Experimental data indicate that the combined effects of wind shear and ground plane attenuation can have a strong influence on sound propagation near the ground even at distances as short as 100 feet. The effect manifests itself experimentally as a noticeable difference between sound propagation upwind vis-a-vis downwind that becomes more pronounced with either increased distance or increased wind speed. Differences of up to 25 dBA were found between into-the-wind and with-the-wind propagation for a listening height of 4 feet and a transmission distance of about 300 feet. Even a very moderate wind (4 mph) produced a difference of 12 dB at 150 feet for the same listening height. A large body of spectral data was also taken under a variety of wind conditions for path lengths of 150 feet and 225 feet. The resulting spectra agreed reasonably well with theoretical predictions for frequencies below 500 Hz, where ground and surface waves predominate; an important observation is that these waves were not affected substantially by wind conditions. Above approximately 500 Hz, the attenuation was frequently more than that predicted theoretically, and it was wind sensitive. The effect of wind was, in many cases, large, and could well mean that much experimentally obtained highway noise data is considerably less useful than previously thought, unless wind shear was taken into account during the measurements. In general, to "hear" the full effect of existing traffic, measurements should be made when the test position is downwind from the traffic.