A cool layer of air adjacent to a cold surface of the earth, where temperature within that layer is statically stably stratified. SBLs can form at night over land when the earth is cooled by net loss of radiation, and they can form at any time when air moves over a relatively cooler land or water surface. Many interacting processes can occur within the SBL: patchy sporadic turbulence, internal gravity waves, drainage flows, inertial oscillations, and nocturnal jets. See nocturnal boundary layer.