大气边界层（Atmospheric Boundary Layer）为大气层中最接近地球表面的空气，其空气的流动受到地表的摩擦阻力、温度差异和地球自转的影响，水平向流速的垂向剖面成不均匀分布，流场较为复杂。大气边界层的厚度约为几百公尺到1000公尺之间，但因为大部份的人类皆居住、活动于大气边界层之中，故大气边界层对人类而言，十分重要。大气边界层之上的对流层，受地表影响较小，地球自转的影响较大，水平流速成均匀分布。

The planetary boundary layer (PBL), also known as the atmospheric boundary layer (ABL), is the lowest part of the atmosphere (extending from the ground to the bottom of where cumulus clouds form) 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.