Crystal Growth in Ice Clouds at Low Temperatures

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This grant supports research on the nucleation, habit, and growth of ice crystals under the low temperature and pressure conditions characteristic of high cirrus clouds and polar stratospheric clouds (PSCs). It is a continuation of work on the physics of ice crystals that Dr. Hallett and his colleagues have done for many years for less extreme conditions. The static diffusion chamber used in previous work will be modified to operate down to temperatures as cold as -100C and pressures less than 100 mb. In the chamber crystals are grown on thin (5-10 microns) glass filaments under controlled conditions of temperature, pressure, and supersaturation. Crystal nucleation and growth are examined by optical microscopy from outside the chamber and video recorded. Likely nucleants will be added to the filament prior to insertion in the chamber. These include nonvolatile soluble particles, such a sulfuric acid, as well as insoluble mineral, meteoritic, and aircraft exhaust material. Cold, high-altitude clouds have important effects on the transfer of solar and infrared radiation, and hence global climate. They also play a part in the production and destruction of ozone. In spite of their importance, the physical properties of these clouds are not well known because of the difficulty of in situ measurements at heights above the ceiling of most aircraft. The laboratory experiments on this project will simulate the extreme atmospheric conditions and provide needed information on the microphysical characteristics of clouds which are nearly inaccessible by other means.