Since it is becoming increasingly difficult to obtain frequency assignments below 10 GHz for ground-to-aircraft wideband data links, frequencies in the window regions of the millimeter wave spectrum are now being considered for this application. In order to estimate system performance at these frequencies, the atmospheric attenuation along the path must be known. In this paper, 440 sets of slant-path attenuation data at frequencies of 15 and 35 GHz are presented. The attenuations are measured with an 8.8 m paraboloidal antenna located at Prospect Hill, Waltham, Massachusetts, using the sun as a source. Meteorological data such as surface temperature, pressure and absolute humidity were recorded at the time of the measurements along with somewhat subjective descriptions of cloud conditions along the path. The systematic measurement and recording of a substantial amount of attenuation data at many elevation angles, under variable atmospheric conditions and at two frequencies, has provided a data base which permits an in-depth statistical analysis of the relationships between attenuation, elevation angle, and atmospheric conditions. The mean attenuations at both 15 and 35 GHz decrease smoothly with increasing elevation angle and at 0°range from 5.44 to 8.80 dB and 15.47 to 24.12 dB respectively for clear to cloudy conditions. At an angle of 5° the mean attenuations at 15 and 35 GHz have a range of .69 to 1.08 dB and 2.45 to 9.02 dB respectively. For regions having a climatology comparable to that of the Boston area, the mean attenuations can be estimated from regression lines derived from the data.