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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.
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. Note, however, that you will either have to replace the PCB inside the mechanically excellent wind direction sensor, or master using the Dallas 1-Wire protocol. (Not impossible. Arduino friendly. Opens many paths to you!). There are more details at the Dallas/ AAG unit. It is well made, on the mechanical level. You can use it with "off the shelf" wiring and software, or you can adapt it, if you are more adventurous. You won't make a better job of the basic mechanical elements. ($75, 12/05. If you don't already have one, you'll need an adapter... DS9490 or DS9097U ($20 or $15). The signals from the AAG unit come down from the roof via the Dallas MicroLan, for 1-Wire chips, unless you embark on extensive (and warrantee destroying) modifications... but MicroLan is pretty cool!!)
See the "Alternative ways..." section of my page on measuring wind speed for a clever idea which gives you both speed and direction from measurements of speed along multiple fixed headings!
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Some old (2005) sources of assembled anemometers-
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Making your own...
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!
Once you have a rotating shaft, detecting how far it has turned has much in common with other "shaft encoder" projects. (In particular, check out the material down towards the bottom of that page.)
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.
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.
My pages are not always well organizied. You may find it worth checking other pages of this site for things which might have appeared on this page, but were either mis-sited or had a more general relevance. In particular, look at what I said on the page about counters.
Page tested for compliance with INDUSTRY (not MS-only) standards, using the free, publicly accessible validator at validator.w3.org