This is a literal question. If you can see mountains outside can you please tell me what temperature they are with your cheap infrared thermometer? (if you have one). Why am I asking? I don’t have mountains outside, I am really curious, and I can’t find the answer online. It’s a dry clear night here in the midwest and I just measured the sky at -40C(-40F). Far away trees and buildings are around the temp outside. In the past clouds going by were 10C less than air temp. The sun has been hotter than outside, but I have only gotten a reading +15C over air temp so far. I think it would be so darn cool if you can take the temperature of mountains 10-20 miles away with a $15 thermometer.
I think the rule of thumb is that every 100 meters lowers the temperature by 1C. So if you know that on the ground the temperature is 10C and you know the mountain is 600 meters then on top of that mountain the temperature would be around 4C. But maybe this rule of thumb doesn't apply everywhere and everytime.