Why Are the Roofs of School Buses Painted White?

Lately, I’ve been seeing a lot of school buses that are yellow everywhere except for the roof.? I was perplexed as to why.? According to this nearly 20-year-old New York Times article, the reason is that white tops are more reflective, lowering the temperature inside the bus by an average of 10 degrees during the summer.? The source of that statistic is a very non-scientific sounding North Carolina pilot study.

This 10-degree difference in temperature sounds preposterous to me.?? Yellow and white just don’t seem very different in terms of heat absorption.

Are there any scientists among our blog readers who can do some back of the envelope calculations to determine whether white-roofed buses would actually be noticeably cooler?

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  1. Forrest Grant says:

    This honestly sounds like something for the Myth Busters to tackle.

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  2. Chris D says:

    You’ll notice the same thing on UPS trucks Although I’m guessin the white vs brown provides more benefit than the white vs yellow.

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  3. David says:

    Related, but different: Roofs of UPS trucks are translucent white, so they don’t have to use interior lighting to see the packages (at least during the day).

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  4. Edward Bond says:

    White paint reflects all light across the visible spectrum. Yellow paint absorbs all visible light except yellow. It is the same reason ice packs are often in silvery packets.

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  5. Alex says:

    The more pressing question is why school districts increasing feel the need to add a tiny strobe light to the top of school buses. Do we really need at strobe lite to call attention to a BIG YELLOW BUS! How safe do we need to be!?!
    Sorry, this is one of my pet peeves.

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  6. DanInesanto says:

    Edward, however, the brighter (and shinier) the color, the more of the light it reflects away out of the entire spectrum, so a bright yellow will reflect a lot more than a darker yellow.

    School Bus Yellow is a particular color (wikipedia school bus yellow) the RGB is approximately 255,216,0. (FFD800 for the web developers out there)

    What you want to compare is the albedo of the different colors. The albedo of white paint will be roughly .7 to .8. The yellow paint’s albedo will be somewhere around .5 to .6. (I’m just guesstimating that based on similar colors) The yellow will absorb about 40-50% of the energy while the white will only absorb 20-30%.

    That is certainly enough to make a noticeable difference in the right conditions. There are way too many variables to completely nail it down here in a blog comment, but it certainly suggests that there is some basis in fact. Whether it’s a full 10 degrees difference under normal operating conditions – I don’t know.

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  7. Jorge says:

    Though I live in Spain, I was told in school 25 years ago that buses, police trucks and other vehicles that could be hijacked, were painted white on the roof to more easily locate them from the air in such event. I have no idea if that was true.

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  8. Josh says:

    It all has to do with a property called absorptance–basically a percentage of how much radiative energy is absorbed vs. reflected. White paints have an absorptance of around .10-.15, while yellows range from .3-.4; although both greatly depend on additives to the paint.
    Also playing a role is the angle of incidence–the angle at which the energy impinges the surface. The roof is subject to much more direct sunlight, and thus the lower absorptance plays a bigger role on the roof.
    10 degrees would be pretty easy to achieve, I believe SecEnergy Chu has suggested we all paint our roofs white for the same reason.

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