Anything cooler than the sun will absorb heat from the sun. Being made of metal has nothing to do with it.
If you're looking for something to keep your computer cool while you're using it, you have a ton of options - virtually anything that's roughly the same footprint as your Mac. But if you're looking for something to keep it cool while it's sitting in your car on a hot day, I'm not sure such a product exists (if it did, car manufacturers would buy them all and install them in seats!).
I also live in Texas and I don't think twice about leaving my MBP in the car for even up to several hours if I need to - but it isn't running under such circumstances, so an external cooler or heat sink really isn't needed, nor would it be any help even if I did have one.
Well, I used it all day last Sunday at a local park. I was in the shade, but it was over 100 degrees in the shade. I think I may have melted some of the internals of my MBP...
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Agree. I likewise live here in Texas and there is no way I would leave any of my Apple devices in my car and turned on while I was out and about. The temp in your car even with a windshield shade can easily reach 120 F in the middle of Summer.
That's what my thermometer says!
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I bought my wife a cooling pad for her MBP at Walmart. Cost about $15, has dual fans, is light and is powered by her USB port. Helps out a lot and has a good pitch to it for laptop use. Also very quiet. Can't recall the brand off the top of my head, but it's a wedge shaped device.
So you think it helps?
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Radiative heat transfer ("under sunlight," as you put it) is completely independent of material - it's based on temperature difference, and only on that. Read more about the Stefan-Boltzmann equation.
But your computer doesn't get hot in your car because the sun is shining on it, it gets hot in your car because the sun's radiation heats the inside of your car, and the glass lets very little of that escape. The effect on your computer is the same as putting into a warm oven; that is interface heat transfer, and it's not a function of the object's material, but of BOTH objects' material.
Sorry, but that is totally wrong!!
My aluminum MBP gets hot in my car MUCH MORE because it was in the sun than because my care was hot.
Doubt this?
Put on some short shorts, and go sit on the metal hood of a car and tell me how it feels!! (Then go sit on a piece of styrofoam - also in the sunlight - and tell me how it feels.
Metal is more dense than stryofoam, thus is CONDUCTS heat much better.
Yes, the closed environment of your car gets hot, but gases do not CONDUCT HEAT the way a solid does.
That is why metal and stone retain heat so much better than wood, styrofoam and so on.
My MBP get wickedly hot in my car first because it was in the sunlight and it is metal, and then secondly because it was hot in my car. (My car seats are clothe, and in a 120 F car they don't feel hot. But even on a Texas Winter day, when my metal laptop is in the sunlight, it gets hot as hell!