Plane-parallel validity from pressure scale height
= Plane-parallel validity from pressure scale height
{title2=$H/R\ll1$}
A <plane-parallel atmosphere> is geometrically accurate only if the relevant line-forming region is thin relative to the stellar radius. An isothermal estimate gives $H=k_BT/(\mu m_ug_{\rm eff})$. Radiative acceleration reduces the effective gravity and increases extension. A small photospheric $H/R$ does not justify a plane-parallel description of wind lines formed far above that photosphere.