Apparent wind in knots and degrees. Note that a number of additional factors come into play when converting the measurements from the masthead anemometer into the true wind if a high degree of accuracy is required, including the following: In the presence of a current, the true wind is considered to be that measured on the craft drifting with the water over the bottom, and wind with respect to the sea bed as the "ground" or "geographical wind". The apparent wind on board (a boat) is often quoted as a speed measured by a masthead transducer containing an anemometer and wind vane
Environmental impact of wind power made no mention of avian deaths per unit of electricity produced, which excluded meaningful comparisons between different energy sources. More importantly, it concluded, the most visible impacts of a technology, as measured by media exposure, are not necessarily the most flagrant ones. Sovacool estimated that in the United States wind turbines kill between 20,000 and 573,000 birds per year, and although he regards either figure as minimal compared to bird deaths from other causes. He uses the lower 20,000 figure in his study and table (see "Causes of avian mortality" table) to arrive at a direct mortality rate per unit