Sky brightness temperature measurements at 135 GHz and 215 GHz

Zenith sky brightness temperature measurements at 135 GHz and 215 GHz have been made on a semicontinuous basis for a period of seven months in the Gunston Cove area of Northern Virginia. These measurements were made using Dicke receivers with noise figures of 8 dB and 14 dB, respectively. A liquid nitrogen cooled load was used to calibrate the measurements. The 215 GHz sky temperature was on the average about 80 K greater than that at 135 GHz. Clouds were found to cause the sky temperatures to fluctuate as much as 150 K in a few minutes. Graphs are presented to outline general trends of the data as well as representative days, including the blizzard on February 11, 1983. In addition, empirical relations between precipitable water vapor, atmospheric water density at the surface, sky brightness temperatures, and zenith attenuation are given for visually clear days.