Sky and solar radiance are of major importance in determining the ground temperature. Knowledge of their behavior is a fundamental part of surface temperature models. These two fluxes vary with elevation and this variation produces temperature changes. Therefore, when using thermal-property differences to discriminate geologic materials, these flux variations with elevation need to be considered. From a representative set of field observations, it was found that the flux variations with elevation can cause in the mean diurnal temperature gradient from -4¿ to -14¿C per km evaluated at 2000 m. Changes in the temperature-difference gradient of 1¿--2¿C per km are also produced which is equivalent to an effective thermal-inertia gradient of 100 W s1/2 m-2 K-1 per km. Thus, exposed bedrock on topographic ridges will appear to have a lower thermal inertia due to the additional effect.