The principle is simple:
The device consists of a transparent cone-shaped tube, open at both
ends. The lower opening is rotated 90 degrees to face the wind. Inside
the tube there is a disc, very light, balanced by the wind pressure and
Due to the conical shape of the tube, the flow of air
circulating inside the tube has variable speed. In its most narrow
section (bottom) air runs faster than in the wider section
(up) due to the conservation of the air flow between the two
When the external wind speed increases, the dynamic pressure on the
disc increases (in proportion to the square of wind speed). The pressure on the
disc is greater than its own weight, then the disk moves upwards.
But, simultaneously decreases the wind speed inside the tube, and
pressure re-balance the weight of the disc in its new position.
Similarly, when the external wind speed decreases, decreases the
pressure on the disc, which is not enough to balance its own weight.
Then the internal disc moves downward, and simultaneously increases the wind
speed inside the tube, until its pressure balances the disk in the new
this is a simplified description of other effects that occur. But for
practical purposes, can be used to make a functional anemometer,
experimenting with different tubes and disks of various
sizes and weights.