You need to drink plenty before you ride not just riding days but everyday and not just during exercise. I ride in heat index of 120, and I'm fine. I drink 2L everyday though. If I haven't ridden for a long time say 1 month, then go out at a high intensity I will feel like crap. I puked while I was cycling - while moving. I immediately felt better after I vomited. That day I was probably dehydrated, drank too much during exercise, and I ate too much and too fast for breakfast right before cycling.