It angers me that dogs still die (or come close to dying) in hot cars due to being left alone during the Spring/Summer months. It does not have to be particularly hot outside to become dangerously warm inside a car in a relatively short amount of time.
Even with a window open the temperature in the car can become life threatening for your dog, so please don’t consider leaving a dog unattended for anything but a very short amount of time. If you are parked in a fully shaded area it is still important to check on the dog/s every 20 minutes or so and provide drinking water as humidity is just as a risk as direct sunlight. If it feels warm in the car, don’t risk it. Equally, conservatories can become very hot on sunny days, so always leave your dog in a cool room if you have to leave them for any length of time.