The mesosphere begins at an altitude of about 50 kilometers (30 miles), as temperature begins to decrease with altitude.
The stratosphere begins about 9-12 kilometers (16-17 kilometers in the tropics) above Earth's surface. The stratosphere is heated from above (primarily as oxygen and ozone absorb solar ultraviolet radiation). Temperature in the stratosphere increases with altitude. In this region (Latin: stratum, layer), mixing is much slower and the ozone layer is found here.
The troposphere begins at the Earth's surface, which acts as a source of heat resulting from absorption of visible sunlight. The temperature decreases with height in the troposphere, and so the air is well mixed in this region (Greek: tropos, a turning). Weather occurs in this layer, and most commercial airliners fly in it.