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<br />Seeding Factors <br /> <br />If a 20 pm diameter particle of hygroscopic material grows to a 5 mm <br />diameter raindrop, the growth in volume is by a factor 1. 56' 107 and, <br />since the starting particle has a density of 1. 3, the mass increase is <br />by 1. 2' 107. The mass increase starting with a 10 pm diameter particle <br />is 9.6' 107, while for 30 pm and 40 pm particles the multiplier is 3.5' 106 <br />and 1. 5' 106 respectively. <br /> <br />Use the figure for growth of the 20 pm particle. and note that 1 acre foot= <br />1220 metric tons. Then ,pne finds that 1 kg of seeding material evolves <br />into 1. 2'107 kg = 1. 2' 10 tons = 10 acre feet of water. One kg of the <br />fertilizer solution costs about 59. so each acre foot of water uses 1/29 <br />of hygroscopic material, if each particle is. effectively processed by the <br />cl ou d. <br /> <br />As a comparison, note that the AgI particle required to initiate a raindrop <br />may be O. 2 pm diameter. hence about one millionth the volume of the 20 pm <br />hygroscopic particle. <br /> <br />The growth actually occurring on an individual hygroscopic particle <br />released near cloud base depends on the particle size and the cloud con- <br />ditions it encounters as it grows - - 'which depends on the cloud characteris- <br />tics and on relative falling speed of the hydrometeor as it grows. <br />Mr. G. E. Klazura of the Bureau of Reclamation, adapting growth form~las <br />from various sources, has computed this growth of an individual particle for <br />various upcurrent strengths (assumed constant with height), cloud base <br />heights and temperatures, and hygroscopic particle sizes. I igure 9 shows <br />the results of one set of calculations- - for cloud base conditions representative <br />of Oklahoma, for a 5 m/ sec updraft in an unmixed core. ln this instance <br />each starting size results in a raindrop which breaks up and generates others <br />by the Langmuir chain reaction. The 40 pm particle rises only 2 km into the <br />cloud while the 5 pm particle trajectory peak is 4 km above cloud base. With <br />a stronger upcurrent the hydrometeor from the 5 pm particle may exit out the <br />top without growing large (at temperature colder than -40C growth decreases <br />greatly) while hydrometeors from larger embryos grow both during ascent <br />and descent. Particles reaching the -15C level are assumed to freeze, and <br />then be able to exceed 5 mm diameter and not go through drop breakup. <br />There are many concepts to be drawn from Klazura's figures, too many for <br />exhaustive discussion here. However, there are some generalizations which <br />deserve mention here. F or weak and moderate upcurrents (1 m / sec to <br />5 m/ sec). all particles from 5 pm to 40 pm are reasonably efficient, with <br />the smallest going highest and having more growth but requiring a larger. <br /> <br />22 <br />