I wear my deerskin leather jacket, up until the overnight lows are about 65F. (18C), then the Joe Rocket jacket comes out for the 'hot season' .
Temperatures here in Phoenix on average reach 100 F (38C) by early to mid May.
Then settle into a predictable 105-110 F (41-43C) temperature range .
There's the occasional temperature 'spikes' to 115-120 F (46-49C), in June and July .
The hottest I've ridden my R65 in, is 128 F (53C), that was in Lake Havasu City, Arizona ( the new home to the London Bridge), in June 1994 .
When the temps get above the 105 F (41C) mark, I only ride to and from work, no long rides, unless you get out by 0500 and are home by 1100 .