Why does it get hot inside your car when the windows are closed?
When I was in high school, one student guessed that the windows magnified the
light from the sun. That's not it. Return to my Mathematics pages
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© Copyright 2002, Jim Loy
Why does it get hot inside your car when the windows are closed?
When I was in high school, one student guessed that the windows magnified the
light from the sun. That's not it.
Glass lets some of the light from the sun go through it, and reflects some. In particular, infrared "light" (which you can't see) will not go through glass. The visible light from the sun goes through the window glass and strikes the interior of your car. Some of it is absorbed and heats up the interior, whether the windows are closed or open. The heated interior radiates infrared "light," which is reflected by the windows and cannot escape the interior of the car. This called black body radiation, as it is the radiation coming from a theoretically perfect black object when it is heated. And your car gets very hot as it accumulates this energy.
That is the greenhouse effect. Greenhouses use this feature of glass to keep plants warm in winter.
Some gasses, carbon dioxide for example, act much like glass and are transparent to visible light and are not transparent to infrared light. So a planet covered in carbon dioxide (Venus) will become significantly hotter than one with little carbon dioxide. The carbon dioxide in the Earth's atmosphere is increasing because of the burning of fuels; and the temperature of our atmosphere may be increasing because of this. This is called global warming.
If you cover the inside of your windshield with a metallic sunshade, your car will be cooler. The metallic surface reflects light and does not scatter it much. So it reflects the light right back out the window without converting much of the light to infrared light. A nonmetallic sunshade will absorbe some of the light and heat up your car. A white sunshade will produce less infrared light, and your car will be cooler. A glossy surface on the sunshade will reflect some of the light and scatter less, and make your car cooler. Those people who face the "Need help, call police" side of the sunshade outward are heating the car more because that side is a duller, darker color which absorbes more light.
My previous sunshade had this warning: "Do not operate vehicle with sunshade in place." Who would be stupid enough to drive with the sunshade up? And what jury would be stupid enough to blame the manufacturer for damages? It makes no sense. But stranger things have happened.