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 “cracked” a bit, effectively prevents convective heat loss.
Back in 2002, 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 and a light-colored car. 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 just about 75̊°F. The greenhouse heating effect becomes dangerous much more quickly when the outside temperature is warmer. If, for instance, the outside temperature is 90̊°F, the inside can get to 110̊°F within 15 minutes.
The table below compares the heating results over time between a car with closed windows versus a car with windows open 1.5 inches. You can see that “cracking” the windows does not make much difference, especially for longer time periods.