The efficiency of silicon p-n junction photovoltaic cells for solar energy conversion is higher in terrestrial use than in space use by an amount which can be as high as 25%. The power output in terrestrial use is nevertheless lower than in space use because of the lower solar intensity. The explanation for the higher efficiency is that, on traversal of the sunlight through the earth’s atmosphere, water vapor and carbon dioxide absorb energy primarily at wavelengths in the near infrared to which the solar cell is not sensitive. In the present paper a quantitative evaluation of this hypothesis is carried out by numerical determination of the total integrated power and of the number of photons/sec capable of creating hole-electron pairs corresponding to known solar power distributions for air masses 0, 1, 2, and 3 and by translation of the results into lossless cell efficiencies. The efficiency for silicon cells at air mass 1 for the case of no absorption bands proves to be the same as for air mass 0, whereas in the absorbing case the terrestrial efficiency is about 10% higher. Also for the absorbing case, as the air mass is increased, the efficiency relative to the space case increases to 15% at air mass 2 and 16% at air mass 3. Measurements of short-circuit current, efficiency, and incident solar power under air mass 2 conditions during the period from mid-November 1975 to early January 1976 have been carried out on a variety of silicon solar cells including conventional satellite cells, violet cells, and COMSAT nonreflective cells. For relatively clear atmospheric conditions, efficiency enhancements relative to space conditions of 11–12% were observed for both direct radiation and the sum of direct and sky radiation. Under hazy atmospheric conditions, efficiency enhancements of from 15–19% were observed. It is concluded than wavelength selective attenuation mechanisms (e.g., infrared absorption by water vapor and carbon dioxide and scattering, predominantly in the infrared, by suspended particulate matter and water droplets) which attenuate the total solar power to a greater degree than the power to which the solar cell is sensitive will produce a higher conversion efficiency for terrestrial use than for space use. However, the higher terrestrial efficiency implies only that, as the sunlight is attenuated by transmission impairments, the cell power output falls off more slowly than the solar power input.