I have just measured under a blue tarp in direct sunlight and got 27C reading with my tiny infrared thermometer. I had just sprayed a toy with white paint on some newspaper. When I put the newspaper over the top of the blue tarp I thought it would be even cooler, but NO, it instantly read 35C. The air & stuff around is 32C, the ground under the tarp is 29C, so I assume heat is radiating UP from the warmer ground to the cooler underside of the tarp. The tarp is actually trying to cool down the ground! If I point the gadget to the cloudless BLUE sky it reads 14C.... not sure if these things measure temp. from outer space too well, (but I am interested in super cooling effects of a clear night time sky on a roof ..... another topic) Why does it instantly show higher temp. from 27C to 35C ie +8 centigrade increase? when I add these extra shade(s) over the top? One or doubled layers of paper made almost no difference. I am thinking the Sun's wavelengths absorb maybe 85% into the blue tarp, it re-radiates some of this heat 50:50, prox. 50% to above to the 14C sky &outer space betyond that, & 50% to the 32C ground below, and some heat you would think would be swept away by conduction of the air gently blowing by... but this is 32C air so it is heating the underside 27C plastic? Is it the BLUE colour radiating really well to the super cold outer space &sky? And when I cover with paper it is this huge loss that is blocked? Can any one explain this? or can't these thermometers be trusted to read from blue colors? A bit difficult to measure with a contact thermo. I should try same tests with different tarp colours? Another test showed paper alone 37C, paper doubled 36C, glass alone 39C, 29C underside Blue Tarp. I forgot to measure the TOP sides of any of these.
closed as unclear what you're asking by John Rennie, Ali, user36790, Steeven, Kyle Kanos Jan 10 '16 at 12:21
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Truckers tarp is designed to insulate as well as waterproof. Blue light spectrum is the hottest and the most common blue tarp reflects that spectrum best.