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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1 A condensation of this paper was presented at the American Meteorological Society's Third Conference on Agricultural Meteorology held in Kansas City, Missouri, 18–19 May 1960.

2 Contribution from the Agronomy and Soils Department, Montana Agricultural Experiment Station, Paper Number 518, Journal Series.