hmm. the gentleman in the video linked is terribly confused in regards to, ahemm. he seems to be clear about the electric humvee and being sponsored, but otherwise, it's all a bit of a mayonaise.Originally Posted by Zeusrex
it's understandable, the way heat is transferred is messed up, i think most engineers are confused about it, not to mention "normal humans".
to make a long story short: if you want dark under the darp, in daylight, you should look for a tarp coated with a reflective/opaque layer (some of them are not that thick or heavy, they are often advertised as having an integrated survival blanket "feature", because that's basically what it comes down to)
the "a bit longer short story":
- the amount of radiative heat transfer which is stopped by the tarp will make a huge difference, especially (but not only) in direct sunlight; if what stops it is absorption or reflection is perhaps less important, in this case (because most camping tarps are too thin to stop much of it reaching the monkey under the tarp, and that's the first thing to focus on);
- the color of the tarp is not directly correlated with how transparent or opaque the tarp is, it's best to keep the two things separate, for ease of understanding.
- if you covered the point above, and you made sure the tarp lets through near 0 IR from the sun, only then the difference between colors can be relevant, but we're talking about the difference between IR color (if that's even acceptable wording); that is to say, it's the IR range which is most important, you can have something that looks nearly black to the naked eye, but reflects most of the IR, so doesn't get much hot, and something which looks very bright in the visible spectrum, but absorbs lots of IR (for instance, a somewhat similar example is folliage of most green plants: they look relatively dark, but tend to not get very hot in the sun (which is only partly due to water evaporation), but at the same time they do absorb relatively high amounts of energy in UV (and they use it to turn CO2 into magic); green plants look very dark in UV, dark-ish green in visible spectrum, and bright hot white in IR; when it comes to energy transfer through radiation, our vision is useless (i guess we evolved to hunt for colorful fruit hidden in the folliage, not for hot blooded animals; but i digress)
this is the very short version, btw. within the IR spectrum, things get even more weird. for instance, there's narrow bands of IR which will transfer energy very efficiently to particular substances, because of the molecular resonant frequencies for the browonian motion of the molecules of that substance, but will be unimpressive when they hit other substances, that prefer (resonate at) different frequencies (this is why good quality IR heating pannels seem magic, they are tuned for the bands (yeah, there's a few) which transfer best to H2O, and us monkeyz contain a lot of H2O); yeah. you don't want to know. deep and weird rabbit hole.
anyway, if the tarp is very absorbant in IR, it will get very hot in the sun quickly, and will radiate some of it back at you, so i'd say it does matter.
if you like to DIY (as it sounds), i suggest to maybe take your time, and experiment a bit: for instance, get a cheap mylar tarp (space blanket/survival blanket), and drape it on top of your usual tarp in direct sunlight, see how/if that makes a difference. keep in mind that such experiment will have very different perceived results on a windy day compared to a still day, due to differences in convective heat loss; relative humidity will also make a big difference (as it influences the thermal confort of humans tremendously, our thermal regulating system is directly reliant on evaporation, and thus relative humidity). ah, btw, don't bother to measure air temperature differences under an "open" tarp, i suppose that goes without saying, instead just see how you _feel_
ahem. well, you asked.<shrug>
have fun, and let us know what you find
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