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Show temperature, hence the following simplification can be made, t = 3. 8 x 10 y (T) -4 (r - r o ) (7) where Y (T) is a thermodynamic function. Figure Al shows a plot of Y (T) and Figure A2 shows the variations of crystal growth at various temperatures. It can be seen that a dendrite should grow to about 1 mm radius in 1000 seconds (17 min) and to 1 cm in 10, 000 seconds (2-3 / 4 hours). Observational data by Nakaya (1954) are also plotted on Figure A2. Similarly, the equations can be solved in terms of mass, dm dt = 2 x 10 2 Y ( T) m O• 5 (8) (9) 2. Fall Velocity of Single Crystals Single plane dendrites have a fall velocity of about 30 cm sec -l, independent of crystal size (Nakaya and Terada (1935), and Magono (1954). In still air the fall time of unrimed or unaggregated plane dendrites, generated at the 6 to 9 km level, is many hours. Hence, single crystals formed at high altitudes cannot significantly contribute to local precipitation unless they are subsequently involved in the particle collection process. 3. Growth by Accretion The rate of mass growth for a falling crystal by accretion (riming) of supercooled cloud droplets is given by, dm dt -- = 1T r 2 EQ C (v-v ) a (10) where r is crystal radius, E collection efficiency, Q liquid water content, and (v-v a) the difference in fall velocity of crystal and supercooled droplet. Collection efficiencies of ice crystals for cloud droplets are not well known. Fletcher (1962) summarizes E for different droplet and crystal sizes. These results are given in Table Al. The cloud droplets are assumed to be 5 µ, radius, which is compatible with observations of supercooled clouds. Jiusto has computed growth rates by riming, assuming a collection efficiency 341 |