\label{sect:soundspeed} Observations of jets and winds from CTTS indicate that typical temperatures are a few thousand K (except in shocked regions) and typical densities are in the range \(10^4-10^6 \mathrm{ cm}^{-3}\) in the optically visible component \citep[e.g.][]{2000A&A...356L..41L,2007ApJ...657..897K}. This might not be the same outflow component that our model describes, but it is the best observational estimate. With those numbers the sound speed \(c_s\) is \[c_s = \sqrt{\gamma \frac{k T}{m_{\textrm{H}}}} \approx 10 \textrm{ km s}^{-1} \; ,\] which is low enough that a strong shock forms even for small \(\psi\).