I'm interested if the brain works better when the temperature is higher than usual and the amount of oxygen in the air is a bit lower than in fresh mountain air. This has been my personal experience.
Warmer temperature are shown to raise aggression level (Anderson, et al, 1995). Citing this study, DeWall, 2009 found a similar correlation between words associated with high temperatures and hostile behavior. This could be perceived as a threat to clear thinking.
Moss, 1996 shows oxygen administration increases memory. However, intermittent hypoxia on developing children led to adverse effects (both mental and physical), according to Bass, 2004. Though, I must say, you can't really determine the oxygen content indoor vs. outdoors without sensors, even if you raise in elevation and who's to say what competing particulates there are between the two environments?
It's known that kinetic motion itself increases neurogenesis, mostly in studies with exercise, like this one. There's been a lot of papers and laymen articles on the subject of exercise and neurogenesis lately. The act of walking or climbing the mountain might contribute a lot to the subjective experience of thinking.