Stability is commonly evaluated by examining vertical soundings. This is sometimes referred to as thermal turbulence. In forced convection the air is mechanically pushed, such as along fronts or mountains. In free convection , air moves vertically without any external help. Forced convection involves neutral or stable air.
Free convection requires unstable air.
Stable air reduces convection and thus has less thermal turbulence. These eddies are caused by surface friction or the roughness of the surface. The degree of turbulence depends on the wind speed, stability, and the roughness of the surface. This type of turbulence is also referred to as mechanical turbulence. Since the vertical motion is in the vertical, gravity plays a major role in restoring the displaced air to is original location.