The growth kinetics for ice particles in the polar summer mesosphere is studied using the density of water vapor, temperature, and total ice volume simultaneously measured by the infrared Fourier Transform Spectrometer on the Atmospheric Chemistry Experiment (ACE-FTS) satellite. The results are based solely on the ACE-FTS retrievals, without using any adjustable parameters. The computed particle formation time is in the range between 2 hours at 150 K and 20 hours at 120 K, during which particles come to equilibrium with water vapor and reach the size of 20–70 nm. The growth rate varies from 0.2 nm/hour to 30 nm/hour in the temperature range analyzed. As it takes ice crystals only 20 minutes to grow by 10 nm at 150 K, the transition from optically subvisible to the visible size range can occur on a time scale of minutes. This could account for fast variations in PMC brightness observed recently.