In the Earth's atmosphere, the planetary boundary layer is the air layer near the ground affected by diurnal heat, moisture, or momentum transfer to or from the surface.

The thin layer of air adjacent to the Earth's surface; ground layer: The atmospheric boundary layer extends up to the so-called anemometer level (the base of the Ekman layer [thin top layer of the sea]). Within this layer the wind distribution is determined largely by the vertical temperature gradient and the nature and contours of the underlying surface, and shearing stresses are approximately constant.