Geometry of transmission spectroscopy in a planetary wind. A planet of radius R_p is surrounded by a bound hydrostatic atmosphere and an outer hydrodynamically escaping region. Stellar rays intersect the atmosphere along chords of impact parameter b, accumulating a wavelength-dependent slant optical depth, τ(b), along the line of sight toward the observer. The effective transit radius, R_eff(λ), is defined by the impact parameter for which τ(b) reaches the reference value, τ_∗. The dashed blue circle marks the sonic radius, r_s, and the approximate location of the XUV photosphere, R_XUV, is also indicated.