For completeness, since this came up again:

>[An infrared thermometer][1] is a thermometer which infers temperature from a portion of the thermal radiation sometimes called blackbody radiation emitted by the object being measured.

To get accurate readings from an infrared thermometer, the emissivity of the body aimed at has to be gauged and applied to it.

> By knowing the amount of infrared energy emitted by the object and its emissivity, the object's temperature can often be determined. Infrared thermometers are a subset of devices known as "thermal radiation thermometers".

>[Emissivity][2]:The ratio of the radiation emitted by the surface at a 
given temperature to the radiation emitted by a blackbody at the same temperature

Air  does not follow a black body radiation curve well and has a very small emissivity, [of order of 0.3 for normal humidity and CO2][3], therefore will be far away from the callibration curve of a given thermometer.

One has to know to what [emissivity value][4] the infrared thermometer is calibrated to really have a temperature measurement.

. 


  [1]: https://en.wikipedia.org/wiki/Infrared_thermometer
  [2]: http://www.kostic.niu.edu/352/Heat_4e_Chap12-Radiation_Fundamentals_SUMMARY-PDF.pdf
  [3]: http://www.biocab.org/Overlapping_Absorption_Bands.pdf
  [4]: http://www.optotherm.com/emiss-table.htm