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Let me generalize and say that there are three general factors when dealing
with how hot an item gets...and how hot it feels...

1) Color
2) Material composition 
3) Surface area/contact area (slightly different but related).
4) Did I say three factors?  If an item is biological it can have extra
factors including abilities to temperature regulate by getting rid of hot
water.  And if an item isn't biological, it may lose heat quicker by
evaporating off part of its surface (think water), keeping itself cooler.

More specifically...

1) Color - it's true, black surfaces absorb and re-radiate more heat as they
are black so they absorb a larger range of light frequencies and re-emit
them as heat.  But we can only compare black apples with white apples so to
speak.  

2) Material composition...Ah yes, remember those chemistry experiments
dealing with "specific heat capacity?" that's all about the material.  How
well it absorbs and re-releases energy.  This is to compare say clear
Plexiglass with clear glass.  Both have similar color and shape, but are of
different materials.

3) Surface area/contact area.  Take a piece of rock and set it in the sun.
Take an identical piece of rock and grind it up so it is sand.  The surface
area/contact area will affect both how it absorbs heat and how it
re-radiates.  More particularly we can feel things as being hotter, if they
contact more of our body with fewer air gaps.  Think of a hot potato - you
don't get burned by the hot potato when holding it gingerly by the
fingertips, but you do get burned when gripping it fully in the hand.  Or
better yet, think of a slab of metal versus if that slap of metal was turned
into a sponge with holes everywhere (hey, it's a thought experiment).  Which
one would burn you more quickly?  The one without all the air gaps since it
has more contact area to burn you with.

-William

William Katzman
Director of Exhibits         
Catawba Science Center
(828) 322-8169 x307
"The Magic of Learning, the Science of Fun" 
 
Now Renting Out
Dinosaurs! (3,000 sq. ft. only $25k)
Body Carnival (2250 sq. ft. only $15k)
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http://www.catawbascience.org/on-exhibit.htm




-----Original Message-----
From: Informal Science Education Network
[mailto:[log in to unmask]] On Behalf Of Daniels, Alissa
Sent: Tuesday, September 16, 2008 11:28 AM
To: [log in to unmask]
Subject: why is sand hot?

ISEN-ASTC-L is a service of the Association of Science-Technology Centers
Incorporated, a worldwide network of science museums and related
institutions.
****************************************************************************
*

We were talking about our new green roof recently, and made the comment that
the green roof is cooler than a traditional tar roof; any kid who has ever
walked on the grass and then on the driveway can relate to that. The black
roof absorbs a lot of heat. 


But then someone said "well, how about the sand on the beach? it's white AND
hot." And we batted around a few ideas ranging from "well, it's still cooler
than the black roof,  and "well, the grass and the soil hold water, so that
keeps it cool" and "it must have to do with the shape of the grains of
sand..."

Anyone else have any thoughts on this? Something to think about if your
September is as quiet as our September.

AD

~~~~~~~~~~~~~~~
Alissa Daniels, Science Program Manager/Senior Educator
Boston Children's Museum
617 426 6500 .342
"The most exciting phrase to hear in science, the one that heralds new
discoveries, is not "Eureka!" (I found it!) but "That's funny..."  " --Isaac
Asimov
~~~~~~~~~~~~~~~

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