  The stable isotopologues of water have been used in atmospheric and climatic
studies for over fifty years, because the temperature-dependent preferential
condensation of heavy isotopologues during phase changes makes them useful
diagnostics of the hydrological cycle. However, the degree of preferential
condensation has never been directly measured at temperatures below 233 K
(-40$^{\circ}$C), conditions of cirrus formation in the atmosphere and
routinely observed at surface elevation in polar regions. (Models generally
assume an extrapolation from the warmer experiments of Merlivat and Nief,
1967.) Non-equilibrium effects that should alter preferential partitioning have
also not been well-characterized experimentally (Jouzel and Merlivat 1984). We
present here the first direct experimental measurements of the HDO/H$_2$O
equilibrium fractionation factor between vapour and ice ($\alpha_{\mathrm
{eq}}$) at cirrus-relevant temperatures, and the first quantitative validation
of the kinetic modification to equilibrium fractionation expected to occur in
supersaturated conditions. In measurements of the evolving isotopic composition
of water vapour during cirrus formation experiments in the AIDA chamber, we
find $\alpha_{\mathrm{eq}}$ several percent lower than has been assumed. In a
subset of diffusion-limited experiments, we show that kinetic isotope effects
are compatible with published models (Jouzel and Merlivat 1984, Nelson 2011),
including allowing for small surface kinetic effects. These results are
significant for the inference of cirrus and convective processes from water
isotopic measurements.
