is normalized to be 1.0 at full face, thereby defining the geometric albedo, and is a decreasing function of $\alpha$. For so-called “Lambert" reflection in which an incident ray on a planetary patch emerges uniformly over the exit hemisphere, $p$ is 2/3 for purely scattering atmospheres and $\Phi(\alpha)$ is given by the formula: $$\Phi(\alpha) = \frac{\sin(\alpha) + (\pi - \alpha)\cos(\alpha)}{\pi}\, .
\label{lambert}$$ However, EGP atmospheres are absorbing and the anisotropy of the single scattering phase function for grains, droplets, or molecules results in non-Lambertian behavior. For instance, back-scattering off cloud particles can introduce an “opposition" effect for which the planet appears “anomalously" bright at small $\alpha$s. This spike might be a useful signature of cloud particle size. Moreover, the light scattered from EGPs is likely to be strongly polarized[@SWS]. The degree of polarization as a function of wavelength and phase angle $\alpha$ can also be used to determine cloud properties. However, polarization will be rather more difficult to measure.

Both $p$ and $\Phi(\alpha)$ are functions of wavelength, but the wavelength-dependence of $p$ is the most severe. In fact, for cloud-free atmospheres, due to strong absorption by molecular bands, $p$ can be as low as 0.03. Rayleigh scattering serves to support 