Nucleation rates of nitric acid dihydrate (NAD) in 1:2 HNO3/H2O solutions between 194 and 183 K were measured by observing the freezing behavior of liquid drops with 7--33 &mgr;m diameter and with concentration of 64 weight percent HNO3 using an optical microscopy technique. The nucleation rate increases from 3.1¿105 cm-3 s-1 at 194 K to 1.3¿109 cm-3 s-1 at 183 K. These results suggest that NAD might not nucleate homogeneously in binary or quasi-binary nitric acid stratospheric aerosols in relevant time scales. ¿ 2000 American Geophysical Union