  Greenhouse gases, most importantly water vapor, increase the emissivity and
decrease the albedo of clouds for thermal radiation. The modifications, which
can be of order 10% for optically thick clouds, depend on the attenuation
coefficient $\alpha^{\{g\}}$ of the greenhouse gases, and also on the
attenuation coefficient, $\alpha^{\{c\}}$, the single-scattering albedo $\tilde
\omega^{\{c\}}$, and the scattering phase function $p^{\{c\}}(\mu,\mu')$ of the
cloud particulates. Cold, high-altitude clouds with low partial pressures of
water vapor have smaller emissivities for thermal radiation and larger albedos
than otherwise identical but warmer low-altitude clouds with higher partial
pressures of water vapor. In $2n$-stream scattering theory, these phenomena can
be quantified with the intensity emissivities $\varepsilon_{i}$ of the streams
$i=1,2,3,\ldots, 2n$, and with upward or downward flux emissivities,
$\varepsilon_{\bf u}$ and $\varepsilon_{\bf d}$. The emissivities are the
ratios of the outgoing thermal intensities or fluxes to those of a reference
black cloud. Emission from optically-thick, isothermal clouds with scattering,
as well as absorption and emission, is limb darkened. Intensity emissivities
$\varepsilon_i$ for streams that are nearly normal to the cloud surface are
larger than those of more nearly horizontal streams. The limb darkening
increases with increasing values of the single scattering albedo $\tilde
\omega$. For fixed values of $\tilde\omega$, the onset of limb darkening with
increasing zenith angle is more abrupt for phase functions with more forward
scattering. Black clouds, which have only absorption and emission but no
scattering, have unit (Lambertian) emissivities, $\varepsilon_i = 1$, for all
stream directions.
