Size of Ice Crystals Formed during Rapid Expansion of Humid Air

The size of the ice crystals formed during the expansion of humid air in a supersonic nozzle is determined by analyzing samples sucked into Pitot probes. Small probes read a higher humidity than is present in the free stream. This increase is a measure of the inertia of the crystals, and thus of their size. For spherical crystals, a radius of 15 Å (1.5 × 10−9m) is predicted which means that a crystal contains about 360 molecules. This result is confirmed by measuring the evaporation rate of crystals behind normal shocks. It is further found that the particles are of fairly uniform size. No variation of the size with humidity was found for water concentrations below 2 × 10−3.