Aerosol measurements from the Halogen Occultation Experiment show stratospheric clouds over England on March 3 and 4, 1996. The measured cloud volume density profiles were compared to equilibrium volumes computed for nitric acid trihydrate (NAT) and liquid ternary H2SO4-H2O-HNO3 aerosols. The measurements on March 4 are confidently identified as NAT, and isentropic back-trajectories show that conditions were favorable for ice formation roughly 11 hours upwind of the observed cloud. These results support the suggestion that ice formation could be a necessary precursor to NAT nucleation. ¿ 1999 American Geophysical Union