A statically stable layer at the top of the atmospheric boundary layer. Although the word “inversion” implies that temperature increases with height, the word “capping inversion” is used more loosely for any stable layer (potential temperature increasing with height) at the top of the boundary layer. This inversion is a ubiquitous feature of the atmospheric boundary layer, formed because the troposphere is statically stable on the average, and because turbulence homogenizes air within the boundary layer, which by conservation of heat requires that a stable layer form at the top of the boundary layer. This inversion traps surface-induced turbulence and air pollutants below it, and causes the free atmosphere to not “feel” the earth's surface during fair weather (i.e., no drag, free slip, no heat or moisture from the surface, and winds are nearly geostrophic). See lid.