The reaction time is determined by the so-called distance constant \(\ell_\circ\).The response time is obtained by \(\ell_\circ\) and the mean-wind speed \(U\) as \(\tau_\circ=\ell_\circ/U\). Let us consider a situation where we have a constant wind speed \(V\) and then have it changed instantaneously to \(V+\Delta{v}\). Then, after the time \(\tau_\circ\), the anemometer will show 63 percent (=1-\(\mbox{e}^{-1}\)) of the final value \(V+\Delta{v}\). We can also express this by stating that a column of air of length \(\ell_\circ\) must pass through the anemometer to get to the same result. The distance constant \(\ell_\circ\) is an instrument constant. However, the time constant is not an instrument constant, since it is inversely proportional to the mean-wind speed \(U\). The instrument has a slower response to changes in the wind speed the larger the distance constant.