A microwave radiometer of the Dicke type for the 6‐mm wavelength range has been built and operated to measure radiation from the sun and attenuation in the earth's atmosphere. The total vertical attenuation in the earth's atmosphere on four clear summer days averages 1.2 db. It is presumably due principally to oxygen, since the 6‐mm wavelength is well up on one wing of the 5‐mm absorption line. The quoted values are in agreement with a line‐width constant of 0.02 cm−1. Attenuation in clouds is highly variable. An additional 1 db above the fair‐weather value is typical for a thin overcast.Measurements during the summer of 1956 indicate an effective solar temperature of about 4500°K. This value is appreciably lower than temperatures reported at 7.5 and 8.5 mm, and it favors a model of the lower chromosphere which assumes temperatures well below the optical surface value of 5700°K.