Found this at
http://www.answers.com/topic/anemometer
The connection between the velocity and the pressure of the wind is one that is not yet known with absolute certainty. Many text-books on engineering give the relation P = 0.005v2 when P is the pressure in pounds force per square foot and v the velocity in miles per hour (equivalent to P = 0.048v2 for P in pascals and v in m/s). The history of this untrue relation is curious. It was given about the end of the 18th century as based on some experiments, but with a footnote stating that little reliance could be placed on it. The statement without the qualifying note was copied from book to book, and at last received general acceptance. There is no doubt that under average conditions of atmospheric density, the 0.005 should be replaced by 0.003, for many independent authorities using different methods have found values very close to this last figure. It is probable that the wind pressure is not strictly proportional to the extent of the surface exposed. Pressure plates are generally of moderate size, from a half or quarter of a square foot up to two or three square feet (3 mÂ² to 32 mÂ²), are round or square, and for these sizes, and shapes, and of course for a flat surface, the relation P = 0.003v2 (or P = 0.029v2 for pascals and m/s) is fairly correct.