To put it another way, the accuracy of your meter depends greatly on the
emissivity of the object it's pointed at. In theory, to get an accurate
non-contact measurement, you must know and correct for the emissivity of the
object of interest. Some thermometers have an adjustment for this, but most
inexpensive ones just assume it's around .95 (which is reasonably close for
most materials).
http://www.engineeringtoolbox.com/emissivity-coefficients-d_447.html
But air has a really lousy emissivity and you aren't correcting for it.
-- Randall
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