The Atmospheric boundary layer (ABL) is the portion of the atmosphere most affected by the sun's heating of the Earth's surface, and by friction. It is typically 1 to 2 kilometres above ground level; the depth is dependent on the temperature of the underlying surface. Warmer surfaces in the summer months will have a thicker atmospheric boundary layer than a cooler surface in the winter months.

The layers from bottom to top in the ABL consist of a surface layer, convective mixed layer and entrainment zone, which consists of a temperature inversion.

Meteorological importance

The ABL is important meteorologically in terms of assessing of convective instability. The entrainment zone at the top of the ABL acts as a lid on rising (and cooling) air parcels due to the temperature inversion. An external mechanism such as geographically forced uplift, vigorous surface heating or drylines, can break the entrainment layer, allowing the capped air parcels to rise freely. As a result, vigorous convection will begin producing severe thunderstorms.