The theory of microwave thermal emission from a nonscattering half-space medium is developed for application to regions with nonuniform subsurface soil moisture and temperature variations. A coherent stratified model is presented which is valid for nonuniform temperature profiles and rapidly varying moisture profiles, under which conditions the commonly used emissivity and radiative transfer approaches become inaccurate. For naturally occurring profiles the stratified model gives more accurate results than the other approaches at frequencies below about 4 GHz. Experimental results from ground-based radiometric observations of a controlled target area compare systematically with brightness temperatures predicted from the theoretical model to within approximately 10¿K. Results of dielectric constant measurements of the sand are given at seven frequencies in the microwave range and for moisture contents in the range 0--30% by volume. By using this model the thermal microwave emission spectrum is computed for a number of representative moisture and temperature profiles in the frequency range 0.25--25 GHz. A regression technique is used to show that multifrequency data can be used to obtain moisture and temperature gradients in the soil when an estimate of the surface temperature is available.