Cloud Optical Depth, τ, tau
It is a measure of attenuation of the light passing through the atmosphere due to the scattering and absorption by cloud dropelts.
The cloud optical depth or optical thickness (τ) is defined as the integrated extinction coefficient over a vertical column of unit cross section. Extinction coefficient is the fractional depletion of radiance per unit path length (also called attenuation especially in reference to radar frequencies).
Radiative Transfer Model
Earth Radiation Budget
Other sources for data