I just finished Caldwell's book Push. The big walls in Yosemite are essentially vertical deserts. Even in winter they're in the full sun much of the day. It would drop below freezing at night, but during the day temperatures would regularly get above 50 °F (10° C). In the summer the heat can be overwhelming. Aside from the difficulty of performing extremely gymnastic climbing in high temps, sweaty hands make tenuous holds impossible. Gymnastic chalk can only do so much to counteract that. It also turns out that the spring and fall shoulder seasons are problematic because ice forms on the wall every night and peels off during the day. After several close calls with baseball sized hunks of ice, they found it slightly less hazardous during colder periods. On the other hand when it got too cold, the rubber on climbing shoes wouldn't grip as well, and they would just take a rest day on their porta-ledge.