Why does a car get hot when the sun is out on a cold day?

Why does a car get hot when the sun is out on a cold day?
This happens through a process that we commonly call the Greenhouse effect.

The infrared rays (heat radiation) can pass easily through the windows and heat up the dashboard, parcel shelf, seats etc'. Some of that heat is then transferred to the air via convection and as the hotter air rises, cooler air moves in beneath it ... that gets heated and also rises. This creates a cycle in which the air gets hotter and hotter until the point where the heat being conducted to the outside skin of the car allows for an equilibrium to be reached.

It can easily be approx 80 degrees F on the outside of a car and build to 120 degrees F or thereabouts on the inside due to this Greenhouse effect.