White vs. Black Helmets

The facts about which helmet color is cooler — white or black — have been hotly debated, typically with a lot of esoteric engineering jargon, supported by a lot of ill-conceived ideas about how heat transfer works. Without wasting your time, here it is: white helmets are cooler (and black helmets are hotter). If you care to know why, read on.

The underlying physics surrounding the debate are certainly complex; heat transfer is probably one of the most difficult engineering disciplines to master. However, the analysis of a white or black helmet should not be overly complicated. If we assume that the two helmets we are comparing are only different in their color, then we can ignore factors such as conduction through the layers of the helmet, and convection at the surface (as we will flat-out assume that the coefficient of convective heat transfer does not vary with surface temperature, although it does, to some degree). After throwing out two components of our conservation of energy equation, we are just left with two remaining terms: 1) the heat contribution from solar irradiance, and 2) the heat loss due to radiation. The energy balance equation follows:

Where (description / units):

• alpha = absorbtivity factor / unitless
• G = solar irradiance / W/m²
• epsilon = emissivity / unitless
• Ts = helmet surface temperature / Kelvin
• Tsur = surrounding air temperature / Kelvin

Looking at the properties of black versus white helmets (see Table 1, below), we see that white helmets are similar, but slightly less capable of radiating heat than black ones, however, their absorptivity is significantly lower. This fact, ultimately, is what allows a white helmet to stay cooler — it just absorbs less heat from the sun.

Table 1. Comparison of radiative/absorptive properties of black vs. white helmets

So what does this mean for you, the rider? Well, if you were a rider in a vacuum with a flat-plate for a head, here’s what your helmet would experience. Spanning a range of solar irradiance from 50 W/m² (moonlight) to 1250 W/m² (think Kona under partly cloudy skies), here are the surface temperatures you’d see at the exterior of the helmet:

Figure 1. Surface temperature vs. solar irradiance for black vs. white helmets

While the model ignores factors related to conduction, convection/ventilation, which will reduce the differences between white and black helmets in the real world, the exercise definitively shows that a black helmet will be hotter than a white one, in any practical situation.

Why is there so much debate on this topic? I can think of a real-life example where the choice to paint something black resulted in cooler surface temperatures: the SR-71 Blackbird spy plane. In this case, Lockheed Martin’s Skunkworks team made the choice to add more than a 1000 lbs of black paint to the plane, for the marginally greater radiative efficiency (higher emissivity). But the SR-71 was in a unique situation: the solar heat gain was modest, and the thermal gain from mere friction of the air against the fuselage while racing at mach 3 was astronomical. Because much of the heating was occurring from the skin of the plane, in addition to the heat dissipated from the jet engines, they needed to find a way to eject as much heat as possible. To do so, they chose a finish with the highest possible emissivity, while sacrificing a higher coefficient of absorbptivity. With the surface of the SR-71 Blackbird reaching over 1000 degrees Celsius, in relative terms, it didn’t matter what the heating from solar irradiance was.

Coming back to the cycling helmets, that’s just not the case. time trials and triathlons occur in sunny places, where the sun’s power can be quite high. The major component contributing to heat rise in the helmet is the solar radiation, and due to the low surface temperatures (<100 C), radiation does not play as vital of a role in cooling the helmet. In this case, we care less about the emissivity value, and a lot more in the absorbptivity, as was demonstrated in the analysis above.

I hope that this discussion was informative, and brings clarity to the helmet color issue. In a future post, I may actually measure the temperatures of helmets to confirm (or disprove) my analysis. Until then, you can have confidence that, for endurance cycling events, you will be better off with a white helmet.