Why does it get warm in a car on a cold day?

Answer 1

If a car is left in the sun, the interior of the car will get warmer than the outside air. (This assumes the windows to the car are left closed.) This is true whether it is a warm day or a cold day.

The cause of this is the same thing that causes global warming, "the greenhouse effect." The phrase "greenhouse effect" was in existence long before the worldwide discussion of global warming became popular because the effect really does happen in a greenhouse.

In a car and in a greenhouse, the sunlight enters through glass. Sunlight contains energy. Much of the sun energy passes through the glass. The sunlight is then mostly absorbed by the material inside the car where it is converted into heat. The heated material then heats the air.

The energy in sunlight is large. If the sun is directly overhead, it supplies 1,000 watts per square meter to the Earth's surface. That is ten times the heat given off by a regular light bulb. A portable electric space heater of the type used in houses in winter typically puts out 1,000 to 1,500 watts.

The amount of glass is many cars is large enough to allow in a significant amount of the sun's energy. That is why a closed car on a warm day can get too hot for humans (or pets) in a matter of minutes.

On a cold day in winter, the sun is not directly overhead and less energy is likely entering a car, but even a fraction of a kilowatt will create a noticeably warmer environment inside.

Aside: One should not confuse the actual warming of the car in a cold environment with the feeling of warmth. A warmer feeling that might result from the stillness of the air and decreased heat loss through the skin inside the car compared to the more rapid heat loss at the same temperature due to wind outside the car.