The IR thermometer works by looking at the spectrum of infrared light
being emitted by an object (blackbody radiation). When you aim it
into the sky, it's just going to collect all of the infrared light it
sees and interpret it the best that it can.
I can see two reasons for getting -15.
One option: It's a weighted average of reflected IR from everything
terrestrial around you and then the IR making it through the
atmosphere. The IR from all the stuff around you looks like about 50
degrees, and the stuff from the upper atmosphere and the rest of the
universe looks like less the farther out you go (Eventually becoming
very, very cold). It's going to see more of the closer stuff, with
some of the really cold stuff in the upper atmosphere (and beyond)
averaged in. Either that works out to be -15, or the thermometer has
a low temp. limit of -15 and it just reads that for temperatures less
than or equal to -15.
2nd option: It's not getting enough IR to its sensor to determine a
temperature and -15 is the default reading in that situation.
-Paul
On Fri, Jan 29, 2010 at 1:20 PM, Elton E. (Tony) Clark
<eltonclark@gmail.com> wrote:
> *I have a little IR temperature meter which I use around the shop checking
> AC in and out temps, motor temps, etc . . one day I was outside checking
the
> wall temp to see how my insulation was working, it was about 50 degrees
> ambient and . . . I aimed the meter at the clear blue sky: It read minus
15
> degrees!!!!!! Is it really measuring something? Is that a default?
> HELP!!!!!*
>
> *Tony*
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