pages - The Black Vault

asis of 500 mb temperatures which are used as an estimate of cloud top temperaturesince no direct measurements of cloud top height have been available. Thedependence of seeding effectiveness on cloud top temperaturc has been well documentedby Grant (1967) and others.Three different temperature divisions were tested with the warm-cold splitoccurring at -180, -200 and -22 0 C. There was very little difference between theseed/no seed ratios of the bands when the temperature division was made at -18 0 C.When the division occurred at -20%C however, a significant difference begins tcbe apparent ;.ith the bands with 500 mb temperatures equal to or warmer than -20 0 C(Figure 9) showing a large area of high seed/no seed ratios in both the primaryseeding area and in the area 60-90 miles downwind. In the warm category 70 stationsshow significance at the 10% level and 38 at the 5% level. The bands with500 mb temperatures colder than -20 0 C (Figure 10) show much smaller areas of highseed/no seed ratios and an area with ratios of less than 0.67 (50% less rain fromseeded bands) appears about 60 miles north of the seeding site. Contrasted withthe warmer category, only one station is significant at the 5% level and nine atthe 10,%' level or less.When the temperature division is made at -22°C, the contrast becomes evenmore evident. The seed/no seed precipitation ratios from the bands with 500 mbtemperatures of -22°C or warmer (Figure 11) are greater than 1.50 over a verylarge area and extremely high ratios (over 20:1 at stations A27 and A31) are observedin the San Joaquin Valley. The ranking of the precipitation distributionswas significantly different at 92 stations at the 10%0; level, 64 stations at the51. level and 17 stations at the Vo level. The bands with 500 mb temperaturecolder than -22'C (Figure 12) show a rather large area with ratios under 0.67in the same region where high ratios occurred in the warm category. Most of thehibfher ratios (over 1.5) occur over the Sierra Mountains more than 120 miles fromthe seeding site. The Wilcoxon (Mann-Whitney) test showed only one station witha precipitation distribution which was significantly different for seeded andunseeded bands and that station(A44) showed a decrease in precipitation fromsei•ded bands significant at the Ur' level. The sample size for that paiticularstation was quite small, however, (six seed, six no seed), so the results mayhie questionable.-17-