The Tropical Tropopause Layer (TTL) and the Asian Monsoon (AM) are important regions for current and future climate. They are the main pathways by which tropospheric air ascends into the stratosphere, and the TTL is the coldest region of the lower atmosphere (at times T < 185 K). Air ascending through the TTL is dehydrated by formation of high-altitude cirrus, which are radiatively important, producing a total radiative forcing of 4 W/m2 over the tropics. The link between deep convection and high-altitude cirrus is an important potential positive feedback in a changing climate. A warmer world may experience more frequent and vigorous deep convection, resulting in more high-altitude cirrus clouds that contribute to warming. ,In-situ measurements of water isotopologues can help reveal how convective detrainment of water vapor contributes to later cirrus formation. Convectively lofted ice is isotopically heavier than surrounding vapor since the heavier isotopologues (e.g. HDO and H218O) preferentially partition into the condensed phases. As this lofted ice sublimates, its isotopic signature is imprinted on the TTL. Both remote sensing and in-situ instruments have measured isotopic profiles that show an increase with altitude in the TTL, likely due to the sublimation of lofted ice. These profiles provide information about the importance of convective detrainment of vapor and ice relative to other sources and sinks in the overall water budget. ,The AM is an especially interesting area for TTL water isotopologue measurements. The AM may contribute up to 75% of the upward water vapor flux to the tropopause in Northern Hemisphere summer. Recent analysis of ACE-FTS satellite data shows significant differences in water vapor isotopic enhancement between the North American and Asian monsoons, suggesting differences in water transport processes, but no in-situ water isotopologue measurements in the AM have yet verified this observation. High-altitude measurements of the AM are extremely limited in general, and StratoClim will provide a valuable new perspective on this important region. ,Here I describe the motivation, design, construction, and performance of an Integrated Cavity Output Spectrometer (ICOS), designed to make the first measurements of the HDO/H2O ratio in the Asian Summer Monsoon. In addition, I discuss the context motivating the construction of this instrument, and describe the results of an investigation into the metastable forms of ice that resulted from previous cloud chamber studies. In the following, Chapter 1 reviews the properties of water and introduces the scientific motivation for studying cirrus clouds and the isotopic composition of atmospheric water vapor; Chapter 2 reviews the principles underlying measurements by absorption spectroscopy; Chapter 3 describes results from an experimental campaign at the AIDA cloud and aerosol chamber using the custom-built Chicago Water Isotope Spectrometer (ChiWIS); Chapter 4 describes the design,of a new version of ChiWIS for use on high-altitude aircraft that relies on Integrated Cavity Output Spectroscopy (ICOS); and Chapter 5 describes a new optical element designed to mitigate poor light collection efficiency in ICOS instruments. The non-axially symmetric optical component described here increases the light collection efficiency of the instrument by a factor of 6. ChiWIS flew aboard a high-altitude aircraft in the 2017 StratoClim science campaign out of Kathmandu, Nepal, and Chapter 6 reviews the measurements and field performance of the instrument during this campaign.