The thermal emission spectrum of the atmosphere near the 118 GHz oxygen resonance has been measured from the NASA Convair-990 aircraft as it flew over clear air and storms. The instrument viewed the ground 45° from nadir with a 7.5° beamwidth. Brightness temperatures were measured in six bands 200 MHz wide centered at frequencies 821–1891 MHz from the line at 118.7505 GHz. The double-sideband super-heterodyne receiver had ∼1 K sensitivity for 1 s integration. Comparison of observed clear air brightness temperatures (from 238 mb) with those computed for a coincident dropsonde yielded agreement within 1.4 K; the retrieved temperature profile agreed with the dropsonde with an average magnitude error of 1.4 K. Observations over precipitation yielded brightness perturbations as large as 30 K.