Sunday, June 30, 2013

Things to Know About Albedo...

Which side of the T&C logo is hotter: the white side or the black?  Does
it matter?  For the answer see the end of this blog post.  
Have you ever heard that it is cooler to wear lighter clothes on a hot, sunny day?  How could this be?  How could the color of your clothes make a difference in temperature?  This couldn't possibly be true, could it?  Well, it is true, and the difference in temperature is explained by albedoAlbedo refers to the proportion of the sun's energy (insolation) that is reflected by a surface.  Albedo is generally expressed as a percentage, and so if it is high (say, 90%), then more of the sun's energy is being reflected by a surface.  The 90% figure means that 90% of the sun's energy is reflected off the surface.  If albedo is low (10% for example) then more of the sun's energy is absorbed.  In this case, only 10% of the sun's energy is reflected whereas the balance is absorbed.  If a surface absorbs more energy, it heats up more.  We can change the albedo of a surface simply by changing its color; light colored surfaces tend to have higher albedo than darker surfaces.  One very good example of this is clothing; on a sunny day here in Hawai'i white shirts can be as much as 30 degrees cooler than dark shirts.  In this post we'll be looking at another practical example of the difference that albedo can make.

A White Roof?

One of the neat things about studying physical geography is that we can see practical applications of what we learn everywhere.  A couple of years ago the property managers of the apartment complex that houses one of this blog's authors installed a "white roof", coating the roof of the building with a white polymer that significantly increased the roof's albedo.  In other words, the white roof reflected a great deal more energy than the old tar roof.  Immediately after the white roof was installed we noticed a big change in the temperature of the was a lot cooler!  Even on the hottest days of the year at the peak of the afternoon sun we no longer needed to run the air conditioner to stay comfortable.  The new roof made a big difference and has saved us money on our electric bill, which in turn leads to decreased greenhouse gas emissions.  In short, everyone wins.  

More recently the management company had the entire building repainted, which was long overdue.  The old color was pale pink, a very light color.  The new color is dark gray.  Based on our experience with the white roof, I was perplexed as to why the management company would go with a dark color, because I reasoned that the darker color would absorb more of the sun's energy, thus making the interior of the building slightly warmer.  I was also curious how much a difference the darker color made to the surface temperature of the building.  So we here at TWITB decided to do an informal investigation: we would measure the temperature using an ExTech infrared thermometer, a handy little tool which uses a laser to measure surface temperatures.    We measured the temperature at approximately 3:20pm, which is approximately the hottest time during the day in Honolulu (why this is the case will be covered in a future post).  Since the painters didn't quite finish the job on the first day there was still some of the old pink surface (see the photo to the left), and so we could compare the results.  We measured the pink surface, the grey surface, and the dark gray doors.  We measured them in the sun and in the shade as well.  Which one do you think we found to be the hottest?

We were really amazed with what we found.  First the pink paint in the shade was 98 degrees whereas in the sun it was 110.  The concrete bricks that had been painted gray were 103 degrees in the shade and 130 in the sun.  The door was most surprising; in the shade it measured 111 degrees, whereas in the sun it measured 148 degrees.  So as you can see the paint really makes a big difference!  Whether this affects the interior temperature is a more complex problem to solve; this depends on the specific heat and conductive properties of the bricks and the door.  

So as you can see the color of a surface definitely does make a difference in surface temperature.  Let's look at some more examples.  In the picture below you will see some cars in a parking lot.  We've placed letters over a few surfaces.  This picture was taken at 3:30pm on a day when the official airport temperature was measured at 88 degrees.  "A" is exposed blacktop.  "B" is a white car in full sun.  "C" is a white parking space marker painted on the exposed blacktop.  "D" is shaded leaves.  "E" is a black car in full exposure to the sun, and "F" is a green car in full exposure.  "G" is blacktop in the shade.  For reference, we also measured a grass patch (not pictured) in the sun; its temperature was 110 degrees.  Match the letters from the picture with the numbers below the picture that correspond to the temperatures of each surface. 

1.  164 degrees.
2.  91 degrees.
3.  143 degrees.
4.  117 degrees.  
5.  114 degrees.
6.  151 degrees.
7.  138 degrees.

You can do similar tests yourself just by touching various surfaces on a hot day.  You'll also notice a big difference between the shade and exposed temperatures for the same surface.  In a future post we'll explain more in depth why color makes a big difference, but for now it's enough to know that darker things are hotter and lighter things are cooler.  As for the T&C logo, check out the pictures below.

White side.
Black side.
Answers: 1. E  2. D  3. F  4. B  5. G  6. A  7.  C  

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