Laboratory experiments are presented which show that liquid stratosphe
ric aerosol droplets under polar winter conditions do not freeze for t
emperatures higher than the water ice saturation temperature (frost po
int). Calorimetric measurements of the freezing of supercooled H2SO4/H
NO3/H2O bulk solutions with concentrations typical of the polar strato
spheric aerosol exhibit very small freezing rates, which excludes the
possibility of homogeneous freezing of the droplets for temperatures a
bove the frost point. Even heterogeneous formation of H2SO4 and HNO3 h
ydrates at these temperatures is a very inefficient process unless the
stratosphere offers nuclei better suited for nucleation than those pr
esent in the laboratory experiments, which appears to be unlikely. Onl
y ice was found to be a potential nucleus suited for the formation of
the hydrates, which could cause the hydrates to freeze at temperatures
below the frost point.