Your present weather is sunny with a steady barometer. A low swell approaches your vessel from the south with crests passing at relatively long periods of about four per minute. This usually indicates __________.

Correct Answer: Option B — a tropical cyclone south of your vessel

Long-period swells, known as precursor waves, originate from the intense wind fields of a distant tropical cyclone. According to Bowditch, these swells travel faster than the storm itself. When arriving from a specific direction under otherwise fair conditions, they serve as a reliable warning of a storm's location.