When performing two-microphone measurements of surface properties, an algorithm is used to find the properties that give the best fit between measured and predicted transfer functions between the two microphones. Recently it was found necessary to adjust certain geometrical parameters, e.g., the separation between the source and microphones, to obtain an acceptable fit between measured and predicted data measured over asphalt surfaces. It was hypothesized that it was necessary to allow that variation to account for the effects of wind and temperature gradients. That hypothesis has been confirmed by using ray tracing techniques to simulate the effect of wind and temperature gradients on two-microphone measurements. It has been found that near-surface gradients have significant effects on both the angle of specular reflection and the path length difference between the direct and reflected arrivals. These effects are particularly important when estimating the properties of high impedance surfaces owing to the rapid change of the reflection coefficient at near-grazing angles in that case. Nevertheless, it will be shown that the impedance of asphalt can be estimated accurately if the effect of ray curvature is accounted for when fitting measured and theoretical transfer functions.