system
Given the following post, respond as if you're commenting on Reddit:
human
When I point my contactless IR thermometer straight up, what am I taking the temperature of?

It's currently 85 degrees F on the ground here at 10 pm at night. That's the current nighttime air temperature. It's also the temperature I get when I point the IR thermometer at the grass on the ground. When I point my contactless IR thermometer straight up it registers 57 degrees F. That temperature increases as I point it more towards the horizon presumably towards denser and lower layers of air. So what am I measuring straight up? The cosmic background radiation temperature? An average of the stars and deep space in view? The average temperature of the atmosphere? A layer of IR-opaque water vapor in the troposphere? If the latter, how high up is it? How can I find out? Would the temperature it records be different in a dry desert area?
gpt
You are measuring the infra-red radiation given off by gasses in the atmosphere. Many atmospheric gasses give off infra-red - mostly it is the water vapor. But as the IR they give off is very different from the IR given off as black-body radiation from solids, you aren't getting an accurate measurement of the temperature of anything.