WMOD00456

• Project Name: Sierra Ecology Project
• Title: The Sierra Ecology Project - Volume III
• Date: 1/1/1982
• Weather Modification - Doc Type: Report

45

SPRING
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CLASS

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( IN)

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5

15 20 25 30
PRECIPITATION

25 38 51 64 76
PRECIPITATION

89102114127140152
CLASS (CM)

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1 3

Figure ll.-Distribution of spring precipitation by 12.7 cm (5 in) class.

In an attempt to develop such a correlation, precipitation amounts for fall, winter, and spring
were regressed in various combinations over the 76 years of record, Three different combinations
were available: fall-winter, fall-spring, and winter-spring, Resulting correlation coefficients are
shown in table 9.
It was hoped that at least some characteristics of the fall precipitation pattern would have a
significant correlation with winter and spring precipitation amounts. Table 9 shows this was not
the case. All coefficients were very low. The fall and winter relationship was slightly negative-
suggesting that an increase in fall precipitation tends to precede a decrease in winter precipitation.
However, the correlation was so low that any conclusions of dependence are questionable, A
correlation using the natural logarithms of seasonal precipitation was also considered. Although
an increase in the correlation occurred, part B, table 9 shows these values to be very low also.
A relationship between fall and the other seasons' precipitation may exist, but the indications
for dependence are pOOL In view of the low correlations, an assumption of independence may be
valid. Using this assumption, an optimal method for deciding whether and when to seed clouds
was developed and will be discussed in the subsection entitled, "Precipitation Type (Simulation)."
Although this method is optimal only if precipitation is independent from one time period to the
next, simulation studies have shown it to be highly effective when applied to the precipitation data
of the last 76 years. Therefore, even if precipitation amounts do show some dependence from one
time period to the next, the correlations are so small that the assumption of independence proves
to be a good working hypothesis.

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