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Factors controlling the microphysical link between distributions of relative humidity above icesaturation in the upper troposphere and lowermost stratosphere and cirrus clouds are examined with thehelp of microphysical trajectory simulations. Our findings are related to results from aircraftmeasurements and global model studies. We suggest that the relative humidities at which ice crystalsform in the atmosphere can be inferred from in situ measurements of water vapor and temperatureclose to, but outside of, cirrus clouds. The comparison with concomitant measurements performed insidecirrus clouds provides a clue to freezing mechanisms active in cirrus. The analysis of field datataken at northern and southern midlatitudes in fall 2000 reveals distinct differences in cirrus cloud freezingthresholds. Homogeneous freezing is found to be the most likely mechanism by which cirrus form atsouthern hemisphere midlatitudes. The results provide evidence for the existence of heterogeneousfreezing in cirrus in parts of the polluted northern hemisphere, but do not suggestthat cirrus clouds in this region form exclusively on heterogeneous ice nuclei, thereby emphasizing thecrucial importance of homogeneous freezing. The key features of distributions of upper troposphericrelative humidity simulated by a global climate model are shown to be in general agreement with both,microphysical simulations and field observations, delineating a feasible method to include and validateice supersaturation in other large-scale atmospheric models, in particular chemistry-transport andweather forecast models

Freezing thresholds and cirrus cloud formation mechanisms inferred from in situ measurements of relative humidity