A swell forecast of "4 ft at 12 seconds" indicates what?

The swell forecast "4 ft at 12 seconds" indicates a wave height of 4 feet and a wave period of 12 seconds. In marine weather forecasting, the term "swell" typically refers to long-period waves that have traveled away from their generating source.

The "4 ft" specifies the height of the waves from trough to crest, which is an important parameter for sailors, as it gives them an idea of the potential challenges they might face at sea. A higher wave height can lead to rougher conditions.

The "12 seconds" refers to the wave period, which is the time it takes for two successive wave crests to pass a fixed point. A longer wave period often indicates more powerful swells that may have traveled across a large distance, which can also provide insight into the state of the sea and the potential impact on sailing conditions.

Understanding wave height and wave period is crucial for sailors, as these factors directly affect the behavior of the vessel in the water. The other options either misinterpret these terms or incorrectly attribute the measurements, thereby not accurately reflecting standard maritime wave terminology.