A planetary boundary layer is the part of an atmosphere strongly affected by exchanges of momentum, heat and material with the underlying surface on relatively short timescales. Surface friction generates shear, while heating or cooling changes buoyancy and hence vertical mixing. Its thickness and internal structure vary with the forcing rather than being fixed by one universal altitude.
A sun-heated surface can drive vigorous convection, whereas nocturnal cooling can create stable density stratification and suppress mixing. This distinction matters for interpreting a weather station: a near-surface wind or temperature observation need not represent the air above the boundary layer. Moisture and suspended material are likewise sensitive to the strength of vertical transport.
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The planetary boundary layer (PBL) is a key part of the Earth's atmosphere that is directly influenced by the presence of the Earth's surface. It is the lowest portion of the atmosphere, typically extending from the surface up to about 1 to 2 kilometers (or approximately 0.6 to 1.2 miles) in altitude, although its thickness can vary depending on weather conditions, terrain, and time of day.