The standard temperature lapse rate means the temperature is decreasing at a rate of 2° C or 3.5° F per thousand feet gained. This holds true up to 36,000 feet msl. So, while your standard temperature at msl is 15° C or 59° F, your standard temperature at 20,000 feet msl will be -24.6°C or -12.3°F.

At an altitude of 20,000 feet (6,096 meters), the air is significantly colder than at sea level due to the "lapse rate," where temperature typically drops by about 2∘C (3.5∘F) for every 1,000 feet of ascent. In a standard atmosphere, if it is 15∘C (59∘F) at sea level, the temperature at 20,000 feet would be approximately −24.6∘C (−12.3∘F). This extreme cold is why commercial aircraft must have sophisticated heating and pressurization systems; without them, the human body would suffer from both hypoxia and hypothermia within minutes. Pilots also monitor "Density Altitude," as the thinner, colder air at this height affects engine performance and lift. While the air temperature remains frigid, the high-altitude sun can be deceptively intense because there is less atmosphere to filter out UV rays. For those flying in small, unpressurized private planes, "oxygen and parkas" are the standard gear at this level, as the environment is effectively inhospitable to unprotected human life.