Q: Even on a cold day, when I park my car in direct sunlight with the windows closed, the temperature inside climbs above the ambient temperature. Why is that?
A: The same process that puts children and pets in danger in a closed car on a hot day is at work under the winter sun: the greenhouse effect.
Light energy, in the form of short, visible wavelengths, passes easily through glass into the closed vehicle. Some of the energy is reflected right back out, again in visible wavelengths.
But some of it is absorbed by objects and surfaces inside, and then reradiated in longer wavelengths in the invisible infrared range — that is, heat.
The heat is effectively trapped by automobile glass, which is not as permeable to longer wavelengths. The air inside the car is also trapped.
Temperatures can rise quickly in a closed, sunlit car. One study, done at warmer temperatures, found typical increases of 20 degrees Fahrenheit above the ambient temperature in just 10 minutes, and 33 degrees in 20 minutes.