A fundamental relationship that is the foundation for all radar is that a target’s range is proportional to an echo delay time. The actual relationship requires knowledge of the velocity of propagation of the signal whose echo delay time is measured. A typical assumption for radar ranging is to use free-space velocity of propagation. However, atmospheric dielectric properties yield a measurably slower velocity of propagation that is a function of temperature, atmospheric pressure, and especially humidity. This results in range measurement errors. A simplified model is developed to estimate the error in range measurements for airborne ground-surveillance radars.