= Planetary boundary layer
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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