Wind Speed Probes

In another response to a question about Wind Probes I put together this short explanation on why cheaper is better!

If you absolutely MUST have one, get a simple Hall Airspeed Indicator. The usual anemometers you see folks holding up at your local ridge are impeller driven (with a little propeller) and what they're measuring is the speed of the air molecules as they pass through the impeller. However, they do not compensate for air temperature or density (density altitude) and so have little bearing on how flyable or unflyable it is unless you happen to be at sea level and the temp is 59ºF. The Hall airspeed indicators will compensate for pressure, temperature, etc. The reason that's important, especially in places where you're likely to be launching at high altitude, is that if you measure the wind speed with an impeller at 10,000 ft you'll get a 15% faster reading than if you measure it with a Hall meter. To better illustrate my muddled explanation, here's an example:

You have a hall meter and an impeller attached to your toes, in clean air flow. You launch from sea level at 59ºF and go hands up on your glider. Both wind probes read 22mph. As you climb, the impeller driven meter will begin to read faster because your glider is moving through less dense air and your true airspeed increases (it has to move faster through the air in order to generate the same lift with fewer molecules). However, the Hall meter will continue to read 22 mph because the pressure being exerted on it remains the same, as it does on your glider.

If you are already stuck with one of those battery eating, propeller spinning, whizzbang wind-gadgets you can do a quick recalculation to get close to the results of a Hall probe. For every 3k feet above sea level subtract 1 Mph from your indicated wind speed. So, for example, you're standing on a 10,000 ft mountain and are getting a reading of 20 Mph. Subtract ~3.3 Mph to get a reading of 16.7 Mph which is what the wind should feel like anyway.

Hall meters are about half the price and don't require batteries!