WMOD00593

• Contract/Permit #: 14-06-D-7052
• Title: Ecological Impacts of Snowpack Augmentation in the San Juan Mountains, Colorado
• Date: 3/1/1976
• State: CO
• Weather Modification - Doc Type: Report

Table 13. Percentage frequency of rain days (;? 0.5 mm) within each type
a) Durango
Type Oct Nov Dec Jan Feb March April May
A 36 24 44 45 39 38 30 31
B 18 17 26 12 13 22 24 18
B 17 18 34 33 24 19 26 13
n-c
E 33 34 40 43 39 37 39 35
m
b) Silver ton
Type Oct Nov Dec Jan Feb March April May
A 46 29 49 46 40 48 39 40
B 23 19 34 23 28 35 31 33
B 17 20 41 36 35 30 46 33
n-c
E 33 34 40 44 39 37 39 35
m
Table 14. Mean percentage contribution of each rain day type to the monthly total precipitation
a) Durango
Type Oct Nov Dec Jan Feb March April May X
A 18 25 14 11 9 10 17 16 15
B 11 14 8 20 16 13 13 13 15
B 12 18 17 16 12 19 14 26 17
n-c
E 16 31 18 16 11 11 12 10 16
m
b) Silverton
Type Oct Nov Dec Jan Feb March April May X
A 14 18 11 10 9 9 10 12 12
B 11 18 10 13 7 . 7 11 12 11
B 9 14 14 19 10 13 9 8 12
n-c
E 12 14 16 11 10 11 12 10 12
m

this would require a more detailed study based
specifically on cloud seeding criteria and taking
into account standard errors and statistical
probabilities.

In terms of cloud seeding operations, it is of
interest to examine the changes in frequency of
these four types over the 30 year period (Figure 32).
From 1941-42 and 1946-47 there was a period of
above average frequency of these types preceded and
followed by periods of below average frequency
(approximately 1930-31 and 1939-40 and 1947-48 and
1953-54). The maximum frequency during the winter
1954-55 is also notable.

SUMMARY

Temperature fluctuations over the last 100-120 years
indicate a cooling trend from the late 1860's to
approximately 1930 when this trend was reversed.
The 19th century cooling was associated with
increasing precipitation. The 1860's were drier or
at least as dryas the period 1931-60. Observational
meteorological records do not support dendroclimatic
reconstructions for the area and possible reasons for
this are suggested.

Synoptic weather types have been examined for the
period 1928-1958 and they have been of some value
in explaining the observed climatic fluctuations.
However, in view of the small area under examination
and the very large area to which the synoptic classi-
fication pertains, a very close correspondence be-
tween the two should not be expected. Conditions
favorable to cloud seeding in the San Juans mainly
occurred during a small number of broad-scale
synoptic types. The pertinent characteristics of
these types have been considered for the period
1928-1957.

Degree-day totals for Durango support the general
conclusions concerning temperature fluctuations in
the area and indicate that a period critical for
growth at high elevations may have occurred in the
late 1920's. The frost free period at Durango has
been shorter since 1942, starting on average 1~
weeks later than in the period 1895-1941 and ending
slightly earlier. This may possibly reflect changes
in broad-scale synoptic events.

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