Editing Anemometer (section)

===Cup anemometers===
[[File:Cup-Anemometer-Animation.gif|thumb|right|Cup anemometer animation]]
A simple type of anemometer was invented in 1845 by Rev. Dr. [[Thomas Romney Robinson|John Thomas Romney Robinson]] of [[Armagh Observatory]]. It consisted of four [[Sphere#Hemisphere|hemispherical]] cups on horizontal arms mounted on a vertical shaft. The air flow past the cups in any horizontal direction turned the shaft at a rate roughly proportional to the wind's speed. Therefore, counting the shaft's revolutions over a set time interval produced a value proportional to the average wind speed for a wide range of speeds. This type of instrument is also called a ''rotational'' anemometer.

====Four cup====
With a four-cup anemometer, the wind always has the hollow of one cup presented to it, and is blowing on the back of the opposing cup. Since a hollow hemisphere has a [[drag coefficient]] of .38 on the spherical side and 1.42 on the hollow side,<ref>{{Citation|url=https://archive.org/details/FluidDynamicDragHoerner1965 |title=Sighard Hoerner's Fluid Dynamic Drag|pages=3–17, Figure 32|date=1965}} (pg 60 of 455)</ref> more force is generated on the cup that presenting its hollow side to the wind. Because of this asymmetrical force, [[torque]] is generated on the anemometer's axis, causing it to spin.

Theoretically, the anemometer's speed of rotation should be proportional to the wind speed because the force produced on an object is proportional to the speed of the gas or fluid flowing past it. However, in practice, other factors influence the rotational speed, including turbulence produced by the apparatus, increasing drag in opposition to the torque produced by the cups and support arms, and friction on the mount point. When Robinson first designed his anemometer, he asserted that the cups moved one-third of the speed of the wind, unaffected by cup size or arm length. This was apparently confirmed by some early independent experiments, but it was incorrect. Instead, the ratio of the speed of the wind and that of the cups, the ''anemometer factor'', depends on the dimensions of the cups and arms, and can have a value between two and a little over three. Once the error was discovered, all previous experiments involving anemometers had to be repeated.

====Three cup====
The three-cup anemometer developed by Canadian John Patterson in 1926, and subsequent cup improvements by Brevoort & Joiner of the United States in 1935, led to a cupwheel design with a nearly linear response and an error of less than 3% up to {{convert|60|mi/h|km/h|abbr=on}}. Patterson found that each cup produced maximum torque when it was at 45° to the wind flow. The three-cup anemometer also had a more constant torque and responded more quickly to gusts than the four-cup anemometer.

====Three cup wind direction====
The three-cup anemometer was further modified by Australian Dr. Derek Weston in 1991 to also measure wind direction. He added a tag to one cup, causing the cupwheel speed to increase and decrease as the tag moved alternately with and against the wind. Wind direction is calculated from these cyclical changes in speed, while wind speed is determined from the average cupwheel speed.

Three-cup anemometers are currently the industry standard for [[wind resource assessment]] studies and practice.