One-way, low-altitude radio propagation measurements at 94 GHz and simultaneous surface meteorological measurements were made on a 40.6 km, over-horizon, over-water path along the southern California coast to assess the effects of the evaporation duct on signal propagation. More than 2000 h of RF and meteorological data were recorded in eight measurement periods from July 1986 to July 1987. On this path, the transmission loss in a standard atmosphere is nearly 280 dB; the median transmission loss measured is approximately 220 dB. This significant decrease in loss is due to the evaporation duct. A propagation model is used to predict transmission loss from observed surface meteorology; predictions derived from 10-min averages of wind speed, air temperatures, sea temperature, and humidity compare favorably to the measured values. On average, the modeling underestimates observations by only 10 dB. The propagation model is used to predict transmission loss from an independent climatology of evaporation duct heights. A comparison of the modeled distribution to the observed distribution is favorable. The accuracy of the propagation model provides a strong justification for using it to assess the propagation characteristics of millimeter-wave communication and radar systems operating in many, if not all, ocean regions. >