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

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1970

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Figure 22. Relationship between Durango mean annual
temperatures (solid line) and mean
annual precipitation (dashed line); nine
year weighted binomial running means.

Figure 23. Relationship between Silverton mean
annual temperatures (dashed line) and
mean annual precipitation (solid line);
nine year weighted running
means.

Dailv temperature data analysis, Durango and
Silverton

Daily temperature data for Durango (1894-1970) and
Silverton (1906-1970) have been used to examine
degree-day totals over time and the length of the
frost free season.

Figures 24a and 24b show degree-day totals a) ~ 0 C
and b) ~ 18 C at Durango, 1896-1970. Clearly shown
is a fall in degree day totals from the beginning

1890 1900

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300
275
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~175
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Figures 24a and
Durango.

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40 50 60 1970

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10 20 30 40 50 60 1970

24b. Degree-day tctals, > 0 C for
Annual totals.

Degree-day totals, ? 18 C for Durango.
Annual totals.

of the record to about 1928 when the trend was
reversed (note different scales of Figures 24a and
24b). Such a trend is not shown in the Silverton
record where cold air drainage into the valley and
consequent inversion development may have obscured
the pattern (see below). It was suggested above
in the literature review that a general cooling
occurred in the San Juan area from approximately
1867 to 1930 when a reversal of trend occurred, and
degree-day totals for Durango support this suggestion
for the period when data are available.

Using a mean summer lapse rate for the area of -0.82 C
per 100 m (328.2 feet), degree days above 18 C
at Durango are approximately equivalent to degree
days above the freezing point at 4,250 m (13,949.7
feet); freezing temperatures are a critical
threshold for growth at tundra elevations. Thus,
Figure 24b gives an estimate of degree-days?O C
at high tundra sites and indicates very low totals
were experienced in the late 1920's. Only in the
last decade has the period normal been reached once
again.

The frost free period is defined as the number of
days between the last minimum temperature < 0 C in
the late spring or early summer and the fi~st
minimum temperature >0 C in the late summer or
fall. The frost free period at Durango was generally
longer than average prior to approximately 1942 and
shorter than average after that. Silverton data
do not show this pattern as clearly and the frost
free period is considerably less than at Durango.
This may reflect the effect of cold air drainage
in the valley where Silverton is located, as suggested
above. In some years frosts occurred throughout the
summer at these two stations.

56