- 1). Set a variable ring marker (VRM) on the edge of the storm that is nearest you by turning the VRM knob until the line touches the edge of the storm. Start your stopwatch. Note the range of the VRM. If the storm crosses the variable ring marker, part of it is coming closer to you.
- 2). Set an electronic bearing line (EBL) on the edge of the storm nearest the center of the screen by turning the EBL knob until the line touches the edge of the storm.
- 3). Set the "track" function of the radar to 60 seconds; this takes a snapshot of the storm's position every 60 seconds, displaying it as an electronic trail of the storm's echo.
- 4). Stop the stopwatch after six minutes--1/10 hour--have passed and adjust the VRM to the nearest edge of the storm again. Note the range. Subtract this range from the initial range to determine the distance the storm moved in three minutes. If the number is negative, the storm is moving past you. If the number is positive, use the formula D/T = S, where D = distance, T = time elapsed and S = speed of the weather. If the initial VRM range was nine miles and the second VRM range (six minutes later) was six miles, then 9 - 6 = 3 miles/.1 hour = 30 knots, the rate at which the storm is moving in nautical miles per hour (1 knot = 1 nautical mile per hour).
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