Editing Anemometer (section)

==Pressure anemometers==
[[File:Britannia Yacht Club tour, burgee, and wind gauge.jpg|thumb|right|[[Britannia Yacht Club]] clubhouse tour, burgee, and wind gauge on roof]]
The first designs of anemometers that measure the pressure were divided into plate and tube classes.

===Plate anemometers===
These are the first modern anemometers. They consist of a flat plate suspended from the top so that the wind deflects the plate. In 1450, the Italian art architect [[Leon Battista Alberti]] invented the first such mechanical anemometer;<ref>{{cite web |title=Windvanes and anemometers |url=https://brunelleschi.imss.fi.it/itineraries/multimedia/WindvanesAndAnemometers.html|publisher=[[Museo Galileo]] - Istituto e Museo di Storia della Scienza |series=Scientific itineraries in Tuscany}}</ref> in 1663 it was re-invented by Robert Hooke.<ref>{{cite book |contribution=A Method for making a History of the Weather |contributor-first=Robert |contributor-last=Hooke |title=The History of the Royal Society of London |first=Thomas |last=Sprat |author-link=Thomas Sprat |date=1746 |orig-date=1663
 |contribution-url=https://en.wikisource.org/wiki/The_History_of_the_Royal_Society_of_London/Chapter_8}}</ref><ref>{{cite web |title=History of the Meteorological Office  |first=Malcolm |last=Walker |publisher=Cambridge University Press |quote=The habit of making weather observations regularly and systematically was encouraged by the Royal Society, and as early as 1663 Hooke presented to the Society his paper titled 'A method for making a history of the weather' |url=https://assets.cambridge.org/97805218/59851/excerpt/9780521859851_excerpt.htm}}</ref> Later versions of this form consisted of a flat plate, either square or circular, which is kept normal to the wind by a wind vane. The pressure of the wind on its face is balanced by a spring. The compression of the spring determines the actual force which the wind is exerting on the plate, and this is either read off on a suitable gauge, or on a recorder. Instruments of this kind do not respond to light winds, are inaccurate for high wind readings, and are slow at responding to variable winds. Plate anemometers have been used to trigger high wind alarms on bridges.

===Tube anemometers===
[[File:Tube anemometer invented by William Henry Dines.jpg|thumb|left|Tube anemometer invented by William Henry Dines. The movable part (right) is mounted on the fixed part (left).]]
[[File:Instruments at Mount Washington Observatory.JPG|thumb|Instruments at [[Mount Washington Observatory]]. The pitot tube static anemometer is on the right.]]
[[File:Pitot tube static anemometer.JPG|thumb|The pointed head is the pitot port. The small holes are connected to the static port.]]

[[James Lind (1736–1812)|James Lind]]'s anemometer of 1775 consisted of a vertically mounted glass U tube containing a liquid [[manometer]] (pressure gauge), with one end bent out in a horizontal direction to face the wind flow and the other vertical end capped. Though the Lind was not the first, it was the most practical and best known anemometer of this type. If the wind blows into the mouth of a tube, it causes an increase of pressure on one side of the manometer. The wind over the open end of a vertical tube causes little change in pressure on the other side of the manometer. The resulting elevation difference in the two legs of the U tube is an indication of the wind speed. However, an accurate measurement requires that the wind speed be directly into the open end of the tube; small departures from the true direction of the wind causes large variations in the reading.

The successful metal pressure tube anemometer of William Henry Dines in 1892 utilized the same pressure difference between the open mouth of a straight tube facing the wind and a ring of small holes in a vertical tube which is closed at the upper end. Both are mounted at the same height. The pressure differences on which the action depends are very small, and special means are required to register them. The recorder consists of a float in a sealed chamber partially filled with water. The pipe from the straight tube is connected to the top of the sealed chamber and the pipe from the small tubes is directed into the bottom inside the float. Since the pressure difference determines the vertical position of the float this is a measure of the wind speed.<ref>{{cite journal|title=Anemometer Comparisons|first=W. H.|last=Dines|journal=Quarterly Journal of the Royal Meteorological Society|date=1892|volume=18|issue=83|page=168|url=https://archive.org/details/quarterlyjourna33britgoog |access-date=14 July 2014|doi=10.1002/qj.4970188303|bibcode=1892QJRMS..18..165D}}</ref>

The great advantage of the tube anemometer lies in the fact that the exposed part can be mounted on a high pole, and requires no oiling or attention for years; and the registering part can be placed in any convenient position. Two connecting tubes are required. It might appear at first sight as though one connection would serve, but the differences in pressure on which these instruments depend are so minute, that the pressure of the air in the room where the recording part is placed has to be considered. Thus, if the instrument depends on the pressure or suction effect alone, and this pressure or suction is measured against the air pressure in an ordinary room in which the doors and windows are carefully closed and a newspaper is then burnt up the chimney, an effect may be produced equal to a wind of 10&nbsp;mi/h (16&nbsp;km/h); and the opening of a window in rough weather, or the opening of a door, may entirely alter the registration.

While the Dines anemometer had an error of only 1% at {{convert|10|mi/h|km/h|abbr=on}}, it did not respond very well to low winds due to the poor response of the flat plate vane required to turn the head into the wind. In 1918 an aerodynamic vane with eight times the torque of the flat plate overcame this problem.

====Pitot tube static anemometers====
Modern tube anemometers use the same principle as in the Dines anemometer, but using a different design. The implementation uses a [[pitot tube|pitot-static tube]], which is a pitot tube with two ports, pitot and static, that is normally used in measuring the airspeed of aircraft. The pitot port measures the dynamic pressure of the open mouth of a tube with pointed head facing the wind, and the static port measures the static pressure from small holes along the side on that tube. The pitot tube is connected to a tail so that it always makes the tube's head face the wind. Additionally, the tube is heated to prevent [[rime ice]] formation on the tube.<ref>{{cite web|title=Instrumentation: Pitot Tube Static Anemometer, Part 1|url=http://www.accuweather.com/en/weather-blogs/clarkb/instrumentation/14828|publisher=Mt. Washington Observatory|access-date=14 July 2014|archive-url=https://web.archive.org/web/20140714135450/http://www.accuweather.com/en/weather-blogs/clarkb/instrumentation/14828|archive-date=14 July 2014|url-status=dead}}</ref> There are two lines from the tube down to the devices to measure the difference in pressure of the two lines. The measurement devices can be [[manometer]]s, [[pressure transducer]]s, or analog [[chart recorder]]s.<ref>{{cite web|title=Instrumentation: Pitot Tube Static Anemometer, Part 2|url=http://www.accuweather.com/en/weather-blogs/clarkb/instrumentation-1/15065|publisher=Mt. Washington Observatory|access-date=14 July 2014|archive-url=https://web.archive.org/web/20140714232314/http://www.accuweather.com/en/weather-blogs/clarkb/instrumentation-1/15065|archive-date=14 July 2014|url-status=dead}}</ref>

===Ping-pong ball anemometers===
A common anemometer for basic use is constructed from a [[ping-pong ball]] attached to a string. When the wind blows horizontally, it presses on and moves the ball; because ping-pong balls are very lightweight, they move easily in light winds. Measuring the angle between the string-ball apparatus and the vertical gives an estimate of the wind speed.

This type of anemometer is mostly used for middle-school level instruction, which most students make on their own, but a similar device was also flown on the [[Phoenix Mars Lander#Meteorological station|Phoenix Mars Lander]].<ref>[http://marslab.au.dk/research/instrument-development/the-telltale-project/ "The Telltale project."] {{webarchive |url=https://web.archive.org/web/20120220080017/http://www.marslab.dk/TelltaleProject.html |date=20 February 2012 }}</ref>

===Effect of density on measurements===
In the tube anemometer the dynamic pressure is actually being measured, although the scale is usually graduated as a velocity scale. If the actual air density differs from the calibration value, due to differing temperature, elevation or barometric pressure, a correction is required to obtain the actual wind speed. Approximately 1.5% (1.6% above 6,000 feet) should be added to the velocity recorded by a tube anemometer for each 1000&nbsp;ft (5% for each kilometer) above sea-level.