Ok, so summer is great, it really is. You can bike to work at full tilt and enjoy the wind whistling through your helmet.
However, just try and remember that when you do that, and it's a heat index of 90F+, with heavy humidity, that you're pushing your limits. Or at least, I was yesterday.
Yesterday on my ride in, I basically put the hammer down and was having a blast, sweating my brains out, literally...
By midday, I had a dehydration induced migraine the size of a freaking jackhammer going off in my head. I promptly left work, went home, drank what seemed like a tank of water, and passed out for about 3 hours. All was well.