Consider the following hypothetical situation. The surface temperature is 10°C and the environmental temperature lapses normally until height ‘X’, then there is an inversion. The surface is heated at a particular location, which causes the temperature to rise to 20°C. The air in this region becomes less dense and starts to rise.
This is what is known as a “thermal.” In this case, the air is unsaturated so it is called a dry thermal.
Because the air is unsaturated, it cools at the DALR — faster than the environment and hence eventually the two lines will intersect. In the hypothetical example, the two lines intersect at X, the height at which the inversion starts.
If the thermal were to continue to rise it would follow the dotted line, so it would be cooler than its environment. Therefore, it will be more dense and no longer has the tendency to rise.
If you were to fly below height X, you would experience turbulence due to the updrafts in the thermal. Above height X, the conditions would be smooth.