^a To attain a fully arisen sea for a certain wind speed, the wind must blow at that speed over a minimum distance (fetch) for a minimum time (duration). When winds are 50 knots or more, the required fetch and duration for a fully arisen sea rarely occur. The wave heights shown in the last column, ''Average Wave Height'' represent what will be found on the average at given wind speeds.

Wave heights refer only to wind waves, and swells from distant or old storms are nearly always superimposed on the wind-wave pattern.

Practical Methods of Observing and Forecasting Ocean Waves, Pierson, Newuman, James, H.O. Pub. 603, 1955.

^b H.O. 118A