Present Climate and Glaciation of Western Montana, U.S.A.

Abstract

In western Montana the presence of active weather stations, snowpack monitoring sites, and modern glaciers allows the regionalization of summer temperature and winter precipitation data through trend surface analysis, and their comparison to the location of modern glaciers and the morphology of their cirques. Summer temperatures at 71 sites are well explained (80%) by a trend surface based on altitude, longitude, and latitude. In contrast, winter precipitation at 261 sites shows coherent residuals to a weaker (65%) regression on those variables. The residuals show the importance of local windflow and regional moisture sources in controlling precipitation.

A trend surface on equilibrium-line altitudes (ELAs) of 88 modern cirque glaciers explains 92% of the variance with latitude and longitude alone. Cool moist air, as recognized in the climatic data, controls present glacier elevations. The residuals to the trend reflect control of cirque glacier elevation by local windflow (7–64%), cirque orientation (20–30%), and backwall height (0–60%).

Glaciers in Montana persist at higher summer temperatures for a given winter precipitation than most glaciers worldwide, a difference which can be explained by the minimal longwave radiation inputs and sensible heat available from warm, but dry continental air. Regression of ELAs against the estimated temperature/precipitation relationship yields consistent residuals which can be explained in part (13–36%) by cirque orientation and backwall height.