The water cycle is expected to respond to rising air temperature, which redistributes moisture in the lowest part of the atmosphere, known as the boundary layer. To understand how boundary layer processes redistribute rainfall across different climate settings, climatologists use stable isotope ratios of oxygen and hydrogen in rainwater. A process known as Rayleigh fractionation—which depends on air temperature and amount of rainfall among other factors—controls the stable isotope makeup of rainwater. For example, in the tropics the isotope ratio depends on the amount of rainfall, whereas in high‐latitude continents it depends on the air temperature.