Yes, dark hair does become hotter than blond hair when out in the sun. To understand this, you need to know two things about light.

The first thing you need to know is that sunlight is actually the combination of many different colors. You can see this by shining light into a prism and watching all the colors of the rainbow come out.

The second thing you need to know about light is that the color that an object "looks" is the colors of light that that object *reflects*. Plants are green because they *reflect* green light, and keep in all of the other colors. White objects reflect all colors of light, and black objects keep in all colors.

Putting these two facts together, dark hair absorbs (keeps in) all colors of light, while blond hair reflects all colors of light. Absorbing more sunlight makes hair hotter, and therefore dark hair will be hotter than blond hair in the sun.

Before getting into the main part of the answer, I should make the distinction between retaining heat, and becoming hotter (absorbing heat). The first can be thought of ashow much heat energy can be contained, while the second is how well heat is taken up. All else being equal, dark-colored hair and light-colored hair should be able to hold the same amount of heat (as an analogy, dark- and light-colored clothes coming out of the dryer will be the same temperature), but dark hair will absorb more heat when in the sun. How much energy a substance can hold depends on the structure, and therefore on how much and how many ways the various atoms/molecules making up the substance can vibrate/rotate/("jiggle"). (I suppose the different colors will arise from different molecules or structure, and that this would have some effect, but the net difference is apparently small.)

As for dark hair becoming hotter, the reasoning is the same as for dark- vs. light-colored anything else: dark-colored things absorb more energy than light-colored things. The reasoning really goes the other way though, and it is that we have chosen to call objects which do not reflect light (i.e. those which absorb the light) as black, and those which do reflect light (and the energy in it) as lighter/white (some discussion here ).

If you would like proof, unfortunately I was not able to find good, scientific information for humans, so hopefully the consideration of springboks (a deer-like animal) is acceptable. In these animals, those with black fur experienced a greater and faster temperature rise.

Things with dark colors absorb (take in) all light and turn it into heat. On the other hand, things that are lighter in color absorb some light but not all light.

For example, a red shirt reflects red-colored light and a yellow shirt reflects yellow-colored light. A white object, for instance a white box, reflects all colors of light and therefore absorbs the least heat of all the colors and stays cool the longest. Black objects, by contrast, absorb all colors of light and becomes hotter than objects of any other color, so the answer is yes, dark hair does become hotter than light-colored hair.

Dark hair absorbs more heat. It also emits more heat, so it will be cooler indoors to be dark-haired and warmer out-doors to be dark-haired. Still, it's better to just wear a hat when spending much time outside.

Here's an experiment that should show how a light colored house stays cooler in the summer than a dark colored house:

house color heat

Darker colors tend to absorb more energy from the sun than objects with lighter colors. Someone wearing a white T-shirt in the summer will find that he is cooler than someone wearing a black or dark-colored shirt. This is true of all materials which have dark colors. Other dark surfaces include blacktops, paved roads or rooftops.

materials absorb energy