From 10 to 30 kilometers in altitude, the steadily decreasing temperatures in the upper troposphere and lower stratopshere cause water vapor containing the hydrogen isotope deuterium—HDO, or heavy water—to preferentially condense out of the air, falling back toward the surface as ice. This selective depletion of heavy water leads, in theory, to a 90% drop in the ratio of heavy water to regular water vapor concentrations as compared to surface levels. The observational record, however, does not always align so well with theoretical predictions. Researchers suspect that skewed heavy water concentrations are due to atmospheric mixing, where deuterium- laden water from lower altitudes is transported upward. By tracking changes in the HDO/H2O ratio, researchers hope to tease out the history of evaporation, condensation, and convection that affect a particular atmospheric region.