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I'm toying with an infrared thermometer - one which you point onto an object, press the button and it instantly measures the temperature by estimating the infrared radiation from the object. It shows more or less the room temperature when I point it to things in the room.

Now I open the window. The temperature outside is 12 degrees Celsius. I point the thermometer to the ground - it shows something near 12 degrees. When I point it horizontally and directed onto a building located several hundred meters away - it shows something like 5 degrees. When I point it onto the sky it shows an error message indicating that the measurement result is below zero Celsius and it can't display it.

This doesn't make sense - the ground has the same temperature as the air and there's a thick layer of air above, so wherever I point the thermometer the temperature is more or less the same and so the infrared radiation intensity should be more or less the same. Yet the thermometer displays largely different results.

Why does the infrared thermometer display much lower temperatures when directed to the outer air compared to when directed onto nearby solid objects?

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2 Answers 2

up vote 6 down vote accepted

The reason for this can be seen by examining how an infrared thermometer works. As you mentioned, it measures the infrared radiation, and uses this to determine the temperature. So, with that in mind, we consider what happens in the situations you mention. Namely, you cannot use it to measure the air temperature, because the emissivity of the air is piss-poor. So it will "think" the air is very cold, when it is not. Pointing at the sky, well, there's no IR emission of any great significance, that the device would register.

This can be seen by examining footage taken with a thermal infrared camera. This shows how the world "looks" in the thermal infrared band that these devices use. Using such a camera, one sees that the sky appears black. There is little emission, and there is practically no Rayleigh scattering of sunlight either at these wavelengths (it scatters better in the visible spectrum, esp. toward the blue, which is why the sky is blue).

For pointing at a far-off building, the distance means the amount of IR light reaching the device will be too small for it to make a proper reading.

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The sky at night (no clouds) typically measures -48c to -21c at my location (Seattle), currently -44c. The clouds measure -5c to -1c right now. The trees across the road are at 2c.

The reason I can measure the clouds is that the atmosphere is largely transparent to IR, so my thermometer "sees" the radiation from the clouds, but not the atmosphere - the intervening gasses don't tend to absorb or emit photons of IR radiation, so don't interfere with the measurement (the technical term for this is "piss-poor emissivity").

Water is a good emitter - so if it were raining (and, being Seattle, I'm surprised it's not), it'd mainly "see" the radiation coming from the droplets (although the temperature reading might also be wildly inaccurate, because your typical cheapo IR thermometer doesn't actually measure the dominant IR wavelength - which would allow an accurate calculation via the Wien displacement law - but instead measures the total IR emitted at a specific wavelength, extrapolating that back to a temperature via the Stefan-Boltzmann law, which is that the amount of radiation emitted is proportional to the fourth power of the temperature).

As for the -44c of the clear sky: either something up there (eg ice/dust particles in the stratosphere) is emitting at that ambient temperature, or the temperature reading is meaningless. I tend to prefer the former view, as the temperature is remarkably consistent depending on the season :)

If your thermometer went below 0c, I presume you'd see the same.

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Addendum to the stratosphere: that temperature's going to be wildly inaccurate too, since my thermometer measures total IR, and for the Boltzmann calculation to be accurate the object being measured has to cover your entire field of view (obviously, there's a lot of space between dust particles). – MikeM Nov 16 at 5:01

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