Re: Heat absortion vs. Color

Hi JJ--

Here's the short answer, direct from an online physics lecture:

"A black car will heat up faster in the sun than a white car will. It will also lose its heat faster in the winter."

Read on if you'd like some details on why this is so.

For this discussion, we don't really care about the subatomic causes of heat or why black bodies are the best absorbers and emitters of electromagnetic radiation (EMR). We only care about three things:

1. Which parts of each car are sunlit.

   The majority of the direct sunlight will build up on the exposed surfaces of each car-- the roof, the hood, the trunk. As a rough estimate, let's say that 20% of the light hits the interior directly. So 80% of the light is hitting the painted outer surfaces of each car.

   Now, about half of the EMR given off by the Sun is in the infrared (IR) range. This is light with a wavelength longer than red light and which can't be seen by humans, but which still causes heating effects. Sunlight also includes a small portion of invisible ultraviolet (UV) light, which has wavelengths shorter than visible light. Keep this in mind as we move on to...

2. What materials are in the sunlit portions of each car, and which of those materials are better at absorbing and emitting heat.

   Basically, there are only two types of material here: metals and non-metals. In any case, we've decided that most of the sunlight is falling on the body of the car, which is metal and paint.

   We know that sunlight includes IR and UV as well as visible light. Certain types of paint may reflect more or less IR and UV light than others. You may have heard of Light Reflectance Value (LRV), which is used to measure how much visible light a certain color reflects, but there's also a Solar Reflectance Index (SRI) which measures how much solar heat (i.e., infrared light) a given material reflects.

   For this discussion, we're only concerned with visible color, so we'll assume that all materials in the two cars reflect UV and IR equally well. Note that this may not be the case in real life.

   So what's reflecting the visible portion of sunlight? Metals are much better conductors, but being naturally shiny, they don't absorb a lot of light. But-- and this is important-- it's the paint on the car which is absorbing the heat, and the metal underneath which is conducting and emitting heat through the entire car. The sheen (shininess) of the paint will affect its LRV, but we'll ignore that for the time being. Only the color matters.

3. Which of those materials has the most impact on the temperature inside the car.

   No contest. A car is mostly metal, and that metal surrounds all the interior areas. If the metal gets hot, the car gets hot.

If we look up the LRVs for the colors white and black, we find that white reflects 80% of visible sunlight, and black reflects only 5%. So we can conclude that, regardless of the color of the interior, the car with the darker paint job will have the higher temperature.

Of course, leaving any car out in the sun for many hours will make driving it later an unpleasant experience. My advice? Install air conditioning.

Hope this helps!
Curtis