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I just got myself an infrared thermometer. I wouldn't have been able to predict what temperature it would give me when pointing at the sky, but it turned out to be -2 °C the first time I measured, and slightly going up and down with the temperature on the ground (say from -6 °C to 2 °C in the course of a day in which the ground temperature goes from 20 °C at night to 30 °C during daytime).

Some details: this was for pointing straight up (as long as its not pointing too closely to the sun), a clear sky and a humidity of 63% according to internet. The range in which it measures radiation is from 8-14 μm and the specified range of temperatures is -50 to 550 °C.

What is it that causes this reading?

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  • $\begingroup$ I believe your question has already been asked: physics.stackexchange.com/q/41399 $\endgroup$
    – ACuriousMind ♦
    Commented Jun 21, 2014 at 23:06
  • $\begingroup$ Thanks @ACuriousMind. That question is very similar indeed. I'd like to point out though that in my case the result does not seem to be due to the lack of any clear signal; I get quite precise and repeatable readings, and much higher than readings inside my freezer for example $\endgroup$
    – doetoe
    Commented Jun 21, 2014 at 23:23
  • $\begingroup$ Interesting. Mine shows between 2C and 6C. $\endgroup$
    – Brandon Enright
    Commented Jun 21, 2014 at 23:32
  • $\begingroup$ @BrandonEnright See mynasadata.larc.nasa.gov/804-2/1035-2 Makes me almost want to go out and buy one of these things to play with. $\endgroup$
    – Selene Routley
    Commented Jun 22, 2014 at 0:45
  • $\begingroup$ I think this question should be kept open. The previous question was why an IR thermometer didn't work when pointed at the sky; this question is about explaining a specific and repeatable result within the instrument's operating range which has a different and more interesting explanation. $\endgroup$
    – rob ♦
    Commented Jun 22, 2014 at 12:34

2 Answers 2

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You are likety to be directly measuring the greenhouse effect of the atmosphere. The fact that your cursory measurements seem to be correlated with the air temperature around you supports this idea: the measurement is higher in the day because the Earth itself is hotter and radiating back into space more powerfully. Greenhouse gasses are thus absorbing some of this radiation, and sending their re-radiated infrared back to your thermometer. The thermometer will also be influenced by clouds and the like, even mist and haze that may be hard to discern as clouds.

I would try noting the thermometer's reading with different atmospheric conditions, noting the sky condition (cloudy, clear), local temperature and humidity. Although I wouldn't be too surprised if the reading were not influenced by the last: humidity at the ground may not give a reliable idea of how much water there is in the various jetstreams and so forth at different heights.

But you're definitely measuring something "real" and repeatable: see the following experiment, which is a formalisation of what you have done, suggested by NASA:

Measuring the Temperature of the Sky and Clouds

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    $\begingroup$ For if you're interested: I had the opportunity already this morning, which happens to be very cloudy. We have 25°C on the ground, and measuring the sky gives 19°C now. $\endgroup$
    – doetoe
    Commented Jun 22, 2014 at 9:10
  • $\begingroup$ Hmm. If one properly(!) kept a long term read of the readings and had a good enough instrument, could one potentially use this to test global warming for oneself? (e.g. plot the data over several years and look for the secular trend) Since you're measuring the greenhouse effect directly, any trend you observe (that is not experimental error!) would be a change in greenhouse effect, and thus would eliminate all proposed claims by global warming deniers: solar flux, "it's just not happening", greenhouse effect isn't real, UHI, whatever in one fell swoop. All 100% independent research. $\endgroup$
    – The_Sympathizer
    Commented Feb 27, 2017 at 9:09
  • $\begingroup$ And conducted on a (relatively) shoe-string budget too, making it impossible for them to claim you're rich or whatever (although they might still anyways but they would be so easily exposed a laughing stock it wouldn't even be funny). $\endgroup$
    – The_Sympathizer
    Commented Feb 27, 2017 at 9:12
  • $\begingroup$ Clear, windy, sunny day, air temperature around 0, thermometer reads -35°C. $\endgroup$
    – SF.
    Commented Feb 6, 2018 at 13:12
  • $\begingroup$ @The_Sympathizer "solar flux" Huh? More solar flux means the earth is warmer, which means it radiates more infrared, which means the atmosphere reflects more back. "it's just not happening" Only people waaay out there claim that there's no greenhouse effect at all. You seem to have a rather simplistic idea about what the debate is about. $\endgroup$
    – Acccumulation
    Commented Apr 5, 2019 at 20:40
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All bodies above absolute-zero emit some radiation. This is "black-body" radiation and it can be correlated to temperature using the Stefan-Boltzmann law. Your infrared thermometer uses this to calculate the temperature by measuring this radiation.

The temperature you measure for the sky is the radiation of an equivalent blackbody at -2°C. This is sort of an "average" of the temperature of the air column above you (but take into account that the Stefan-Boltzmann law has a power of four in there so it is not a linear average).

I come from a colder climate and I can tell you that snow on the ground will persist when the air temperature is above 0°C for days or even weeks. One reason is that the sky is below the temperature of the snow and the surrounding air. The snow can emit more radiation to the sky than it absorbs, offsetting the heat gained from the surrounding air.

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