Spaceborne remote sensing of aerosol particles, evaluation of the climatic effects of aerosol, and atmospheric corrections of spaceborne imagery of the Earth's surface are based on an assumed relationship between the spectral aerosol optical thickness and the spectral path radiance. Path radiance is the radiance detected by a spaceborne sensor above a nonreflective surface and is the result of backscattering to space by particles and molecules in the atmosphere. In specific measurement conditions the path radiance can be measured also from the ground. Simultaneous measurements of the path radiance and the optical thickness from the ground are reported at over 30 locations spread all over the world. All the measurements are performed with a single eight-channel portable sunphotometer/radiometer in the 0.44- to 1.03-μm range. The observations are taken for constant solar and view directions, resulting in a constant scattering angle of 120¿, which resembles space observations. One set of measurements is used to develop empirical relationships between the aerosol spectral optical thickness and the scattered spectral path radiance. A second independent set is used to test these relationships. It is shown that simple measurements performed from the ground can yield empirical relationships that can be used to check some of the common but not validated assumptions about the particle homogeneity, sphericity, composition, and size distribution used in remote sensing models and in estimates of the radiative effects of aerosol. Results are summarized in Table 4 and used to test concepts of atmospheric corrections and remote sensing of aerosol from space.