The accuracy of wind‐speed estimates at sea

Abstract

Most wind reports at sea come from merchant ships without anemometers. The Beaufortscale is used, the units of which are now denned principally in terms of sea disturbance. A study of 1315 pairs of measured and estimated winds made on U. S. weather ships shows that the estimated speeds corresponded to the measured within 1–3 knots (same Beaufort number) in 6pct of the cases; 2pct were underestimated and 1pct overestimated. The greater frequency of underestimates than of overestimates is apparently owing mainly to the fact that the anemometers (at 20–30 m) on the weather ships are higher than the (10 m) elevation for which the speed equivalents of the Beaufort scale were established, but partly also to the extra mileage of the anemometer owing to the rolling and pitching of the ship.Changes in wind speed occur so often that lag in the reaction of the sea is the most common cause of error in estimates. Another source of trouble is the sea—air temperature difference, a cold wind relative to the sea producing larger waves than a warm wind. Decreasing winds that are also cold are especially likely to be overestimated, and increasing ones that are warm, underestimated.