The mutual relation and the observed correlation between reflective and thermal IR images of ground terrain are presented and discussed. The joint distribution density function of thermal and reflective images and its diurnal behavior are presented and analyzed. An approximated analytic expression of the correlation coefficient between thermal and reflective images derived from the explicit form of the joint distribution function is calculated and presented, but it fails to predict the observed correlation between images in these two bands. A modification of the distribution function to consider the influence of local ground topography in the scene is introduced. The prediction of the modified joint density function and correlation coefficient shows a much better agreement with the experimental data. The meaning and the practical applications of the model and the experimental data are discussed.