Friday, February 15, 2013

Understanding wave data

Developing the Surf Today IOS apps had led me on a merry chase around the internet in search of off shore buoys providing near real-time data for the various states of Australia. Here in WA, we are very fortunate that the marine section of the Department of Planning's website translates the standard wave data in to a most immediately readable form using the top graphic and the lower chart. The top graphic displays the height, direction and interval of the swell and the local sea which can make a huge difference to whether or not there is a ridable wave. If you know the effect a specific direction of local sea has on your breaks then you will have an excellent set of data on which to base your decision to go surfing. The bottom chart, only available on the larger screen of the iPhone 5, provides a great visual representation of the movement of the swell and sea over 5 days with the useful addition of the yellow line providing an overall wave height figure. You maybe wondering how it is derived especially if you have managed to decipher the swell and sea heights in the top graphic which are 0.96 for the swell and 1.80 for the sea. Ummm, that adds up to nearly 3 metres but the overall wave height has been set at 2 metres.

I am now going to explain how that figure is derived using the wave charts that are produced in NSW.
Oceanographers use 6 labels to describe wave height but we only really need to know about 3 of them. H, Hsig and Hmax.
In the chart to the left, the green Hsig line shows the average of the highest one third of the waves recorded.
The suitably chosen red Hmax line shows the highest wave you should be alert for and that is two times the height of Hsig!!
Now you need to do a quick calculation as the figure that provides the average of all the waves recorded is H, which is not displayed in this style of chart.
H equals 2/3 of Hsig and is the wave height you are most likely to encounter during your session.
So this explains why the yellow line displaying the overall wave height in the 1st chart is 2 metres and not 3 metres.
To summarise:
Hsig = 3 metres
H (2/3 of Hsig) = 2 metres the most likely average wave height
Hmax ( 2 X Hsig) = A 6 metre wave is possible so watch out!

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