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The grand name for a wind speed sensor is "anemometer".
Many anemometers have something that spins around when the wind blows, and creates some kind of electrical signal.
Two types are common: One closes a switch once per revolution (I think this is the way to go.) The other makes different voltages depending on how fast the cups are spinning. The former is a "digital" signal in the best sense of the term, and it is no accident that digital electronics long ago outstripped analog. Digital electronics is reliable and inexpensive.
Once upon a time, when I was young, poor and foolish, I would have encouraged you to try to make your own sensor head. (I have done it!) And I'll talk about that in a moment. But first...
I really like, on various grounds, the illustrated simple, inexpensive speed sensor which is available from various sources. That link will open in a new window or tab, so just close it to come back here, if it isn't the one for you. It is often sold as part of a "set" of sensors... speed, direction, rainfall. But you can find it for sale on its own.
In August 2011, I bought a very nice unit from Inspeed, with 100' of good cable already attached. I've used the unit with no hassles since then until today, 1/17, when I am doing a "catch up" edit of this page. I expect to use it for years to come.
(Attaching cables at the sensor head has to be done right: All of that is going to be out in bad weather... which includes bright, plastic destroying, sunshine.)
With shipping, etc, The Inspeed sensor came to $71.70. The order process was easy; they were willing to ship to a post office box without extra hassle. You can save a little money if you buy a unit with less cable... a sensible range of choices is on offer. It had a good mounting bracket which I merely screwed to my already exiting mast. Inspeed sell a number of solutions. All I wanted was a well made basic (switch type) sensor, and I got it in their Vortex Sensor.
Also consider: Once upon a time, you could get a nice combined unit with wind speed and wind direction sensors. They still turn up on eBay, from time to time... watch for the "AAG" brand, or mention of Dallas's "1-Wire" chip family.
The Argent Data Systems weather sensing package has a speed sensor, which is why there is an entry here. But it also has a direction sensor, and rainfall counter. So I gave the Argent Data Systems weather sensors array a page of its own.
Alternatively, and perhaps not for every weather hobbyist, but you might be interested in an alternative way of measuring wind speed: A wire is exposed to the elements. It is heated by passing a current through it. The more wind there is, the harder it is to achieve a particular temperature-above-ambient. There are several available, e.g. the one from ModernDevice.com Electronics hobbyists, especially those using an Arduino might want to consider this sensor. At 8/11, assembled, I believe: $17 + p&p.
It is also possible to calculate wind speed and direction by having several ultrasonic transponders "squeeking" at detectors in a small unit which measures the time taken for the sounds to travel different directions through a horizontal plane. Measurements of Doppler shifts, and some hairy geometry calculations will tell you the wind speed and direction. A sensor using this technology is described below.
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Some old (2005) sources of assembled anemometers...
Peet Brothers, in Florida, did what looked like a nice combined wind speed / wind direction sensor. Price at 12/05: $90+p&p. It was also available in a heated version, for places where freezing rain is a problem. They had a very clever way of determining the wind direction... clever in that it is achieved with very simple mechanical elements in the sensor head, so the sensor is likely to be reliable. There just two switches. The frequency of pulses on one gives you the wind speed. The relationship between the pulse trains from the two switches tells you the wind direction... if both switches go on at the same moment, the wind is from the north. If one goes on half-way between two "on"s from the other, the wind is from the south. If you have ideas you'd be willing to share here for how you interpret this data easily, or links to the same, please contact me? (Peet Brothers, obviously, will sell you a box which takes care of that... but some of you will share my urge to do things the hard way.) (Peet also offer a tipping bucket type rain gauge for $90 (12/05), and a fancier rain gauge for $190.)(and other things!))
Back in 12/05 Weathershop.com sold a nice looking unit, which is seemed to be based on the "Vortex" sensor I recently purchase in "simple" form at Inspeed. (See above). At 12/05, price was $80+p&p.
Back in 2005, Rainbird offered what looked like nice anemometers and rain gauges... and they seem to be available still at 8/11. The anemometer looks like the Vortex unit- see above. I may come back to these devices when I have more time, but I couldn't (in the time I was willing to spend) discover where the company is headquartered... I'd guess somewhere in the US... nor could I be sure that both devices gave outputs equivalent to a momentary switch opening and closing. (Rain gauge seemed to be switch based. Rain collecting opening only 3.8"... for US Weather Service work, I think 8" is the standard.) The "spec sheet" was disappointing... more a marketing glossy than geek heaven. Neither could I find prices! The company seems to have its roots in the world changing (I'm not being snide... I believe this) impact sprinkler that we've all seen, and probably given too little respect. There's an interesting note on the site under "Company History".
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Making your own wind speed sensor...
Visit any marina, and you will see wind sensors at the top of many masts. I've yet to discover where you can buy one. I also strongly suspect that the actual rotors/ vanes are frequently damaged in regions where the boats are pulled from the water for the winter, but, again, I can't discover where to buy just rotor or vane as a replacement part. Yes... I have asked at a marina and searched Google!
The actual cups are tedious to make... you might want to spend your energies tracking down a commercial manufacturer of wind speed sensors who will sell you just the cups.
Don't skimp on the mechanicals! In even a moderate wind, the anemometer will spin very rapidly. Bad bearings or an unbalanced device will degrade your instrument's reliability. Also, you want it to last.
Once you have a spinning shaft, detecting how fast it is turning has much in common with other "how fast is it spinning" projects. Click through to this page, and look down towards the bottom.
A brief note of encouragement: One day, fed up, I said "I'll make my own". It went surprisingly well! Very crude, but not entirely useless. A little wood, some wood dowels (about 5mm diameter), some coat hanger wire, a reed switch, a magnet, and some plastic balls from a toy store.
I used a small bit of wood to mount three horizontal arms to a vertical shaft. (Arms and shaft: Dowel wood). Stuck in the bottom of the shaft: A vertical pin. It "rides" on a horizontal surface. It is kept from drifting horizontally by a collar of coat hanger wire. (Essentially, it is a home made screw-eye). Near the top of the shaft, there's another screw eye, to keep the shaft upright. This is the weakest link in the design.
The cups are made from hollow semi-rigid plastic balls cut in half. Bits of wire keep them from flying off the ends of the spokes... a problem which I underestimated on my first attempt at this device.
PS: Didn't last many months. Sigh. However, one I made in 1982, with a set of injection molded cups, is still running fine! (I've had to replace the reed switch a few times.
At 12/05 http://www.wondermagnets.com sold a nice set of anemometer cups at a reasonable price, but they don't seem to stock them any more at 8/11. Know a source of just the triangular cup assembly?
A UK retailer, Weather Front, used to sell a nice set of cups, at 12/05, £22 + p&p. They were replacement cups for the Davis weather station. At 8/11, I didn't find them on the site, but I might have overlooked them. At 1/17, I found all sorts of interesting stuff... complete direction/ speed "heads", among other things. Maybe worth further research, or a quick email if you want just cups or or vane.
First at 8/11, and still at 1/17 I found encouraging things in the "replacement parts" section of the Peet Brothers Company website... anemometer rotors and wind direction vanes. . If you want to try a deep link, http://www.peetbros.com/shop/category.aspx?catid=15 might take you someplace useful. Peet Brothers have been selling weather sensors since at least 2005.
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