As long as the interior of the car stays cool enough on the cool day then you can leave the dog in the car for an hour or so.
Just don't leave the dog in the car longer than 1 hour and 30 minutes.
For example if you have to run into the store and will be out within the 1 hour time then it's okay to leave the dog in the car on a cool day.
It's also a good idea to leave the window cracked a bit to allow some fresh air into the car and also parking the car in the shade would be a good idea if possible.
However if you really don't need to bring your dog with you then it's best to leave the dog at home instead of leaving the dog in the car.
On hot days you should never leave the dog in the car and never leave a child in the car as they can be kidnapped, overheat and die etc.