Cats, being descended from desert-dwelling African Wildcats, tend to have better heat tolerance than other pets.
As long as the temperature isn't higher than 100 degrees and your cat's carrier is out of direct sunlight, she should be fine. You can take additional measures by providing her with plenty of moist food before the trip (scared cats generally don't eat and are hesitant to drink water). You can also cover the carrier with a moist towel when it gets hot, which will not only help with the heat, but will reduce stress-inducing sights and sounds.
Reducing stress while moving cats can also be done by keeping the cat in as dark and quiet a place as is practical while moving and providing a blanket/tshirt/towel with familiar smells to lay on. I've had very good results with using Feliway hormone with my extremely nervous cat while moving, as well.