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Weird Science: Communicating Wave Sizes—Local Scale

NGSS Crosscutting Concepts:

<p><strong>SF Fig. 4.6.</strong> Local scale wave height</p>

Describing the height of waves has been controversial for a long time. And, surfers tend to use local scales that often underestimate the height of waves. For example, the fictional surfer girls in SF Fig. 4.6 are riding a “double overhead” wave, which means the wave has a face height of about 12 feet. However,  the wave might be called “six feet”, on a “local scale” that minimizes wave height. Similarly, a head high wave, with a six-foot face, might be called "three-feet" on a local scale.

 

There are three main reasons why surfers use local scales. First, people use local scales to psychologically minimize the height of the waves. In other words, a surfer can say to their friend, “The surf is not that big. It is only a meter high” when the waves are actually breaking overhead and really about two meters in height.

 

A second reason for local scale is that some surfers like to measure the back of the wave rather than the face. However, estimating the size of waves from the back is not a reliable measurement of wave height. This is because waves can break in deep water onto a shallow shelf in such a way that the wave is much smaller on the back (i.e., 0.5 m) than the front (i.e., 3 m). The famous Teahupo‘o wave in Tahiti is an example of this type of “backless” wave.

 

<p><strong>SF Fig 4.6.1.</strong> A crouching surfer on a breaking wave in Mexico, illustrating why it can be difficult to agree on wave size</p>

Lastly, surfers use local scales to provide a relative, local measure of surf energy or size for breaking waves that are difficult to measure. SF Fig. 4.6.1 illustrates how it is difficult to agree on the size of breaking waves. In this figure the surfer is crouching down, making it hard to estimate his size and the relative size of the wave. The wave is also folded over, creating a barrel. If the wave face were straightened out, it would measure about 16–20, but if only the rideable face is measured, the wave would only be 8–10 feet.

 

Despite the convenience of local scale for surfers with intimate knowledge of their home breaks, surf forecasters are moving toward more universal measurements for consistency and for safety; it can be very dangerous to underestimate the size of surf, especially in public forecasts where readers might not understand the nuances of a particular local scale. To avoid confusion and to be as accurate as possible, Exploring Our Fluid Earth refers to the height of the wave as measured from the crest to the trough on the front of the wave.

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Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.