This project will provide new information on the infrared absorbing properties of water vapor and ice clouds at the very low temperatures present in the upper troposphere. The approach is to measure these quantities experimentally using ground-based equipment at the South Pole Station, where temperatures and vapor amounts approximate those in the high atmosphere at midlatitudes. Working in the natural atmosphere rather than in a confined laboratory space, it is possible to use long experimental path lengths and thereby obtain more precise data than possible otherwise. The principal instrument in these experiments is a high-resolution interferometer covering the wavelength range from 2 to 40 *m. Infrared radiation will be transmitted out and back over a round-trip distance of 1 km and the interferometer will be able to estimate the absorption along the path. Instruments to measure the air temperature, vapor content, and type and concentration of ice crystals will provide data in support of the interferometer measurements. The result of the work will be a better understanding of the radiative properties of ice crystals and water vapor at very low temperatures. This information is needed to improve the accuracy of (1) radiative transfer calculations, (2) remote sensing observations of the atmosphere from satellites, and (3) the approximations for
