You can crack the windows, but it won't help much. Dr. Ernie Ward demonstrates--and personally experiences--that on a hot day, the interior temperature of a car can reach 117° F. It's a miserable experience. But for a pet, it's even worse. Dr. Ward says:
What I really wanted to set out to do was to see how it felt to be left in a parked car if I were a dog. And I know what's going on. I'm in control of the situation. What I can't imagine is how helpless and frightening it would be for a dog to be left in a car, not knowing when you're going to come and slowly but surely having the energy and the life just burnt out of them by this heat.
-via Daily of the Day