Editing Planetary boundary layer (section)

==Constituent layers==
[[File:20120629 atmospheric thermocline.JPG|thumb|A [[shelf cloud]] at the leading edge of a thunderstorm complex on the [[South Side (Chicago)|South Side of Chicago]] that extends from the [[Hyde Park, Chicago|Hyde Park]] [[Community areas of Chicago|community area]] to over the [[Regents Park (Chicago)|Regents Park]] twin towers and out over [[Lake Michigan]]]]
As [[Navier–Stokes equations]] suggest, the planetary boundary layer turbulence is produced in the layer with the largest velocity gradients that is at the very surface proximity. This layer – conventionally called a [[surface layer]] – constitutes about 10% of the total PBL depth. Above the surface layer the PBL turbulence gradually dissipates, losing its kinetic energy to friction as well as converting the kinetic to potential energy in a density stratified flow. The balance between the rate of the turbulent kinetic energy production and its dissipation determines the planetary boundary layer depth. The PBL depth varies broadly. At a given wind speed, e.g. 8&nbsp;m/s, and so at a given rate of the turbulence production, a PBL in wintertime Arctic could be as shallow as 50&nbsp;m, a nocturnal PBL in mid-latitudes could be typically 300&nbsp;m in thickness, and a tropical PBL in the trade-wind zone could grow to its full theoretical depth of 2000&nbsp;m. The PBL depth can be 4000 m or higher in late afternoon over desert.

In addition to the surface layer, the planetary boundary layer also comprises the PBL ''core'' (between 0.1 and 0.7 of the PBL depth) and the PBL top or ''entrainment layer'' or ''capping inversion layer'' (between 0.7 and 1 of the PBL depth). Four main external factors determine the PBL depth and its mean vertical structure: 
# the free atmosphere wind speed;
# the surface heat (more exactly buoyancy) balance;
# the free atmosphere density stratification;
# the free atmosphere vertical wind shear or [[baroclinicity]].

{{Further|Ekman layer}}