Influence of Liquid Water Content and Temperature on the Form and Growth of Branched Planar Snow Crystals in a Cloud

How liquid water content (LWC) and temperature affect the growth of branched planar snow crystals is poorly understood. To address this issue, a vertical supercooled cloud tunnel was used to grow 167 individual snow crystals for 10 min under nearly constant conditions. The LWC varied within 0.07–0.76 g m−3 and average temperature varied from −12.4° to −16.3°C, with the latter varying by at most 0.2°C per run. The crystal habits are divided mainly by temperature, warm to cold, into nine regions: sector above −12.5°C; then broad branch to −13.0°C; and then stellar, dendrite, and fern to −14.5°C. Then the pattern reverses, with dendrite to −14.8°C, stellar to −15.7°C, broad branch to −16.1°C, and finally sector. From −13.3° to −13.8°C, stellar changes to dendrite with LWC increase as well as with temperature decrease. From −13.8° to −14.5°C, dendrites coexist with ferns below an LWC of 0.25 g m−3, but only ferns exist at higher LWC. At other temperatures, a higher LWC does not produce greater side-branch development. Nevertheless, an increase of LWC produces heavier crystals (particularly crystals larger than 1 mm across), despite the crystal diameters being independent of LWC. Stellars at −14.9°C have the maximum mass and diameter, whereas ferns at about −14.3°C have the maximum basal-plane area and minimum fall speed. The apparent crystal density varies with temperature, with two local minima for stellars near −13.3° and −15.3°C and a local maximum for ferns near −14.2°C.