Planetary Boundary Layer

The planetary boundary layer is the layer of the atmosphere directly affected by the Earth’s surface, where turbulence generated by friction and surface heating mixes momentum, heat, moisture, and pollutants within about an hour. Over land its depth swings with the diurnal cycle, from tens of meters in a stable nocturnal layer to 1 to 3 km in a convective daytime mixed layer; over the ocean it is steadier. Wind increases and veers with height through the layer following the Ekman spiral, reaching the geostrophic flow at its top. It is the term used interchangeably with the atmospheric boundary layer.