It is not uncommon to hear about someone who left their dog in a parked car, even for just a few minutes, and came back to find them suffering from heatstroke or already dead. If temperatures are about 70-80ºF a car can heat up to 100ºF within minutes. If the temperature is 85ºF and the windows are slightly open the temperature inside can reach 102ºF in ten minutes and in 30 minutes it can reach 120ºF.
On hot and humid days the temperature inside a car parked in the sun can increase more than 30ºF each minute. That quickly becomes lethal since at 110ºF a dog is in danger of heatstroke.