Pointed my IR laser at the sky today, and it showed about -12.2 celsius. Is there no blackbody radiation in the air? Or there is such gases like Nitrogen? Argon? Also, how far does the laser reach into the sky?
In: Physics
The air does emit some infrared, but not very much due to its low density. It’s also largely transparent to infrared, so what the sensor is reading is a bunch of different wavelengths of IR corresponding to air molecules at different altitudes. Call it a weighted average of sorts of air temperatures along the thermometer’s line of sight.
The bottom two layers of the atmosphere- the troposphere and the stratosphere- account for the vast majority of the atmosphere’s mass (anywhere from 90-99% depending on whose estimates you’re using).
The troposphere transitions from an average of 15C at sea level to about -50C at the boundary with the stratosphere. The stratosphere from there warms slightly with altitude until it reaches about -15 C at its upper boundary.
So in a certain sense, what your IR thermometer is telling you is that average temperature of the air in the sky is pretty cold- and guess what, it is! And the value you got, ~ -12 C, fits squarely within that range.
The value itself is probably a bit off due to the thermometer not having the correct emissivity assumption for air, but that’s a bit beyond an ELI5.
In short, your infrared thermometer, when pointing it up at the air, is telling you that the air is cold on average up there…
…because it is!
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