* Solar Radiation: Sunlight contains a significant amount of infrared radiation (heat). A significant portion of this radiation passes through the windows and is absorbed by the interior surfaces (dashboard, seats, carpets, etc.). These surfaces then re-radiate that heat as longer-wavelength infrared radiation, which is less likely to escape back through the glass. This is the greenhouse effect in action, on a smaller scale.
* Infrared Absorption by Glass: While some infrared radiation passes through, much of it is absorbed by the glass itself, further contributing to the heating.
* Conduction and Convection: The heat absorbed by the interior surfaces is then conducted to other parts of the car's interior, and also warms the air inside through convection. The air trapped inside the car can't easily circulate and dissipate the heat.
* Color of Interior Materials: Darker colored materials absorb more heat than lighter colored materials. A dark-colored dashboard and seats will contribute significantly to the temperature increase.
* Lack of Ventilation: Closed windows prevent air circulation, trapping the heated air inside and preventing it from escaping. Even with the windows slightly cracked, this effect is still significant.
In short, it's a combination of solar radiation being trapped inside the car, the materials inside absorbing and re-radiating that heat, and the limited air circulation that leads to a significant temperature increase in a parked car, even on a relatively cloudy day.
