= Sonic anemometer
A <sonic anemometer> measures wind from acoustic <travel times> between transducers. For <path length> $L$ aligned with <velocity> component $u$, the idealized transit times are $t_+=L/(c+u)$ and $t_-=L/(c-u)$, giving
$$
u=\frac L2\left(\frac1{t_+}-\frac1{t_-}\right).
$$
The reciprocal-time difference removes the common <sound speed> in this model. Several paths recover a <velocity vector>. Transducer wakes, path averaging and precipitation can affect the <measurement>, so <geometry> and <calibration> matter when using it for turbulent <fluxes>.
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