A few months ago, I recieved a piece of literature on a solid state
anenometer.  It had an ultrasonic transmitter and receiver on each corner
of a horizontally oriented 18" equalateral triangle.  The transmitters
sequentially emitted a tone burst at a rate I believe of about 10 Hertz.
The time that it takes for the sound emitted from a given transmitter to
reach the other two sensors depends on the velocity of the air.  By doing a
little math and trig, the device calculated wind speed and direction.
Clever, huh?.