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 m/s, and so at a given rate of the turbulence production, a PBL in wintertime Arctic could be as shallow as 50 m, a nocturnal PBL in mid-latitudes could be typically 300 m in thickness, and a tropical PBL in the trade-wind zone could grow to its full theoretical depth of 2000 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. Principal types Convective planetary boundary layer (CBL) A convective planetary boundary layer is a type of planetary boundary layer where positive buoyancy flux at the surface creates a thermal instability and thus generates additional or even major turbulence. (This is also known as having CAPE or convective available potential energy; see atmospheric convection.) A convective boundary layer is typical in tropical and mid-latitudes during daytime. Solar heating assisted by the heat released from the water vapor condensation could create so strong convective turbulence that the Free convective layer comprises the entire troposphere up to the tropopause (the boundary in the Earth's atmosphere between the troposphere and the stratosphere), which is at 10 km to 18 km in the Intertropical convergence zone). Stably stratified planetary boundary layer (SBL) The SBL is a PBL when negative buoyancy flux at the surface damps the turbulence; see Convective inhibition. An SBL is solely driven by the wind shear turbulence and hence the SBL cannot exist without the free atmosphere wind. An SBL is typical in nighttime at all locations and even in daytime in places where the Earth's surface is colder than the air above. An SBL plays a particularly important role in high latitudes where it is often prolonged (days to months), resulting in