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Visual estimates will normally be based upon the appearance of the surface of the sea. The wind speed is obtained by reference to the Beaufort scale and the specifications for each number. The specifications of the Beaufort scale numbers refer to conditions in the open sea.
NOTE

- Factors which in general must be taken into account in estimating wind speeds are the lag between the wind increasing and the sea getting up, the smoothing or damping down of wind effects on the sea surface by heavy rain, and the effect of strong surface currents (for instance, tidal currents) on the appearance of the sea. Sea criteria become less reliable in shallow water or when close inshore, owing to the effect of tidal currents and the shelter provided by the land.