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The key is how much of the radiant energy is absorbed by 
the sand, keeping in mind that our eyes see only the 
visible light.  White sand absorbs very little visible 
light, while black sand absorbs most of the visible light, 
so white sand tends to be significantly cooler than black 
sand.  But I think both types of sand absorb infrared 
radiation well, and that's half the solar energy hitting 
the beach.

Sand, and concrete, tend to reflect visible light well, 
but absorb infrared light.  Foliage, on the other hand, 
tends to absorb visible light well, but reflects infrared 
light.  You can see this in infrared photography.

It might be interesting to put samples of sand and foliage 
under lights that radiate primarily visible, infrared, or 
both, and compare how hot the samples get get under the 
different lights.  I'd predict that white sand stays cool 
in visible light, but heats up when there's infrared.

-Richard

On Tue, 16 Sep 2008 11:28:15 -0400
  "Daniels, Alissa" <[log in to unmask]> 
wrote:
> 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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