How do I determine "Prevailing Wind"?

Question

How do I Determine the Prevailing Wind on a small area 4-5 ACcers?

Answer 1

http://www.usatoday.com/weather/wanemom.htm
it is difficult to determine the true direction and speed of the prevailing wind but here is a site to show how it is measured

Answer 2

The prevailing wind is the average direction weighted by wind speed. Measurements should be taken at least 2 or 3 times daily for a year. One way of computing the average is to multiply each bearing by its wind speed, add the products, and divide by the sum of the wind speeds.