Today it was 102°F (38.8°C) here in eastern Georgia.
I submit that Stig is correct when he says that riding down the road in this heat (albeit I was at 52mph) nothing keeps you cool. The worst part of it for me, is that the inside of my brain bucket gets so sweaty to the point I can almost wring out the pads. Sitting at a traffic light on blacktop is the absolute worst. Utterly miserable, but not as bad as coming home from work last Wednesday evening. Just as I pulled out on to the main road from my workplace, the bottom fell out of the sky. I made it maybe 3/10 miles, before I pulled off the road to wait it out under the shelter of an abandoned car wash. Got treated to a magnificently beautiful lightning show, but the rain was so bad that visibility even sitting still was less than 100 yards. And it hurts when it hits you when riding above 25mph. I had to sit in the empty car wash for over 90 minutes before it finally stopped.
Last February, I rode to work at 7:30am in 14°F (-10.1°C) weather. It was miserable, but I have a well-insulated ski bib, a very warm winter jacket (Port West S464 modified with armor in the elbows, shoulders, and back) with a fleece zip out lining, and the absolute best cold-weather gloves I have ever seen. Port West A726 Aqua-Seal. They're so good that they can't be worn if it's over 45°F (7.2°C) otherwise your hands will sweat. Both are lovely high-viz...
To me, the best riding is March, April, October, November, and some of December. All the rest is one form of discomfort or another, only made tolerable by the machine on which I am sitting...