How to Use Marine Radar for Storm Tracking
The Coast Guard's weather broadcast on your boat's radio may warn you of impending "heavy weather," but tracking an approaching storm's progress on your boat's radar allows you to take substantial action early enough to avoid the property damage and loss of life that a storm on the water can bring. Even after you make port, if you remain aboard your vessel (assuming your boat has a weather-tight cabin), you can still track the storm with your radar unit; should the storm blow itself out, either abandon your activities or enjoy the calm seas that follow.
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Instructions
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Change your radar's range to 48 nautical miles when you become aware of the storm visually or by radio broadcast. At 48 miles, your radar won't pick up anything below 1,200 feet in height, so you'll see the largest part of the storm. Once you've ascertained the general size of the storm, reduce the range of your radar until the edge of the storm is visible.
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Turn the "VRM" knob on your radar to set your radar's Variable Ring Marker on the edge of the storm. This will display a single ring that can be moved with the knob. Its range will be displayed in one corner of the screen. Note the range to the storm's edge.
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Start your stopwatch for a three-minute count and note the range of the variable ring marker. If the storm enters the area in less than three minutes, stop your stopwatch and decrease the range of the VRM until the line is again touching the edge of the storm. Note the new range of the variable ring marker.
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Subtract the new range from the original range to the storm; this is how far the storm traveled in the time recorded by your stopwatch. For example, 12.5 miles (original range) - 10.2 miles (new range) = 2.3 miles. If it took the storm 2.5 minutes to go 2.3 miles, then divide the distance traveled by the time and multiply the result by 60. For example, 2.3 (the distance traveled) / 2.5 (the time it took for the storm to travel the distance) = 0.92. Multiply the result by 60 to determine the relative speed (the storm's speed relative to your boat) of the storm. In this example, 0.92 x 60 = 55.2 miles per hour.
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Note the storm's apparent direction of movement, relative to your course, on your radar by turning the knob marked "EBL," or electronic bearing line, until the EBL parallels the course of the storm. Note the bearing indicated (also in the corner of the screen, similar to the VRM). This is the relative bearing of the storm.
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Subtract the relative bearing of the storm (the EBL bearing) from your course with your calculator. If your course is 245 degrees, and the electronic bearing line shows the course of the storm as paralleling the EBL set at 290 degrees, then 245 - 290 = -45. Add this to your course to determine the true course of the storm. In this example, 245 + (-45) = 200 degrees.
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Tips & Warnings
If your variable ring marker's setting is less than the distance to the dock, the prudent mariner would consider the condition of the boat and crew and turn to the dock at the earliest possible moment.
The prudent mariner should learn everything possible about weather before attempting to track weather with the boat's radar.
