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For this, anemometers are used, devices similar to weather vanes. They are calibrated in wind tunnels so that the number of turns of their blades corresponds to a specific speed. It works something like this: in the wind tunnel, a powerful current of air is created at a fixed speed. Inside it is placed an anemometer that begins to have the blades rotated. If, say, the tunnel is blowing wind at a speed of 10 km/h and the blades rotate 100 times in one minute, the technicians can already program the device to indicate 10 km/h every time it reaches 100 rotations per minute. Even so, the question remains: how to know the speed of the air current before calibrating the anemometer? One of the techniques used is to place a hot wire inside the tunnel. “When the wind starts, the yarn gets colder. So, the greater the cooling, the greater the air velocity”, says meteorologist Amauri Oliveira, from USP.
It is an operation that follows a mathematical formula: knowing the temperature of the wire without wind, the scientists calculate how fast the air current will be when the wire cools down. With this information in hand, all you have to do is calibrate the anemometer and go around measuring natural winds.
The force of the wind propels the shells and makes an axis turn.
In some devices, there is a tachometer. The more turns the axis makes, the greater the air speed.
In other anemometers, there is an energy generator, powered by the gyres. The more power generated, the higher the wind speed