Light coming from a cloud to the detector of a multispectral satellite (Landsat) consists of light reflected from the cloud and light scattered by the layer of atmosphere between the cloud and satellite. Because of the strong wavelength dependence of Rayleigh scattering, the comparison of the signals in two spectral channels allows one to find the thickness of the scattering layer and hence the height of the cloud top.
Yuri Mekler and Morris Podolak, "Estimation of cloud top heights from satellite imagery," Appl. Opt. 24, 2419-2422 (1985)