The Planetary Boundary Layer (PBL) is the lower part of the Troposphere which
is directly influenced by the surface of the Earth.
The planetary boundary layer can be between 300 m and 3,000 m in depth depending on the terrain.
Surface friction, terrain, and solar heating all influence, to varying degrees,
the part of the atmosphere closest to the surface, causing turbulence, convective activity,
and changing wind direction.
Above this planetary boundary layer, turbulence is less marked and winds follow
the geostrophic flow along Isobars;
this part of the atmosphere, where flow is not influenced by the surface,
is known as the Free Atmosphere.
Wikipedia: "The planetary boundary layer (PBL), also known as the atmospheric boundary layer (ABL),
is the lowest part of the atmosphere and its behavior is directly influenced
by its contact with a planetary surface.
On Earth it usually responds to changes in surface forcing in an hour or less.
In this layer physical quantities such as flow velocity, temperature, moisture etc.,
display rapid fluctuations (turbulence) and vertical mixing is strong.
Above the PBL is the "free atmosphere" where the wind is approximately geostrophic
(parallel to the isobars) while within the PBL the wind is affected
by surface drag and turns across the isobars.
The free atmosphere is usually nonturbulent, or only intermittently turbulent."