It doesn't have to be super hot outside for your car to heat up.
Temperatures can rise to dangerous levels in just minutes, putting your dog at risk of heatstroke. The inside of a vehicle parked in 70-degree weather can reach 100 degrees in just 20 minutes. On very hot days, temperatures inside parked cars can climb to 140 degrees Fahrenheit in under an hour.
But what if you slightly open a window? The answer is simple: you should NEVER leave a dog alone in the car, even with the windows cracked. According to the American Veterinary Medical Association (AVMA), studies have shown that cracking a window changes these figures very little. A parked car with the windows cracked heats up at almost the exact same rate as a car with the windows rolled up, putting pets in serious danger.