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60 of 55 50 "''''--..... 45 40 ,*,\ " /.. ",*' \ /-\ ; \ /\ I ,- \, \,,'''..1 \ : \ ,,I \\../ \............................" '/'Jv.~v" 35 1855 60 70 85 90 1895 65 75 80 Figure 18. Nineteenth century records of mean annual temperature, 1852-1899. Solid line = Fort Garland, dotted line - Hermosa, small dashed line - Fort Union, New Mexico, two dots and dashed line = Fort Lewis, starred = data of doubtful quality. 23 27 c: ~ '" .:: 26", " .2 Q; I- 25,-=.' '" '" r: ~ '" o 24 lL '" '" ~ 0' '" o Ul22 '" '" r: ~ '" .:: 21 " , '" I , , , ,/ \.... , , / ,-" r \ / \ ,......_, ^ /,,,/ " '''vI' \.,/ \ , I \-' '" '" '" 0- '" o 20 1910 20 30 40 50 60 1970 Figure 19. Mean winter temperatures for Telluride (solid line) and Silverton (dashed line); nine year weighted running means. Note different scales for each graph. station. Silverton is particularly subject to cold air drainage conditions which would not affect a number of other station sites. Thus generalizations are somewhat difficult. Most stations show a fall in winter temperatures from the turn of the century to approximately 1930, followed by a warming trend to the present. Peaks around 1905 and 1956-1958 for Durango (Figure 20) are of approximately the same magnitude, almost 2.2 C (4 F) warmer than the 1930 low. The transitional seasons of spring and fall in general show warming trends from the 1930 low, or earlier, to the early 1960's. This trend is also seen in some summer records although the magnitude of the warming trend is not as great. For example, Silverton shows a mean warming of approximately 2.2 C (4 F) since 1910 during the fall season, 3 F in the spring, and 0.8 C (1~ F) in the summer. In winter the trend is less than 0.5 C (+1 F) for the entire record. There is evidence of an apparent cooling during the late 60's during the fall, spring, and summer months but this does not appear in all the winter season records. However, such a reversal of the main trend may only be a short-term fluctuation within the longer-term upward trend. Summer Temp, 67 OF 15 oc 13 66 Fall Temp. 55 \SUMMER ,,' 1\"" 65 , I \ i /', I \ !, ' I' \ .:oj \ ....:J1".:/ \ / .:/ " ...., ,,' I \ / \.. / \...' II OF ! /'1 il \ II \ r">FALL I I , I \ 1\ /\ f / ~, 'v-' \ I \ 'I \} ", !I ,'___ \ \ i/ , - \ \:1 l \ \ \/, ! \ \ I ! \ \j SPRING \ / \ ,'\ ,/\/1. ,,"'\ \, \"" " \ / \.,...,,! \..- \ ',,,,,,l \ 48 9 54 64 7 53 Spring Temp. 50 5 3 " ,/ \ , , , , , , .. ' 52 49 31 47 30 29 1900 20 10 30 50 40 19 0 Figure 20. Seasonal mean temperatures for Durango; nine year weighted running means. Dotted line = summer, large dashed line = fall, small dashed line spring, solid line = winter. Note scales at sides for each graph. The marked warming trends noted in the spring and fall seasons indicate a general lengthening of the growing season. In this regard it is interesting to note that the dates of thawing of Vallecito Reservoir indicate that breakup of the ice has been occurring earlier by approximately 1 day every 2 years since records were first kept in 1949 (Figure 21). ~IO :0 r30 '~20 <l Llo 1940 51 55 57 53 59 61 63 65 67 1969 Figure 21. Dates of thaw of Vallecito Reservoir, 1949-1970. A comparison of the temperature and precipitation records of several stations indicates an inverse relationship; warmer periods are associated with low precipitation as one might expect (Figure 22). Of particular interest are the annual precipitation and mean annual temperature records of Silverton. The precipitation indicates a mean decrease over the period of approximately 3.8 mm per year (0.15 inches per year) while the temperature record shows a mean warming of approximately 0.17 ClIO yrs (0.3 FII0 yrs) (Figure 23). 55