I have noticed when I’m reading the surf report say at 5ft 12seconds yet when i look at the buoy data the size is usually very close to the report but period 7-8 seconds based on the example we are using ,Im aware the buoy data is taken at every hour and there may be alot of different swell trains off ground and wind swells all interacting and affecting the data, but I thought the dominant groundswell would show up and cancel out the inferior windswells,could you explain why the period always smaller than surf report says.
Many thanks jonathan.
Short answer the wave buoy reports ‘average’ period while the forecast shows ‘peak’ period. To better understand what this means I’ve gone into a bit of depth below.
As you’re aware there can be more than one swell running, but also an individual ‘swell’ is simply a collection of waves of different heights and periods that originate from the same storm. The way both the forecast and a wave buoy work is to look at collections of these waves (real for the wave buoy and predicted for the forecast of course) and group them by period. We explain this in a bit more detail here. To give you a good, real world example from today actually, check out this graph from a local wave buoy:
The graph shows the amount of energy at each swell period, so we can deduce pretty easily what sort of day it must be. The big peak of energy around 12 seconds (highlighted by the red line) is a reasonable quality ground swell. In fact you can see that there will be waves right up to around 15 seconds. You can also see a smaller peak at around 10 seconds. Possibly just about distinct enough to qualify as a separate swell although most likely waves from the same storm looking at the shape of the graph. Clearly around 6 seconds there’s a defined separate peak, at this sort of period you can imagine that what’s happening, along with the groundswell, is locally windy conditions. Checking out the webcam you can see exactly what this looks like in the real world.
No huge surprises, strong onshore wind and wind sea obscure all that potentially interesting ground swell. Now if we check out the forecast for the same day:
You can see we break out both this longer period swell AND the wind swell and show that strong onshore wind.
Enough background anyway – to actually answer the question now the scene is set is pretty easy. Your forecast shows peak period. This is really easy to understand, check out the graph again and you’ll see the red line highlighting the period at which there’s the most wave energy. This is the peak period. It reads off at about 11.8 seconds, near as you could expect to the three hour forecast of 11 seconds. However most of the wave buoys we use (and other sites reporting NDBC data) don’t display the peak period at all but the AVERAGE period. For this you sum up all the energies for all periods and try to find the centre point, the overall average or the ‘middle’ of the graph. With this particular reading you can see the average period would be around 8-9 seconds thanks to that wind sea.
So short answer the wave buoy and the forecast report different kinds of period. They’re not directly comparable and, especially in situations where there’s often wind swell present the average will almost always be lower than the peak value. Both are ‘right’ because they’re showing different things.
We’ll introduce greater detail to our wave buoy readings shortly that’ll help with this, in the meantime our new forecast pages splitting out all the separate swells for you are really the clearest way to understand exactly what’s going on.