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Joseph M. Caprio

Two methods of estimating average last-freeze dates as applied in Montana are described. Both methods are based on the observation that day-time-temperature differences between Weather Bureau climatological stations in close proximity and at the same elevation are much less than night-time differences in temperature. A procedure for drawing 100-ft-elevation-interval maps of the date when the mean maximum temperature reaches 70F in the spring is described. The first method of estimating freeze dates is based on the statistical relation between average date of last spring freeze, the date when the mean maximum temperature reaches 70F and average diurnal temperature range at the time. The second method incorporates the same general parameters but is based entirely on mapping procedures. A method of mapping average last-freeze dates is described employing the unit area concept, which can be applied in connection with either method of estimating average last-freeze dates.

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Daniel R. Cayan, Susan A. Kammerdiener, Michael D. Dettinger, Joseph M. Caprio, and David H. Peterson

Fluctuations in spring climate in the western United States over the last 4–5 decades are described by examining changes in the blooming of plants and the timing of snowmelt–runoff pulses. The two measures of spring's onset that are employed are the timing of first bloom of lilac and honeysuckle bushes from a long-term cooperative phenological network, and the timing of the first major pulse of snowmelt recorded from high-elevation streams. Both measures contain year-to-year fluctuations, with typical year-to-year fluctuations at a given site of one to three weeks. These fluctuations are spatially coherent, forming regional patterns that cover most of the west. Fluctuations in lilac first bloom dates are highly correlated to those of honeysuckle, and both are significantly correlated with those of the spring snowmelt pulse. Each of these measures, then, probably respond to a common mechanism. Various analyses indicate that anomalous temperature exerts the greatest influence upon both interannual and secular changes in the onset of spring in these networks. Earlier spring onsets since the late 1970s are a remarkable feature of the records, and reflect the unusual spell of warmer-than-normal springs in western North America during this period. The warm episodes are clearly related to larger-scale atmospheric conditions across North America and the North Pacific, but whether this is predominantly an expression of natural variability or also a symptom of global warming is not certain.

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