Mesospheric water vapor measured from ground-based microwave observations

An atmospheric emission line centered at 22,235 MHz has been detected repeatedly over the period of March–August 1975 with a radio telescope located at Westford, Massachusetts (42.5°N, 71.5°W). The line has also been detected in absorption against the sun's microwave continuum. From its exact coincidence in frequency with the well-known 1.3-cm line of the water molecule the line is identified with atmospheric water vapor. A mixing ratio distribution between altitudes of 50 and 80 km is derived from the measured line amplitude and shape. Mixing ratios as large as 15 ppm by volume, about 2 times larger than the maximum predicted by photochemical equilibrium calculations, are found at these mesospheric altitudes.