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
Infrared guns show below minus Celsius temperatures in the sky because they detect the infrared radiation emitted by objects. The sky, especially on a clear day, emits very little infrared radiation due to its low temperature, often resulting in readings that are well below freezing.
Blackbody radiation is from an idealized literal “black” body. For clear air, no IR is emitted or absorbed.
You can actually cool things to sub-ambient temperatures on clear nights because of this.
The laser is just there to help aiming the thermometer, it’s not actually involved in measuring the temperature.
Sensor is measuring infrared radiation and unless you point it straight at the sun there aren’t any notable sources in the sky.
Even then, it’s not going to tell you the Sun is 6000 C, the IR thermometers are built with assumption that black-body radiation is mostly in IR range so they tend to measure -50 to 500 C stuff. When things are visibly glowing your IR thermometer is out of its depth.
IR thermometer “guns” pick up infrared light from whatever they’re pointed at. The laser just helps you see where it’s aimed.
While the sky offers some infrared, especially any CO2 gas up there, much of what it will detect (or *not* detect) is infrared from space.
Being mostly empty, there isn’t much infrared from space, so it appears cold to the thermometer.
There is blackbody radiation in the air, but colorless gasses like nitrogen, oxygen, and argon in our atmosphere are very bad at interacting with infrared light. Things at -12°C emit very little infrared light through blackbody radiation, so since the thermometer gets very little infrared, it assumes it must be at that temperature or below
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!
One thing that I haven’t seen mentioned yet is that an infrared thermometer needs to be adjusted for the emissivity of what you are trying to measure. Different substances emit different amount of IR at temperatures. If you don’t adjust for this you can be way off.
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