"In that advanced season, the party soon passes out of the moist, temperate regions of the foothills into the dry, cold ,bracing air of the Sierras."
Explain what it shows about poker flat and the old west. "The Outcasts of Poker Flat" by Bret Harte
Have you ever been to the Sierra foothills and then the Sierra Nevada Mountains in California?
This is describing the geography and the climate of those regions.
http://www.google.com/search?rlz=1C1GGGE_enUS379US379&sourceid=chrome&ie=UTF-8&q=sierra+nevada+foothills+climate