Global climate change is one of the defining issues for the study of the Earth system in the 21^st century. The work presented herein is motivated, at the highest conceptual level, by the need to deepen our understanding of past climate systems, and to drive more accurate and reliable predictions of short-term climate variability and longer-term climate change. Water vapor is the most radiatively active greenhouse gas and the process of water vapor feedback is a significant lever of climate control. Accurately defining the atmospheric distribution and transport of water vapor within the Upper Troposphere and Lower Stratosphere (UT-LS) is essential if global climate models are to confidently produce a realistic climate picture.

Consistent differences in the detection of water vapor in the critical UT-LS atmospheric region raise the question: "which data set is correct, if any?" There is an urgent demand for a higher standard of in situ detection in this region. In an effort to move in this direction, we have developed a new instrument that leverages advances in electronic design and signal processing to promise a lightweight and compact measure of water vapor with high accuracy, precision, and reliability. The Harvard Herriott Hygrometer (HHH) is a multipass Herriott cell that measures water vapor via direct detection. Predicted accuracy and precision are ± 3–5% and ± 0.05 ppmv H₂O, in the lower stratosphere, for a 10-s integration time, respectively. The theory and application of HHH as a water vapor instrument are laid out in the context of making accurate measurements traceable to laboratory standards. In conjunction with the Harvard Water Vapor (HWV) instrument, HHH will establish ultimate credibility via three, independent detection methods in-flight and five for laboratory and in-field calibration. A multi-detection, calibration system of this nature is beyond the scope of any in existence today.

Because HHH promises such high reliability and slight margins of error, the data acquired by this instrument should minimize the uncertainty associated with natural and anthropogenic climate forcing. HHH may serve as a prototype instrument for the use of miniaturized, TDL systems as in situ quantifiers of atmospheric gases via the straightforward method of direct detection, thus extending the scientific payback of this new system.