Ice Crystals Produced by Expansion: Experiments and Application to Aircraft-produced Ice

Abstract

The production of ice crystals as a result of the expansion and cooling of moist air was investigated by laboratory experiment. In particular, the warmest expanded air temperature that produces crystals was sought as a function of the initial temperature. The results fit the standard theory of homogeneous nucleation of water droplets, as long as the droplets remain at a cold enough temperature for sufficient time to from by homogeneous nucleation. Experiments were also carried out with “dry” air (dewpoint less than −40°C). Quantitatively different results were obtained, namely, that much colder expanded air temperatures were required to produce crystals with essentially no variation in numbers of crystals produced as the initial temperature varied. These results are also consistent with the same homogeneous nucleation theory. These idea were applied to the production of aircraft-produced ice particles by means of the adiabatic expansion and cooling that occur near the propeller blades of the aircraft; this mechanism is sufficient for the production of such particles in some flying situations. In particular, the difference from one aircraft to another seems a less important factor than the variations in Right conditions for a given aircraft. Situations that require large thrust from the propellers (for example, climbing, icing, or flying at very slow speed with flaps down) are most likely to produce ice particles and should be avoided in all cloud passes made when repenetration is intended. Various actual cases of aircraft produced ice were examined, and in some the same larger-than-average thrust condition was met.

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