A feasibility study is presented for an airborne, laser-based hygromet
er, which is intended to be part of the STRATO 2C, the German high-alt
itude research aircraft. A unique feature of the system considered is
an open absorption path, located outside the aircraft. The optical, me
chanical and electronic design of the instrument is presented as well
as model calculations describing the expected system performance betwe
en ground and 26 km altitude.