The temperature of a car parked in the sun with windows cracked will rise approximately 1 degree per minute over the temperature of the outside air. A car parked for 10 minutes in 90 degree heat will be at 100 degrees in 10 minutes. It will be over 120 degrees in 30 minutes. These temperatures are fatal very quickly. Don't take a chance. It is just not worth it. There is a chart that lists these temperatures at the AVMA website.
I returned to my car one sunny day to find the car parked next to mine had a puppy in the backseat. There was water for the puppy and the car windows were cracked. But the puppy had vomited and defecated all over the car and was in great distress. The owners of the puppy returned with their children and groceries to find the sheriff and medics working to save their puppy. I am sure it was not the outing they had planned for that beautiful day.