What
is a Great Circle? See the Wavebasics
tutorial, about 2/3 rds of the way down the page.
How
are these charts produced? Stormsurf used the excellent GNU
map drawing tool call Generic
Mapping Tool (GMT).
What
are these charts used for? In combination with the output from
the QuikSCAT satellite, one can determine if the winds inside
a storm (fetch) is aimed at your beach. If sufficient wind is
blowing parallel to a great circle path that terminates at your
beach, then a swell could arrive. Conversely, if the wind is
blowing at a significant angle to the great circle path to your
beach (greater than 45 degrees), then the odds of a swell making
it to your beach with it's maximum potential size starts decreasing.
The great circle really becomes important if the fetch is greater
than 1000 nmiles away from your beach. Look at the summer great
circle paths for Northern
CA, and you'll begin to get a sense of how strange (and
important) this tool is. Secondarily, these Great Circle paths
are constructed to identify the swell window for many breaks.
If the fetch is outside the bounds of the great circles depicted,
then it's likely land will block the swell from arriving at
your beach.
Why
are there not charts for my home break? Each chart
takes about 3-4 hours to produce. Every data point is manually
adjusted and positioned on the map. Needless to say it took
an extraordinary amount of time to produce the images contained
currently in this site. We tried to provide a well rounded selection
of charts for the areas with the highest concentration of viewers
serviced by Stormsurf. But we just ran out of time and energy.
There are still broad areas we want to work on, notably South
America, and we.cgian to expand as time permits. Conversely,
the charts provided are fairly serviceable as-is if you live
near a break already documented. For the high level charts,
there is little change in swell direction from the points on
the chart to another break 100 miles up the coast (maybe only
a degree or two at most). So you can visually transfer the great
circles (within reason) to get a rough sense of such paths for
nearby breaks.
How
do the 'Close-up' charts correspond to the high level charts?
What we did was first produce a high level chart for one location,
say Southern
Australia - Victoria. Then we used the exact same starting
points (the ones over Antarctica) and zoomed the end-point perspective
in, constructing individual charts for Port
Cambell, Bells,
and Ocean
Beach etc. Notice that the swell angles change just slightly
from one break to the next, even though the starting points
are mostly the same. As we moved through the breaks the swell
window changed slightly, with some great circles starting to
cross over land. So we either added, deleted or moved a path
or two to fit within the swell window. In some instances we
left paths over land if it seemed reasonable that the swell
would wrap into the nearby beach. Granted, we don't have enough
site specific knowledge about every break depicted to know how
much wrapping actually can occur. It's a best theoretical guess.
Why
are the swell angles/great circles not depicted in even units
of say 5 or 10 degrees? In an attempt to build
an aesthetically.cgieasing image, and to ensure the paths don't
drive over land so that the real swell window could be defined,
we chose to build the images as you see them. In this way, one
can see the true swell window, accurate to 0.5 of a degree.
Note: Swell angles were calculated using the WGS84/NAD83/GRS80
earth model, which accounts for the fact that the earth is not
a perfect sphere.
Why
are distances not depicted, say in units of 600 nmiles?
Because we don't forecast that way and don't advocate that any
one else should either. The most accurate way to determine distance
is to capture the starting and end coordinates and.cgiug them
into a great circle distance calculator like ours here.
Why
are some locations swell windows separated into subcategories?
Notice that in many instances the charts are broken
down by seasonal swell windows. For exa.cgie, for Southern California
there are charts for Summer and Winter, and within the Summer
category there are Southern Hemisphere and Hurricane charts.
This was done mainly as a way to drill down/zoom in and get
to the peculiarities of each. If we chose to produce only one
chart for each location, then we couldn't get enough detail
to depict all the special conditions associated with each seasons
swell windows. In the end, we chose to go for detail and accuracy
rather than a broader approach.
