape of radio pulse received by the Seasat altimeter, showing the influence of ocean waves. The shape of the pulse is used to calculate significant wave-height. From Stewart (1985).

Synthetic Aperture Radars on Satellites These radars map the radar reflectivity of the sea surface with spatial resolution of 6-25m. Maps of reflectivity often show wave-like features related to the real waves on the sea surface. I say "wave-like" because there is not an exact one-to-one relationship between wave-height and image density. Some waves are clearly mapped, others less so. The maps, however, can be used to obtain additional information about waves, especially the spatial distribution of wave directions in shallow water (Vesecky and Stewart, 1982). Because the directional information can be calculated directly from the radar data without the need to calculate an image (Hasselmann, 1991), data from radars and altimeters on ERS-1 & 2 are being used to determine if the radar and altimeter observations can be used directly in wave forecast programs.

[WaveJunkie]

Personally, and I'm new to all this, I'd measure wave height from the "flat area" in front of the wave to the "crest" of the wave.

But then this is entirely pragmatic. In a kayak this is the part of the wave I need to get past. On a board this is the section of the wave I need to get over if heading out, and it's the height of the surface water I'm riding when riding the wave.

Finally, if it lands on me . . . I get the "whole nine yards" -- which is ALL the water that got sucked up in the trough in front of the wave. *LMAO*

Besides . . . much better at the pub later to say you stood on a 9 footer then to admit that some 3' dribble held you against the bottom 'til you felt like you might drown. *G*

[babyboarder89]

well, the way im judging this is, id consider surfing it, so it must be less than 5/6 foot. cos im a wimp!

[Brent]

But for surfers to slighty under-estimate wave size is part of the culture & modern history of the sport. thus the difference between the American method of measuring from the front of the wave or the body method (waist, chest, head high etc) and the Island & southern hemisphere method of measuring wave height from behind. Thus a measurement that's about half. The numbers are smaller & it's easier to be objective (that's my opinion).
Besides there is a bit of false modesty involved there too. "na it wasn't 5 feet..it was only 3 feet Brah"..

[deathfrog]

I heard it was invented so when someone heard it was only three foot no one went out, but it was really more like six...

[calisurfer]

hahaha what a good idea

[MadeInNewJersey13]

Around Chest to Head High, Thats a really nice wave though!!

[Laguna]

I would say it is 4-5ft!!

[flex639]

head high

[manick]

Gotta be about 3ft ish (maybe 3.5ft), about 4.5ft is my comfort limit at the mo, much bigger than that and for me it gets a bit hairy. Solid 5ft (2ft+ overhead) doesn't sound much, but when you see it and hear it even from the back of the wave it's fleshin(???) scary. what do the rest of you think??

Am i a wuss???

manick

[oslo]

From behind; it's a three foot wave. From the front, it is what I call a head high wave.

[oslo]

Just had a second look. Shoulder high is right.

[slightly stoked]

5.75 feet give or take .02 feet lol.

FIRST POST!!! SICK!!!

[dmb72]

3 maybe 4...depends on the break you mostly surf. From a beach break surfer's perspective it doesn't look very big, but I can see how some of you point break surfers are calling it head high. One thing it isn't is a six foot wave. No way.

[Dopey]

[sinistapenguin]

See, this is interesting, cos I don't differentiate between face & wave height!

I would put that last posted wave at around 12 ft.

[Broosta]

Yep deffo double overhead, not so sure about how many feet tho, could call it anything from 8-12ft really depends on testosterone levels...

Actually I think the descrepancy in heights comes from what part of the wave people are looking at - some people would be wanting the wave to be as big as possible when calling it (maybe cos it would be the biggest wave they surfed so far) and so these people would be looking at the tallest bit of the wave which is the peak just before it breaks (usually), while others may have ridden bigger so its no big deal to them and they might judge the average height of the wave ie not the peak but half way along the ride which is usually a fair bit smaller .

[Brent]

In this part of the world that wave would be called a nice solid 6-7 foot. I'd like to be on that wave right now actually :-)))
Where is the spot?

[Dopey]