planetary boundary layer (PBL)

The atmospheric layer closest to a planet's surface. On Earth, for average mid-latitude conditions, the PBL is about 1 kilometer deep. The layer is characterized by mechanical eddies which mix momentum down from aloft and thermal eddies which mix moisture and heat up from the surface. Therefore, highly stable environments may have only 30 meters deep PBL while highly convective environments may have a 3 kilometers deep PBL. The PBL can be further subdivided into a bottom surface layer and the top Ekman layer.