Measurements of the stratospheric aerosol by SAM II during the northern and southern winters of 1979 showed a pronounced increase in extinction on occasions when the temperature fell to a low value (below 200 K). In this paper we evaluate, from thermodynamic considerations, the correlation between extinction and temperature. As the temperature fails, the hygroscopic aerosols absorb water vapor from the atmosphere, growing as they do so. The effect of the temperature on the size distribution and composition of the aerosol is determined, and the optical extinction at 1 μm wavelength is calculated using Mie scattering theory. The theoretical predictions of the change in extinction with temperature and humidity am compared with the SAM II results at 100 mb, and the water vapor mixing ratio and aerosol number density are inferred from these results. A best fit of the theoretical curves to the SAM II data gives a water vapor content of 5–6 ppmv, and a total particle number density of 6–7 particles cm−3.