Most hot-air balloon flights near Earth’s surface take place in the troposphere.
The troposphere is the lowest layer of Earth’s atmosphere. It begins at the surface and extends to an average height of about 12 kilometres, although its upper boundary varies with latitude and season. Weather, clouds, and most atmospheric water vapor occur there.
Hot-air balloons normally operate in the lower troposphere, where the air is dense enough to provide useful buoyancy and where pilots can fly without specialized high-altitude life-support equipment. The balloon’s heated air is less dense than the surrounding tropospheric air.
The stratosphere begins above the troposphere and contains the ozone layer. Special high-altitude balloons can enter the stratosphere, but ordinary passenger ballooning is overwhelmingly a tropospheric activity.