We discuss the emission spectra of natural underlying surfaces measured from an airborne platform. The spectra were measured with the resolution about 0.07 mkm in the wavelength range from 2 to 5.5 mkm. The principal components of the covariance matrices of logarithms of spectral brightness are calculated for two types of the surface: clouds and the ground. It is shown that the subspaces of principal variations in the spectra of the surfaces of both types are close to each other. The spectral behavior of principal components is briefly interpreted, as is its relation to the physical factors that influence the radiative field.