Water isotopes serve as tracers of hydrological processes and as proxies for past climates archived in ice cores. The isotopic signal is acquired throughout the hydrological cycle—through evaporation over the oceans, precipitation, which occurs as moisture is transported from lower to higher latitudes, and during post-depositional processes in which isotopic exchange between snow and atmospheric moisture occurs. Owing to these multiple influences, the relationship between isotope ratios in ice and local temperature varies across Antarctica, and distinct relationships are found when evaluating isotope ratios and temperature across space (for example, in surface snow) compared with temporal correlations at the same site (for example, in precipitation). Here we report measurements of water vapour isotopic compositions from a traverse across East Antarctica, as well as at two fixed sites: the coastal station Dumont D’Urville and Dome C on the plateau. Combining snow and vapour isotopic data, we demonstrate that the temporal and spatial isotope–temperature relationships are distinct because of differences in how the rainout fraction varies across time and space. Our findings support a shift from thinking about the isotope–temperature relationship in terms of distinct temporal and spatial slopes to recognizing that the relationship varies along a continuum based on known dependencies between circulation dynamics and mean climate state. By distilling moisture along moist isentropic transport paths, we can predict the isotope–temperature relationship across either time or space using a physical understanding of large-scale moisture transport under different climatic conditions.