The stratosphere is sandwiched between the troposphere -- the lowest level of atmosphere above the Earth -- and the mesosphere. The stratosphere rises from about five miles above the planet to 31 miles up. The altitude of the lower levels varies and is determined by the latitude and changing seasons. The area in which the troposphere meets the stratosphere is known as the tropopause and is where a phenomenon called "temperature inversion" begins.
Unlike on Earth, in the stratosphere the air temperature rises as altitude increases. This is temperature inversion and is the opposite to the situation on the planet itself. This happens because heat from the sun is absorbed by the ozone layer. The warmest air at the top of the stratosphere, an area called the stratopause, has a temperature of about minus 3C or 29.6F. Vertical stratification means the stratosphere is stable, with very little turbulence.
Civilian aircraft routinely travel in the lower reaches of the stratosphere in the region of five to 10 miles(26,000 feet to 39,000 feet) above the surface of the planet. There are various benefits from flying in the lower stratosphere, among them the lack of drag as a result of thin air density, thus saving on fuel. Because the stable environment reduces turbulence, this makes flying more comfortable for passengers and crew.
It may be tempting to think there is no life so high up but there is and it remains part of the biosphere. The principal life forms are bacterial, but birds have been known to fly in the lower reaches of the stratosphere. Indeed, geese are known regularly to fly over the top of the world's highest peak, Mount Everest, which has its summit, at more than 29,000 feet, in the lower stratosphere.