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I I I I I I I I I I. I. I. I. I. I. I. ~. I. I - 123 - and riming continued on that same face. For hexagonal plates, riming generally occurred on the prism faces or occasionally on the leading basal face. Since riming dominated in the lowe~ levels, plate crystals had generally passed out of their diffusional growth regime prior to the onset of riming. Consequently, the riming was concentrated at the edges of the crystals. Occasionally, riminr; occurred below -lOoC and the rimed droplets were then observed embedded in the crystals. The observations of rimed crystals at the ground confirmed those made in the air. Cold region crystals such as columns, side planes, radiating assemblage of plates, sectors, dendrites, and bullets were not rimed to nearly the same degree as crystals formed at temperatures above -200C. Dendrites, needles, sheaths, and warm columns were heavily rimed. Plain plates were often observed without much riming, as were other sub-water saturated crystals such as crystals with sector-like and broad branches. Thus, the crystals which were heavily rimed most commonly were those which formed at or above water saturation and above -200C. Those crystals not heavily rimed were cold region crystals, both water saturated and sub-saturated types, and sub-water saturated crystals above -200C such as hexagonal plates. At times aggregates of crystals were also heavily rimed. It was deduced from ground measurements and trajectory analyses that the crystals became rimed below about 9,000 feet above the Cascade Mountains and, in many cases, much closer to the surface than this (see ~5.5). In order for riming to occur water droplets must, of course, be present in the air. In general, the concentrations o~ droplets increased with decreasing altitude. In the first few thousand feet above the ground over the Cascades,