Richard J Kinch wrote:

What the radiation thermometer reads, looking at the background of outer
space, overlaid with a transparent but blackbody-radiating layer of
atmosphere...

Which absorbs IR...

No, like all matter, the atmosphere both emits and absorbs radiation...

No? This is unusual logic. It both emits and absorbs radiation,
but it doesn't absorb IR? :-)

When you point your Raytek at the clear night sky, you are measuring that
emission...

I don't think so. Try it. I've pointed my Raytek at night skies and daytime
skies, with clouds, and read similar very cold temperatures. When I point
a $1K Exeltech at the same cloudy sky, I read a much higher temp. I think
the Raytek measures emissive power in a narrow wavelength band that excludes
the strong water vapor absorption bands around 1, 1.4, and 1.8 microns, but
the Exeltech looks at more of the spectrum.

Nick