Even on cooler days, the temperature inside a vehicle can rise as much as 30-40 degrees within an hour. That's even with windows left slightly open.
Persons who leave pets inside of vehicles in the heat can be charged with an infraction or misdemeanor. In the event the animal dies, the person can be charged with felony animal cruelty.
If you come across a pet inside of a vehicle, check to see if it appears to be in distress. These signs include heavy panting, thick saliva, lethargy, restlessness, excessive thirst, and lack of coordination. If you believe the animal is in distress please call your local police and report it immediately. If the pet doesn't appear to be in distress, have someone monitor the pet (if possible) and attempt to locate the owner and then contact the police department.
Please remember that not only is leaving your pet in the car illegal, it could be deadly!