Variations in the ratios of different isotopes of hydrogen and oxygen in water have long been used to trace water cycle processes, and the isotopic composition of water vapor in the atmosphere is known to change with weather events. Farlin et al. measured the isotopic composition of atmospheric water vapor in San Diego, Calif., at hourly intervals during February 2011 using a ground-based sensor. The time period included severe rainstorms and Santa Ana winds. They also used a general circulation model that included simulation of isotope variability to investigate how atmospheric and hydrologic processes affect isotope composition of water vapor near the surface.