Results of measurements made with radars from 35 to 217 GHz of near-earth propagation in rain, fog, snow, and humidity are presented. The effects of water vapor are characterized by an attenuation coefficient that is a function of vapor pressure or absolute humidity. Backscatter contributes significantly to the adverse effect of rain on mm-wave radars because the droplet sizes become comparable to signal wavelengths. Path losses from fogs, haze, or clouds, are caused by both absorption and scattering by water droplets. Attenuation due to falling snow is difficult to link to and single characteristics.