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Animation by HAnis (Tom Whittaker)
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Instructions
1) Hover over the control buttons at the top of the tool to display details concerning each buttons use. 'Start/Stop' start and stop the animation. '<' and '>' allow you to move forward or backward one frame at a time. Rock/Loop lets you control the forward and backward direction of the animation. The '- +' controls affect the speed of the animation. Click 'Zoom' then click on the image one or more times to zoom in. 'Refresh' reloads all the images in the animation sequence. 'Show Loc' displays the coordinates of wherever your mouse is placed on the animation frame. 'Show Dist' displays distances (see details below).
2) To determine if a fetch is producing energy aimed up the great circle paths to your location, click in one of the little boxes below the control buttons to display great circle paths emanating from your beach (or nearby ones). For optimal results, the fetch must be aimed within 30 degrees of a path arriving at your location. Any greater of an angle, and less swell size will result.
3) To determine distance and specific swell angle: Select the 'Show Distance' control. Stop the animation on a frame that displays an interesting fetch area or area of high seas. Place your mouse over your beach then depress the left mouse button and drag your pointer to the core of the fetch/high seas. Notice that swell angle and distance (in nautical miles) is automatically and continuously calculated and displayed on the screen.
4) Swell Angle: If you draw the line from your beach to the fetch area, the resulting angle represents what the swell direction will be (when it eventually arrives) at your beach. This is what is classically know as 'Swell Direction'.
5) Though the line drawn on the screen is straight, the math used to calculate distance between the two selected points is actually computing the great circle distance. Do not assume the swell travels in that straight line. Use the great circle overlays to better illustrate the actual swell path. Also follow that path to identify any potential obstructions or topographic features which would shadow the swell (i.e. islands).
6) Write down the date/time of the forecast frame, the distance the fetch is from your beach and the swell angle. Based on either the wind speed or significant wave height, one can determine the resulting swell period (e.g. 20 ft seas = 13 sec period). Swell travel time in nautical miles per hour = approximately (period X 1.5 - that's the swell group speed for those more oceanographically inclined - or the speed of a set of waves). So a 13 sec period swell travels at 19.5 nmiles/hr. Divide distance (from above) by the estimated period speed to determine swell arrival time (in hours). Example: 2000 nmiles distance with a 13 sec period = 102.6 hrs (2000/19.5 = 102.6 hrs). If the date/time of the forecast image was 06Z Saturday June 4, and the travel time is 102.6 hrs, then the swell will arrive in 4 days, 6.6 hrs or on Wed 12.6Z.
7) Notice the animation contains 9 days of hindcast images plus 7.5 days of forecast images, all in 6 hour increments. The hindcast cache is larger than what is provided in the regular wave and weather models (found using this icon  on the wave model menu). This is to ensure there is sufficient 'retroactive view' to enable one to locate the source of swells that have traveled from one hemisphere to another (e.g. southern hemi to northern hemi).
Disclaimer: This is a high resolution - high power forecast tool. It is designed to be used on a desktop computer. But - with the advent of html5 and sufficient memory, CPU, and signal, it will now run on mobile devices. It is klunky, but it works. But if you have a recent model Samsung Note, the SPen works very much like a mouse and the tool is much more functional. |
