planetary boundary layer

A planetary boundary layer (PBL) is a layer of a planet's atmosphere adjacent to its surface, which includes that part affected by the surface due to friction between the surface and wind, due to the day/night heating and cooling of the surface (which contributes to convection), and due to evaporation at the surface. The atmosphere above the PBL is termed free atmosphere and the convection mixes the two layers. Within the PBL, wind is chaotic and generally from regions of high to those of low pressure, but above the PBL, within the free atmosphere, wind tends to follow general circulation patterns and align with isobars rather than blow crosswise toward them. On Earth, clouds tend to sit at the top of the PBL. During the night, the PBL shrinks, losing height generated by the convection resulting from the daytime heating.