It's generally not a good idea to leave your dog alone in the car for long periods of time, as the temperature inside the car can rise quickly, which can be deadly for dogs. The length of time that is safe for a dog to be left in a car will depend on the temperature and humidity outside. In general, if the outside temperature is above 70 degrees Fahrenheit, it's not safe to leave your dog in the car for more than a few minutes. At 80 degrees Fahrenheit, the inside of a car can reach 99 degrees Fahrenheit in just 10 minutes, and at 90 degrees Fahrenheit, the inside of a car can reach 109 degrees Fahrenheit in just 10 minutes. If you need to stop and run an errand and can't bring your dog with you, it's best to leave them at home in a cool, comfortable place.