Anemometers are used to measure either wind speed or air pressure, depending on the style of anemometer. The most familiar form, the cup anemometer, was invented in 1846 by John Thomas Romney Robinson and features four hemispheric cups arranged at 90-degree angles. Making this form of anemometer is a common school science project, but reading it requires a bit of mathematical skill.
Mark one cup as the reference cup by painting it or tying a piece of yarn to the rod to which it is attached.
Place the anemometer out in the wind. The cups are already arranged to catch the breeze.
Calculate the distance the reference cup will travel to revolve around the axis once by taking the length of its rod from the axis, doubling it and multiplying by the value of pi. (This is the formula for finding the circumference of a circle from its radius.) Convert this distance to either feet or meters for convenience.
Count the number of times the reference cup makes a complete revolution around the axis in a minute.
Multiply the distance traveled in one revolution by the number of times the reference cup revolved around the axis. This will produce an approximate wind speed in feet per minute or meters per minute.
Convert this value to distance per hour by multiplying by 60. If the distance is measured in feet, divide by 5,280 to produce an approximate wind speed in miles per hour. If the distance is measured in meters, divide by 1,000 to produce an approximate wind speed in kilometers per hour.