Dogs die in cars every year in the warmer months. Did you know that even at 60 degrees, it is too hot to leave your dog in the car? It usually takes under 20 minutes for your pet to suffer severe effects or death from heat stroke.
What do you do if you see a dog in a hot car? In most states, get the location, make, model, license plate, etc and call 911. Familiarize yourself with the laws/steps to take in your state. 
A new bill was just passed in Tennessee, which protects animals along with children, allowing good Samaritans to break into vehicles without civil consequence if either is in imminent danger. Please read the law for the steps you must follow before you act. This law will only protect you in Tennessee, and only if you follow all the steps accordingly. Other states will hopefully soon follow suit.