At 35,000 ft. (11,000 m), the typical altitude of a commercial jet, the air pressure drops to less than a quarter of its value at sea level, and the outside temperature drops below negative 60 degrees Fahrenheit (negative 51 degrees Celsius), according to The Engineering Toolbox.
Just so, how cold is it at 30 000 feet in the air?
On aircraft which fly above 30,000 feet, the temperature can often can get to temperatures below Jet-A's freezing point. For example: at 36,000 feet the standard outside air temperature is -56.5°C (-69.7°F) and is well below Jet-A's freezing point of -40°C/F.
Why do airplanes fly at 40000 ft?
The cruising altitudes for short flights are usually less as compared to the cruising altitude for longer flights. High altitude means less power is needed to propel the plane and at 40,000 feet the engines are just above idle power. These were just some reasons why aircrafts travel high up in the air.