Back Off Man, We're Scientists!

Suppose you saw a big old guy in a broad brimmed hat walking around in a small forest of odd looking objects on tripods, holding aloft a long, slim strip of toilet paper. Sometime standing here or there with the toilet paper directly overhead, sometimes moving about or rotating slowly, with the tp held at arms length, first to one side and then another.

You might well think you were looking at a wacko, but were you really?

Once upon a time (and sometimes even now), winds were measured with propeller anemometers. These instruments look a bit like a miniature airplane fuselage on a stick, with a propeller in front and a tail behind. The drag on the tail keeps the instrument pointed into the wind while an angle sensor records the wind direction. The rate of spin can be decoded to yield the wind speed. These instruments are slow of response and not terribly accurate.

A more modern technology is the sonic anemometer. It consists of two or three pairs of usually orthogonal tubes containing ultrasonic transducers pointed at each other across an air gap. It measures the wind speed by measuring the travel times of ultrasonic pulses back and forth across that air gap. They have much higher sampling rates than the propeller types, and are fairly easy to keep in calibration.

The propeller types do have an advantage though. By looking at one you can see the wind direction and get an idea of the speed. Sonics, on the other hand, just sit there, and if your real time readout isn’t operational, even a weatherman can’t tell which way the wind is blowing.

Unless, of course, he tears of a thin, lightweight strip of toilet paper and parades about watching which way the wind is blowing it. Smoke is even better, but who smokes nowadays?

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