Hi Luca,
From a different angle and perhaps easier to build is to get hold of an old VCR , photocopier or printer, light running steppermotor, and attach it to the anemometer propellor, and drive an electronic speedo from a car with it.
You get impulses directly proportional with the windspeed.
I have an old Holden speedo cluster which works of 12 Volts. An extra pin at the speedo requires a pulsetrain which you might be able to collect from the steppermotor, and directly read the windspeed in km/h. As extra bonus is that you get actual kilometres recorded as well on the odometer.
You may need to use a schmitt trigger to clean up your pulses and a 4017 to get the pulsecount correct.
This example is an idea I have already for quite a while but haven't had the time to implement it yet.
I know that the speedo works very precisely when the steppermotor is turned around by hand.
Regards and good luck with the project ,
Raymond