Each country publishes its own transition altitude. A transition altitude is the altitude above sea level where pilots must set their altimeters to standard pressure. When flying with the altimeter set to standard pressure, the altitude is expressed in flight level. Flight level is obtained by removing the altitude's three rightmost digits. For example and altitude of 21,000 feet will be expressed as FL210.
Air density is a major factor in aerodynamic performance and engine efficiency.
An increase in air temperature, altitude and humidity causes a decrease in air density. Since lift and drag are inversely proportional to the air density, a loss of lift is caused by reduced air density. The propeller efficiency is decreased for the same reason. Power decreases because the engine does not get enough air to support an optimum combustion. In jet engines, a lesser mass of gasses is pushed out through the exhaust nozzle.