I have learned out here in Hawaii, where the sun can rip the hide off of you, that even beach umbrellas come with SPF ratings now. The cheaper ones with material that reminds me of those blue tarps, have only a 5-15 spf rating. That was good to know, as I promptly went out and bought a 1000 spf beach umbrella!

I guess my point is that I doubt that those blue tarps are really sun-proof.

You could do the same experiment that was done hundreds of years ago to prove the existance of non-visible frequencies of light... (recall, I work for an astronomical observatory). What the guy did (can't remember his name) was to put two thermometers next to each other. One was just past the violet strip of the output of a prism (where UV light waves would hit) and the other was adjacent to it under a truely opaque (spelling?) cover. He discovered that the thermometer that was near the prism output, had a higher temperature than the one with the opaque cover, proving that even though he couldn't see light, that there was energy of some sort being deposited there.

So, anyway... If you put a thermometer under your blue tarp, and another thermometer adjacent to the first but with a truely opaque cover (scrap of wood, high enough above it to not affect air flow. If the temperatures are different, then i'd say the blue tarp is leaking some frequency of sunlight thru.

Chances are no-one wants to know the answer bad enough to do this experiment, but I think It would be interesting.

Just another of my screwy ideas... :-)

Aloha! Paul

> Probably very very very close to 100% until it starts to crumble.
> They typically last about a year. At the price, it's not a bad deal.
>