As the moisture-laden Pacific winds are pushed up the windward
slopes of Glacier’s mountains, the air cools and water vapor
condenses, forming fog or clouds. Rain or snow begins to fall as the
air continues to rise and cool. By the time the air mass reaches the
crest and flows down the leeward slopes, most of the moisture has
been lost.
Western slopes average about 70 cm. of precipitation at elevations
between 900 and 1,100 m. Upper elevations average 200 to 250 cm.,
mostly in the form of snow; and 300 to 500 cm. is common.