I ride year round here. I've taken 200+ mile trips in the full heat of summer, always wear a jacket, Draggin' Jeans and full face helmet. Mesh jackets above 70 degrees. The trick is keeping the sun off your skin and keeping hydrated. Like Adey mentioned, drink water even if you're not thirsty. I drink about a quart every 15 minutes to half hour when I'm riding in the heat, plus grab a large gatorade every couple of hours. You know it's hot when you've drank three gallons of water over a three hour period and don't have to pee.
