Data from the visible and UV spectrometers on the Solar Mesosphere Explorer are used to derive the color ratios of the reflectance at 265, 296, and 393 nm of light scattered from polar mesospheric cloud particles. This analysis extends the spectral coverage into the visible region of the spectrum. The data reduction technique compared the cloud brightness to the brightness scattered from the background atmosphere at the same wavelength. The ratios determined in this way are independent of systematic errors in instrument radiometric calibration. The data are analyzed using theoretical determinations of the color ratios from the Mie theory of small particle scattering, assuming a lognormal distribution for the particle size dispersion. Here 'size' means the average radius of the sphere having the same ice volume. The present results confirm earlier findings that the effective sizes of polar mesospheric cloud particles are less than 70 nm. Still, there exists a small number of measurements which result in particle sizes of the order of 80 nm. Even for these large particle sizes the required vertical column content of water vapor does not exceed limits imposed by the available atmospheric water vapor concentrations.
