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

It is of interest to know whether a shorter frost
free season after 1942 is due to a delay in the
start of the season or an earlier termination date.
Table 5 shows that for the period 1895-1941 the
frost free period at Durango began on average on

Table 5. Average dates of first and last frost
(Tmin ~ 0 C) at Durango

1895-1941 1942-1970

Mean date of last frost in
spring May 24 June 3
Mean date of first frost in
fall Sept 23 Sept 19

approximately May 24th, whereas for 1942-1970 the
mean date was June 3, 10 days later. Similarly in
the fall, the dates are September 23rd and
September 19th, respectively. Thus the season in
recent years has started later and ended slightly
earlier.

Figure 25 suggests a possible explanation for a
shorter frost free period beginning around 1942.

110
100
90
80
~70
c:
~ 60
f:T
~ 50
u..
40
30
1928

32

36

40

44

48

1956

Figure 25.

Frequency of zonal types - spring,
summer, and fall (April-October).

The frequency of zonal synoptic types is shown for
April-October, 1928-54; prior to 1942 the frequency
of zonal types is above average for the period,
whereas from 1942 to 1954 the frequency is below
normal. A decrease in the number of zonal types
indicates a concurrent increase in meridional type
frequency, i.e. a higher incidence of north/south
airflow types compared with zonal west/east flow.
Thus the shorter frost free period beginning in the
early 1940's may be related to more frequent in-
cursions of cool northerly airflow during the months
of April to October. However, the temperature
record indicates that ~ temperatures generally
fell from 1870-1930 followed by a reversal of this
trend post-1930. In looking at the frost free
period we are examining minimum temperatures only.
A more likely explanation, therefore, is that in the
period following 1940, there was a higher incidence
of anticyclonic activity with increased meridional
flow from the south. Cloudless skies, particularly
early and late in the season, would result in rapid
radiative heat loss and consequent cool evenings,
occasionally with frost. Such an explanation may
also explain a trend towards less precipitation in

recent decades, particularly in spring and fall,
though one might expect more convectional activity
during summer months. Some temperature records in-
dicate a slight cooling in the late 1960's and the
frost free period is shown to increase, perhaps
indicating less anticyclonic activity. It should
be noted, however, that the synoptic type classifi-
cation only extends to 1954 (for summer months) and
thus at present one can only surmise about synoptic
changes after this time.

Temperature-elevation relationships

Temperature-elevation graphs have been prepared by
the Forest Service for recording stations in the
San Juan National Forest. A relationship showing a
decrease of 0.59 C/100 m (3.25 F/1000 ft) in the mean
annual temperatures is apparent (Figure 26). The
highest elevations are again not represented in
constructing the regression.

90

80

70

60

20

...
;50

~o
o
30

---------

~08-3
--.~~__!..cO.917
--

10
"
.
~
o~
o

20

-10

6

7 8 9 10'
Elevation in Thousands of Feet

12

13

Figure 26 .

Mean annual temperature - elevation
relationship, San Juan National Forest.
(prepared by National Forest Service)

DENDROCLIMATOLOGY

The work of Fritts (1965) cited previously suggested
conditions of relatively low. precipitation and high
temperatures through the 1855-1900 period with a
subsequent reversal of these conditions culminating
around 1915.

Observational records for this period do not support
Fritts' dendroclimatic reconstruction in this area.
Nineteenth century climatic data indicate increasing
precipitation to the late 1890's accompanied by fall-
ing temperatures (Le. "moist") (c.f. Figure 3). A
marked minimum in precipitation did occur around
1900 followed by a rapid rise to around 1905-11
or later in some cases, but temperatures continued
to fall throughout the period to approximately 1930.
Fritts' dendroclimatic reconstruction identified the
major period of high precipitation and low temper-
atures in the early 1930's and also the period of
low precipitation at the turn of the century, but
the 19th century reconstruction is erroneous
indicating almost the reverse of what was ac~ually
occurring. Why should the dendroclimatic recon-
struction of 19th century climate be so poor compared
with that of the 20th century? In view of the fact
that Fritts' original multiple regression was carried
out using climatic data for the period 1896-1964, the
reconstruction of early 20th century climatic
fluctuations can be expected to be good. However,
any climatic extrapolation on the basis of tree
growth increments assumes that the climatic parameters

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