The stratosphere is a layer of the atmosphere of the Earth located between 14 km altitude approximately and 60 km altitude. It is a part of the atmosphere where the temperature gradually increases with increasing altitude. The stratosphere is separated from the troposphere below it by an intermediate zone called the tropopause.
In the upper part of the temperature profile below, from 14 to 35 km, the temperature of the air increases with altitude. A mass of air in the stratosphere will not be tempted to rise, but rather remain at a particular height. This leads to the creation of layers – or strata – of air with identical thermal properties, hence where the name stratosphere comes from.
In a stratum of air, the air can only mix with neighboring air masses with the same properties, so only horizontally, without passing over or under another layer of air.
The small fluctuations visible in the temperature profile of the stratosphere is due to the stratosphere’s layered structure.