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The general propagation characteristics of 3.3-mm waves through the atmosphere are determined. The measured absorption of the atmosphere is in fair agreement with the Van Vleck-Weisskopf theory. An approximation for the zenith attenuation in decibels through the atmosphere is , where is the precipitable water in centimeters. The effects of varying attenuation on the determination of the brightness temperature of extraterrestrial sources is considered. The usual radio refraction expression appears adequate for 3.3-mm waves for zenith angles less than 70 degrees.