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
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.