In reply to Chris H:
Hottest: +55C in the Oman desert
Coldest: -48C in Canada
Hottest on a mountain: +48 C, walking up an ophiolite mountain directly behind Muscat, Oman in mid summer. Rock way too hot to touch.
Hottest rock climbing: +42 C at Enchanted Rock, Texas, mid-summer. Gave up at lunchtime when carabiners became too hot to touch.
Hottest bivvy on a mountain: +43 C. Inadvertent night out on a summit near Muscat Oman in the height of summer, when the sun set very fast and there was no moon and I couldn't see the way down. My only accessories or companions: shorts, T-shirt, water-bottle and a tarantula!
Coldest on a mountain: -35 C at Top of the World ski lift at Lake Louise with windspeed of 35 mph - wind chill right off the scale. (I had a small thermometer for skiers with a windchill chart attached, and these conditions were completely off the chart about another whole chart away, diagonally!)