<P> In meteorology the planetary boundary layer ( PBL ) , also known as the atmospheric boundary layer ( ABL ) , is the lowest part of the atmosphere . Its behavior is directly influenced by its contact with a planetary surface . On Earth it usually responds to changes in surface radiative 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 . </P>
this is the layer in the atmosphere where friction is important in determining wind speed
planetary boundary layer