= Atmospheric spectral-feature amplitude
An <exoplanet transmission spectrum> measures a wavelength-dependent <transit depth>. In a simple opaque-disk geometry with uniform stellar brightness, an effective planetary <radius> $R_p$ gives $D=(R_p/R_*)^2$, where $R_*$ is the stellar <radius>. If stronger atmospheric <absorption> increases that radius by $\Delta R=N_HH$, define $H$ as the <atmospheric scale height> and $N_H$ as the number of scale heights separating the two spectral levels. The additional occulted annulus gives
$$
\Delta D=\frac{(R_p+N_HH)^2-R_p^2}{R_*^2}
=\frac{2R_pN_HH+N_H^2H^2}{R_*^2}
\simeq\frac{2R_pN_HH}{R_*^2}.
$$
The last approximation requires $N_HH\ll R_p$. For instance, $R_p/R_*=0.1$ and $N_HH/R_p=0.05$ give exact $\Delta D=0.001025$ and leading estimate $0.001$; these are stipulated geometric ratios.
Since $H=k_BT/(\mu m_Hg)$ in the thin <isothermal atmosphere> model, larger <temperature>, smaller <mean molecular weight> or weaker <gravity> can increase the signal. The factor $N_H$ is set by the atmospheric <opacity>, composition and cloud structure; it is not a universal constant. <Limb darkening>, stellar heterogeneity and instrumental effects also affect the observed <transit depth>.<Theoretical Transmission Spectra During Extrasolar Giant Planet Transits by Sara Seager and Dimitar Sasselov>[Transmission absorption through the atmospheric annulus; weak planet-to-star contrast and cloud sensitivity]{ref}
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