What is my infrared thermometer measuring when I point it at the sky?

Question

I have a Fluke 62 IR thermometer, and I've asked fluke. They told me the measurements are useless, but If I point it at clear sky, I get -4 to -15C, at clouds, I get more typical cloud temps like 10-15C. Obviously this isn't just gibberish, It's measuring something. But, -15 seems a little warm for space. (I imagine the warmth is coming from particulates in the air, etc)

Answer 1

If you place the blackbody radiation curves for Sun & Earth at its average temperature of about 15 °C on the same graph, you'll find the they meet around the wavelength of 4.3 microns - which is between the peaks of solar (.47 microns) of earth IR (10.7 microns) emission. The Fluke 62 measures IR radiation at 6.5-18 microns:



http://support.fluke.com/find-sales/download/asset/2437622_a_w.pdf



Thus solar radiation is probably insignificant - especially pointed away from the sun early or late in the day - obviously no problem at night.



So yes - the temperatures of low & mid level clouds are often about 10-15 °C - which is probably what you are measuring to a large extent. They emit IR radiation very similar to blackbodies at that temperature. It may even register a bit too high in that case because the emissivity of the clouds is greater than objects the instrument is intended to measure, and because of aerosols below. When you point it to the sky, you are most likely getting an integrated average of sorts of the temperature of particles in the atmosphere to space. The average temperature of the atmosphere over the globe is about -20 °C, but the particles are concentrated near ground. I would bet it does indeed register higher when the air aloft is warmer and lower when the air aloft is colder. Play around with it, look at the weather maps, and see. You need nights with absolutely no clouds and probably a similar amount of aerosols and humidity.



You seem to have a pretty good idea about what is causing the readings you are getting, and the numbers seem pretty good - may even be more accurate than pointing it at an object right in front of you - just kidding :) :) :)



Regarding the post below. Water vapor satillite images are from measurements at specific bands wihtin the range of about 6-8 microns and not a spectrum as you are measuring, and are a view down from space - which differs from what you are doing. You can easily test the water vapor only theory during a night with a variable low cloud cover - clear & cloudy periods would be the same. From what you already posted I seriously doubt that.

Answer 2

all infrared thermometers do is interpret the wavelengths of infrared light that it receives.



Fluke is right that it is "useless" because your picking up all sorts of radiation pointing at the sky.



Pointing it at a localized heat source (the way it was designed to be used*) narrows the field of view to an infrared concentrated area, which will either cancel out or cover over the signal "noise" your are detecting in the sky.

Answer 3

It purely measures the duct. That pronounced, the temp of the grill ought to adventure the air after a jiffy. the different ingredient you're able to do is delay a bite of paper in front of the air and factor it at that.

Answer 4

It's not useless... at least to a meteorologist. Your device appears to be measuring wave lengths from water vapor particles around 500 mb... or ~20000 feet...this time of year. Meterologists use IR satellites to monitor water vapor temps from around 300-400 mb, or about 25k-30k feet.



Check out this image/loop. It's a water vapor loop used by mets to track storms.



http://hadar.cira.colostate.edu/ramsdis/online/RSOgeir3.html

Answer 5

probably water vapor... i have this same thermometer, and find that it registers inane values after a certain distance...

Answer 6

Um...Up!