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?


Forrest Grant

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

Chris D

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.

David

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).

Edward Bond

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.

Alex

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.

DanInesanto

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.

Read more...

Jorge

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.

Josh

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.

John Trask

Check out this website http://www.cis.rit.edu/mcsl/outreach/faq.php?catnum=6

AaronS

One of Toyota's (maybe Ford's) new SUV vehicles has a white roof.

Houses that have white roofs are cooler than those that do not.

So it does seem to follow....

It seems to me that another way to handle this would be to put in that attic foil stuff in the tops of buses, to even further reflect any absorbed heat (supposedly it "reflects" back around 97% of RADIATED heat--or cool, for that matter).

As for the strobe light, it is not because buses aren't big, but because fog can obscure a bus (many run during early morning darkness). A strobe light, even if it provides just an additional 1% of safety, seems a small thing to install when you're talking about kids.

cjc

It's not just paint. Google "bus kote."

"Bus-Kote is a bight white, acrylic elastomeric, insulating ceramic, waterproof coating designed specifically for Buses and recreational vehicles. This high build rubber like coating waterproofs, insulates, soundproofs, beautifies and protects with a ceramic shield that expands and contracts with varying hot and cold temperatures plus resists thermal shock. Bus-Kote offers superior mildew resistance and ultraviolet ray reflectivity.

Originally designed as an insulating, reflective coating for school districts to use on the roofs of school buses to lower the interior bus temperature. This energy saving coating will reduce interior temperatures when hot and make interior temperatures more constant when cold due to ceramic insulation."

AaronS

An additional thought....

We know that some colors can create a greater degree of stress/agitation, while some colors are calming. It seems that the INSIDE of buses ought to be the most appropriate color.

When the bus is taking the football team somewhere, it ought to be blood red, I suppose. While if it's taking them on a long trip, it ought to be pastel, so that the bus driver survives the trip.

Makes a Ton of Sense

So during winter, they'll use more energy to keep the buses warm?

Steven Bearden

Is it more important for officials to lower the temperature in the bus or to be perceived as caring about the children who are on that bus? Also, what separates bus manufacturers, if gas mileage and seating is similar, the one with the "cooler" roof may stand out (in more ways than one). Though the roof may lower the temperature in the bus, is that the question to ask or is it reflective of the officials and the bus company?

PaulD

@ Makes a Ton of Sense
Unless the bus is electric powered you get the heat for free.

Jeff

I was gonna say the same ting that cjc said.

The problem with the linked article is that it doesn't go into enough detail. Its costs a couple of hundred dollars more for the white tops because its not the same type of paint as the normal yellow paint, its a more reflective type of paint, in addition to being white.

Geoff

Some areas of North Carolina have year round schools. Instead of a traditional calendar with a summer vacation, there are 4 tracks of 9 weeks in school, 3 weeks out. This strategy is used to help solve problems with school overcrowding but it means that some group of kids are in school in June, July and August when it can easily be 90-100 degrees with high humidity. Most of the buses used in the summer are air conditioned but it is quite plausible that the white roof also helps reduce temperatures inside the bus.

Ian Kemmish

Maybe they are delivered white all over. Who is going to waste time and money painting the roof of a bus?

D. Johnson

Looks like the Mythbusters already took a crack at a similar question:

SPINOFF: A black car heats up faster than an identical white car. (From Pilot 2)
confirmed

A fan wrote in and asked a follow up question: "Does the color of a car affect the way it heats up?". The MythBusters used two identical cars, one black the other white and left them both out in the summer heat with thermometers in both. By mid-afternoon the black car had heated up to a temperature of 135 ?F while the white car topped off at 126 ?F, almost 10 degrees cooler.

http://mythbustersresults.com/episode38

Makes a Ton of Sense

@ PaulD

In that case, wouldn't you get cool air for free too? (I'm not a technician, so I have no idea about how either one works.)

Oh I know, they can save money by not having to wash the buses as often since the roof will camouflage with the bird cra...