A planetary boundary layer (PBL) is a low layer of a planet's atmosphere where the surface effects the atmosphere, due to the effect of friction with the ground on wind, due to the day/night heating and cooling of the surface, contributing to convection, and due to evaporation at the surface. The convection vertically mixes the atmosphere within the PBL. The layer above the PBL is termed free atmosphere. 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 due to the daytime heating.