You can get a decent idea of that by considering the conditions at 16 km altitude, i.e. above exactly 90% of the atmosphere: AvgTemp = 217 Kelvin = -56 celsius = -70 Fahrenheit This is a 71 Kelvin drop versus average temp. at 0 altitude; the greenhouse effect is the main reason. I think, as RWG pointed out, the "greenhouse effect" is poor nomenclature since greenhouses don't actually work that way, so this just deceives suckers like me. Better name would be the "atmospheric insulation effect." More insulation with same heating power ==> you get hotter, that is all it is.