WMOD00265

• Project Name: Evaluation of Operational Seeding in Western Kansas
• Title: Evaluation of Perational Cloud Seeding in Western Kansas
• Prepared For: State of Kansas
• Prepared By: USBR
• Date: 11/1/1987
• State: KS
• Weather Modification - Doc Type: Report

.

One would not consider the area just northeast of the target area as a control area because it is
downwind of the target area during many storm situations. We therefore laid out control areas to
the west and northwest and reran the Wilcoxon tests for the grid cells in the target area, replacing
the actualloss-cost ratios by the extent to which those loss-cost values exceeded expected values
based on hail experienced at the control areas. The results were not distinguishable from those of
figure 4.3, and therefore are not reproduced here. This lack of impact of the controls is due to their
poor correlation with the target area. Despite the poor correlations between target hailfalls and
those in specific control areas, examination of the general patterns of figure 4.3 is of some interest.
It is apparent that areas of "significant" differences between seed and no-seed years show up at
many places, so the mere existence of areas shaded dark blue in the eastern part of the target area
does not prove that hail was suppressed. However, a study of how patterns vary across the target
area is useful.

Comparing the hail experience in-the target area to that to the southwest and to the northwest shows
it to be compatible with the idea that seeding led to a hail suppression effect. The years 1975-85
were bad hail years just upwind of the target area. The results become more favorable as one
moves eastward from the western edge of the target area and, as noted above, a few grid squares in
the eastern part of the area had significantly less hail during the 1975-85 period than during the
no-seed years.

Comparing the maps of statistical results with the maps showing where the seeding material was
released indicates that the most favorable results in the target area occurred about 100 km (60
ml1es) east of the area of heaviest seeding. However, we cannot conclude that that is where seeding
had its maximum effect because of the evidence from the large-scale analysis that the northeast part
of the target area was a part of the naturally favored area centered to the northeast of the target
area.

4.6 Calculation of HailIRain Ratios

The area of apparently favorable hail results in the target area is also an area where there was less
rainfall during the seed years than during the no-seed years. One must consider the possibility that
the apparently favorable h8i1 results were due merely to a scarcity of stonns, which resulted in less
than normal rain and hail both.

As a check on this possibility, hail/rain ratios were computed for all of the study area. To bring in
the results for the no-seed years, we calculated double ratios

hail <loss cost) / averasze rainfall (seed years) .
hail (loss cost) I average rainfall (no-seed years)

r

Hail/rain ratios have been used previously as a measure of hail suppression effectiveness (e.g.,
Schleusener [11 n.

The double ratios for the study area are shown in figure 4.5. They indicate that the shortage of
rainfall over the northeast part of the target area during the seed years was not sufficient to account
for the favorable hail experience there during those years. The fraction of the summer precipitation
falling as damaging hail was less during the seed years than during the preceding years in several
grid cells, a result which is consistent with the hypotheses upon which the program was operated
and also with the fmdings of Schleusener [11] in his studies of hail suppression programs in the
Dakotas.

33