1. The transition region between the turbulent surface layer and the normally nonturbulent free atmosphere. This region is about 1 km thick and is characterized by a well-developed mixing generated by frictional drag as the air masses move over Earth's surface. This layer contains approximately 10% of the mass of the atmosphere. Also called the "atmospheric boundary layer" or "frictional layer. "

2. That layer of the atmosphere from a planet's surface to the geostrophic wind level including, therefore, the surface boundary layer and the Ekman layer. Above this layer lies the free atmosphere. Also called friction layer, atmospheric boundary layer.