A study on how hot it can get
Automobiles parked in the sun will heat up, sometimes dangerously, because of the greenhouse effect. The visible light from the sun heats up the interior of the car and the heat (infrared radiation) cannot escape because it cannot pass through glass. The closed interior, even with the windows opened a bit, effectively prevents convective heat loss. An automobile or greenhouse structure act quite differently from the atmosphere which has no barrier to convective heat loss.
The Department of Geosciences at San Francisco State University conducted an experiment to see how much vehicles would warm. (See full paper here) Below is a graph of results for a dark-colored car (red line) and a light-colored car (yellow line). You can see that the cars heat quickly to uncomfortable and even dangerous levels. So, even if you think “I’ll just be gone a minute” don’t leave children or pets in the car.
The graph above shows results when the outside temperature was about 75 F. The greenhouse heating effect becomes dangerous much more quickly when the outside temperature is warmer. If, for instance, the outside temperature is 91 F, the inside can get to 117 F within 10 minutes.
Table 1 below shows temperature versus time for a closed vehicle.
Table 2 compares temperatures versus time in closed vehicles and vehicles with windows open 1.5 inches. The vehicles with open windows don’t get quite as hot but still get dangerously warm.
The warning about not leaving children or pets in cars is not restricted to Arizona. Last year there were two child deaths from overheating in Canada.
In 2013, there were 44 heatstroke deaths of children left in cars and a total of 607 reported deaths since 1998. See more complete statistics at http://ggweather.com/heat/index.htm
Only twenty states have laws specifically about leaving children in cars (Arizona is not one of them), but all states have laws against endangering the life of a child. Tucson, however, has an ordinance that calls for a thousand dollar fine or 24 hours worth of parenting classes for leaving a child unattended in a car.